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Virtual Plant Training Manual –

Introduction to Simulator

Based Operator Training

Hydromantis ESS, Inc. 6 March 2017

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Table of Contents Preface ...................................................................................................................................................... 6

Chapter 1: Introduction to the Activated Sludge Wastewater Process ............. 7

Learning Objectives ...................................................................................................................... 7

History of the Activated Sludge Process .......................................................................... 7

Why do we need to treat wastewater ............................................................................... 8

What Happens during Wastewater Treatment? ........................................................ 11

Chapter 2: Use of Process Simulators in the Wastewater Process .................... 15

Learning Objectives .................................................................................................................... 15

Building a Process Model ......................................................................................................... 16

Overview of Treatment processes ..................................................................................... 18

Influent .......................................................................................................................................... 18

Preliminary treatment ........................................................................................................... 18

Primary treatment ................................................................................................................... 19

Suspended growth processes ........................................................................................... 21

Attached growth ....................................................................................................................... 22

Secondary clarifiers ................................................................................................................ 24

Tertiary treatment ................................................................................................................... 25

Biosolids treatment ................................................................................................................. 26

Uses of Simulators....................................................................................................................... 27

Flight Simulators in Wastewater Treatment ............................................................... 29

Chapter 3: About OpToolPro™ .................................................................................................. 31

Learning Objectives .................................................................................................................... 31

The Two Modes in OpTool Pro .............................................................................................. 33

Wastewater Treatment Plant Details ............................................................................... 34

Starting OpToolPro™ .................................................................................................................. 35

Navigating the Interface ......................................................................................................... 37

Clickable Wastewater Treatment Plant Unit Processes ........................................ 37

Accessing More Operational Details & Information ................................................ 38

Bringing units In and Out of Service ................................................................................ 39

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Instructions/Questions Panel .............................................................................................. 40

Simulation Input Panel ............................................................................................................. 41

Simulation Output Panel .......................................................................................................... 42

Additional Process Performance Variables ................................................................... 44

Simulation Control Panel ......................................................................................................... 45

Running Simulations .................................................................................................................. 46

Step 1 - Select a Unit Process........................................................................................... 46

Step 2 - Make Changes to Input Parameters .......................................................... 47

Step 3 - Start the Simulation ............................................................................................ 47

Step 4 - View the Results .................................................................................................... 48

Switching Between Training and Testing Modes ...................................................... 49

Exiting the Simulator ................................................................................................................. 49

Chapter 4: Meeting Effluent Targets – Optimizing Plant Operations ................ 50

Learning Objectives .................................................................................................................... 50

Optimizing Plant Operations ................................................................................................. 56

Adjusting Dissolved Oxygen levels ............................................................................... 56

Optimizing F/M by Increasing Mixed Liquor Suspended Solids ................... 60

Changing the WAS Rate: ...................................................................................................... 65

Effect of sludge wastage on solids generation at the plant ........................... 70

Using External Carbon Sources ....................................................................................... 76

Chemical Phosphorus Removal ........................................................................................ 84

Chapter 5: Exploring Seasonal Effects on Wastewater Operations ................... 98

Learning Objectives .................................................................................................................... 98

Wet weather challenges ........................................................................................................ 101

Step 1: Reduce TP by Increasing Ferric Dosage .................................................. 104

Step 2: Methanol Addition ................................................................................................ 105

Summer conditions ................................................................................................................... 107

Effect of Winter Conditions .................................................................................................. 109

Chapter 6: “What if” Scenario Analysis with Simulators ....................................... 111

Learning Objectives .................................................................................................................. 111

Taking aeration basins offline ............................................................................................ 112

Optimizing the plant at a lower flow .......................................................................... 119

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Taking Clarifiers Offline ......................................................................................................... 123

Optimizing air flowrate ....................................................................................................... 126

Removing 2 Clarifiers from Service ................................................................................. 134

Understanding the relationship between COD, TSS and VSS in the effluent ......................................................................................................................................... 135

Removing 3 Clarifiers from Service ................................................................................. 143

Chapter 7: Tapered Aeration ................................................................................................... 146

Learning Objectives .................................................................................................................. 146

Case 1: 50% (Zone 1), 25% (Zone 2), 15% (Zone 3), 10% (Zone 4) ...... 149

Case 2: 25% (Zone 1), 25% (Zone 2), 25% (Zone 3), 25% (Zone 4) ...... 151

Case 3: 10% (Zone 1), 15% (Zone 2), 25% (Zone 3), 50% (Zone 4) ...... 153

Chapter 8: Evaluate Impact of Temperature ................................................................. 155

Learning Objectives .................................................................................................................. 155

Chapter 9: Effect of Inflow & Infiltration (I&I) ........................................................... 165

Learning Objectives .................................................................................................................. 165

Case 1: 1 MGD I&I ..................................................................................................................... 169

Case 2: 2 MGD I&I ..................................................................................................................... 171

Case 3: 3 MGD I&I ..................................................................................................................... 173

Case 4 I&I....................................................................................................................................... 176

Questions ............................................................................................................................................ 180

Chapter 1 Questions (Overview of the wastewater treatment process) . 180

Chapter 2 Questions (Use of simulators in wastewater treatment) ....... 181

Chapter 3 Questions (Introduction to OpTool Pro) ........................................... 183

Chapter 4 Questions (Using OpTool Pro to optimize facility operations) .......................................................................................................................................................... 187

Chapter 5 Questions (Exploring Seasonal Effects) ............................................ 190

Technical Support .......................................................................................................................... 192

References.......................................................................................................................................... 192

OpToolPro™ Testing Mode Challenges .............................................................................. 193

Challenge 1: High Effluent COD (50 Points) ............................................................. 193

Challenge 2: High Effluent BOD5 and NH4 (75 Points) ....................................... 193

Challenge 3: High TKN with Energy Limit (100 Points) ...................................... 194

Challenge 4: High Effluent TN in Cold Weather (50 Points) ............................. 195

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Challenge 5: High Effluent TP (75 Points) .................................................................. 195

Challenge 6: High BOD5, NH4 and Cold Temperature (50 Points) ............... 196

Challenge 7: High Total Nitrogen (TN) (75 Points) ............................................... 197

Challenge 8: Low SRT and MLSS (75 Points) ............................................................ 198

Challenge 9: Energy and Chemical Cost Management (125 Points) ........... 198

Challenge 10: High-Strength Wastewater Treatment (50 Points) ............... 199

Challenge 11: SRT Control (75 Points) ......................................................................... 200

Challenge 12: Energy Management (75 Points) ...................................................... 201

Challenge 13: Total Nitrogen Removal (50 Points) ............................................... 202

Challenge 14: Clarifier Maintenance (50 Points) .................................................... 202

Challenge 15: Cold Weather, No DO Controller (50 points) ............................. 203

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Preface Training is critical to developing competent and motivated water &

wastewater operators. As the water industry gets faced with challenges that

include the growing complexity of the processes used in the treatment process,

increasingly stringent regulations and an ageing workforce, hands on, simulation

based platforms that allow training to be done in a realistic operational

environment will become even more important.

SimuWorks™ OpTool uses the most advanced process modeling solutions

that are commonly utilized for designing water and wastewater treatment

processes to create high fidelity, realistic 3D simulator platforms that replicate all

of the operational realities of an actual water and wastewater treatment process.

We hope that operators, trainers, researchers, educators and engineers will

find the OpTool training and testing platform useful.

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Chapter 1: Introduction to the Activated Sludge Wastewater Process

History of the Activated Sludge Process

The activated sludge process is over 100 years old. It was developed by two

researchers in the city of Manchester, England called Ardern and Lockett, in 1914.

In the one hundred plus years since the activated sludge process was developed,

it has remained largely unchanged. Bacteria are still at the core of the process.

Bacteria (or Bugs as they are called in the industry) are microorganisms that

feed on organic wastes. Wastewater contains a lot of organic wastes. Organic

wastes have a plant or animal origin and contain carbon. All living things need

carbon. When organisms feed on organic materials, they are able to get the

energy they need for their life activities (metabolism), as well as the carbon

necessary for building their cells.

Learning Objectives

• History of the Activated Sludge Process • Processes Used in Wastewater Treatment • Major Factors that Impact Wastewater

Composition • Drivers for Regulation of Wastewater Discharge • Roles of Different Stakeholders in the

Wastewater Treatment Process

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Carbon is a key part of living cells, and most bacteria get their carbon from

organic matter. Organisms that get their carbon from organic matter are called

Heterotrophs. Most of the microorganisms that are found in the biological

activated sludge wastewater treatment process are heterotrophic.

Some microorganisms are able to get the carbon they need from non-

organic sources of carbon such as carbon dioxide. They are called autotrophs and

include organisms such as algae and nitrifying bacteria.

The activated sludge process is focused on the use of bacteria for removing

organic wastes (chemical oxygen demand or biological oxygen demand) and

nutrients like Nitrogen and Phosphorus.

Figure 1: Simple Wastewater Treatment Facility

Why do we need to treat wastewater

Wastewater comes mostly from sewage that originates from homes, industries

and run-off. Eventually, sewage drains into a receiving water body, which might

be a lake, stream, river, or even ocean.

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Figure 2: Sewage Discharge Flowing into a Receiving Body of Water

If the organic waste and nutrients in wastewater are not effectively

removed, these enter into the receiving water body. The presence of organic

matter encourages bacterial growth, eventually leading to the proliferation of

bacteria and the exhaustion of the oxygen content in the receiving water body.

Figure 3 shows the connection between drinking water and wastewater

treatment.

If sewage is not treated, or is ineffectively treated, contaminants will enter

into the receiving water body. This can cause health hazards to people who might

be using the water for recreational purposes such as swimming and fishing. It can

also cause the quality of the water used in the drinking water treatment process

to be compromised.

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Figure 3: The Connection between Drinking Water and Wastewater Treatment

As the bacteria grow on contaminants, oxygen is consumed and if the

organic load is high enough, teeming bacteria growth can cause health hazards

from potentially pathogenic bacteria. Also, sewage contains high levels of fecal

bacteria, these are pathogenic, and if left untreated, can cause significant health

hazards

Unfettered bacterial growth can cause oxygen to be depleted from the

surface waters leading to a condition known as hypoxia.

There is a tight link and connection between sewage treatment and having

clean and sanitary water supply for our homes and businesses1. Drinking water

treatment plants usually pull water from sources that can include surface waters

1 https://www.linkedin.com/pulse/beyond-smart-cities-george-haynes-5948936451461824512

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These surface water sources tend to be the same ones that the sewage

treatment plant discharges to. If the treatment at the wastewater plant is not

done thoroughly, then there is a risk that the insufficiently treated wastewater

laden with organic materials and pathogenic bacteria can harm people who use

these surface waters for recreational purposes such as swimming, fishing etc.

Even worse, the water quality can deteriorate so badly that the source

water quality to the water treatment plant is so thoroughly compromised that it

can begin affecting the operations of the drinking water treatment facility

Effective wastewater treatment is not just about meeting permit

requirements. It is the key to safeguarding community and population health.

Poor sewage treatment results in unhealthy surface waters and ultimately causes

the quality of the water that goes into homes to deteriorate.

What Happens during Wastewater Treatment?

A wastewater treatment facility is a collection of processes that allows for

the removal of contaminants. The contaminants in wastewater includes all of the

rags, toilet paper, stones, gravel, bacteria, soap suds, fats, oils and grease,

embedded in the water.

The various treatment processes are designed and intended to allow for

different contaminants to be removed.

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Process Materials Removed Mechanism of Removal

Bar screens Rags, paper, large debris Screens are used. The screen

mesh size is used to remove

materials that exceed the

screen size from the water

flow.

Grit Sand, gravel Gravity based settling is used

to remove dense materials

Primary Clarifier Organic solids and floating

scum and oils

Gravity based settling. Solids

fall to the bottom, oils and

scum are lighter than water

and float at the top of the

clarifier and skimmed off

Aeration Tank Soluble organic contaminants

e.g., COD, BOD

Bacteria consume organics

Secondary Clarifier Organic solids Gravity based settling

Table 1: Processes in a simple wastewater facility

Figure 4: Rags, paper and other debris on a bar screen

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Figure 5: Grit materials of different sizes2

The activated sludge process used bacteria embedded in wastewater to

facilitate the treatment of wastewater. By feeding on the organic matter,

microorganisms are able to convert the chemical oxygen demand into carbon

dioxide and new cells. The excess (new) cells are removed from the process as

waste activated sludge.

For the 100 years since its invention in 1914, bacteria enabled treatment is still

at the core of the activated sludge process. It is a very well understood process

that comprises of three major steps

1. Removal of influent solids using bar screens, grit system and clarifiers

2. Biological removal of contaminants using bacteria

3. Separation of treated water from solids using secondary clarifiers and solids

filter systems

Activated sludge plants are built based on designs that are put together by

consulting engineers. The designs work because the science behind each of the

2 http://gritthefacts.com/grit_definition_profiles.php

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treatment steps that take place in each treatment process is very well

understood. Those scientific principles can be turned into mathematical equations

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Chapter 2: Use of Process Simulators in the Wastewater Process

The reason why well designed wastewater treatment systems can

consistently undertake the treatment they are rated for, is because the processes

that are utilized are very well understood. Over the years, the chemical, biological

and physical principles that underlie all the processes that occur in wastewater

treatment have been compiled by researchers and practitioners, and these are

the basis of the principles and formulas that are used for designing and operating

wastewater treatment systems.

Wastewater treatment brings together many complex biological, physical,

chemical and hydraulic processes. Biological wastewater treatment is governed

by biochemical relationships that determine how bacteria respond to

environmental conditions such as temperature, pH and the concentration of

contaminants. Equilibrium chemistry relationships determine how effective

chemicals like Ferric and alum are in removing phosphorus through precipitation.

Learning Objectives

• Role of simulators in the wastewater treatment process

• How process models of wastewater plants get developed

• Processes that are covered in simulators

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These principles can be found in design guides, manuals of practices,

research papers and in specialized computer software called process simulators.

When wastewater treatment plants are being designed, engineers carry out the

necessary calculations for sizing the various process units (e.g., clarifiers, aeration

tanks, blowers, etc.) using these specialized computers programs or simulators.

Examples of these simulators include computer software like GPS-X™ and

SimuWorks™. These are the two programs on which this training annual is based.

GPS-X™ and SimuWorks™ allow computers to be used to model, to do computer

based mock-ups or replications of an actual wastewater plant.

Building a Process Model The first step that is required when developing a simulation of a plant is to

first build a mock-up of the plant in the computer. This involves placing symbolic

icons and objects that represent each process in the plant on a board.

The process of placing, selecting and putting in the relevant information

into each of the process units in a simulator is the act of Building a Process Model.

For instance, Figure 4, which is reproduced below, is a process model of a

wastewater plant.

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Figure 4: Process Model of a Plant

Based on this process model, we can figure out the following details about the

facility

1. It has a common headworks with one grit system

2. It has two treatment trains

3. Each treatment train has its own primary and secondary clarifiers

4. Each train has anoxic and aerobic biological tanks

5. The secondary clarifiers are dedicated to each train

6. It has no solids handling system. All the sludges (primary and secondary) are

mixed and sent out in trucks.

We could determine all of this information by simply reviewing the facility’s

process model.

Simulators contain all of the major processes in a wastewater treatment

system. Various process categories exist, and each one would normally have

multiple objects included in them.

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Overview of Treatment processes Influent

This refers to all the streams that enter into a treatment plant. It includes the raw

influent wastewater, solids streams that come from outside the plant,

supplemental carbon (e.g., methanol and glycerol used in nitrogen removal), and

other chemical dosage such as acid, alkali and nutrients

Figure 5: Influent Objects and Chemical Injection

Preliminary treatment

This refers to all the processes that come before the primary clarifier and are part

of the headworks. They include treatment systems such as bar screens and grit

removal systems. The goal of the preliminary treatment system is the removal of

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large objects that can clog up the treatment system (e.g., rags, tissue, etc.), are

inorganic and cannot be treated in a biological process (e.g., grit, sand, etc.).

Figure 6: Preliminary Treatment Units

Primary treatment

The primary treatment processes are designed to remove large solids and

immiscible materials. Bacteria are very small objects, and in order for the bacteria

to be able to consume organic materials, those contaminants must be very small

and must be dissolved inside of the water. A single bacteria cell is very small,

ranging in size from about 0.2 to 10 microns. When the bacteria join together to

form floc, they can get larger. One way to think about what a primary treatment

system is intended to do is to imagine how humans eat.

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Figure 7: Bacterial cell floc3

A primary treatment systems helps to remove the larger organic materials

that are still left, so that only the ones that are small and therefore likely to be

effectively degraded by bacteria in the wastewater process remain. Generally,

clarifiers are designed to remove objects that are greater than about 50-100

microns.

Another function of clarifiers is to remove materials that do not mix well

and tend to float. Fats, oils and grease tend to be lighter than water. In order

for bacteria to feed on contaminants, such wastes must be in contact with the

bacteria. This means they must be miscible, which implies that they should be

soluble in water. Immiscible materials that are lighter than water, tend to float

to the top of a clarifier, while large solids settle in the bottom.

3 Evolution of Size Distribution and Transfer of Mineral Particles between Flocs in Activated Sludges: An Insight into Floc Exchange Dynamics. Water Research 36(3):676-84 · March 2002. Chaignon et al.

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Rake arms sweep the scum from the surface of a clarifier, while the solids settle

to the bottom and are discharged through pipes to solids handling systems.

Figure 8: Primary Treatment Units

Primary treatment objects include processes such as primary clarifiers and

micro screens.

Suspended growth processes

When the bacteria are all contained within a well-mixed liquid system, such

a process is called a “suspended Growth” process. This is because the solids are

contained in the liquid, and are therefore “suspended”. The bacteria are not

attached to anything, and they can move around freely. Most wastewater

treatment systems are of this type, and they include commonly used systems

such as completely mixed tanks, sequencing batch reactors (SBR), and oxidation

ditch.

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Figure 9: Suspended Growth Processes

Attached growth

The second type of biological treatment process is the attached growth

system. In these systems, the bacteria are not suspended within the liquid, but

they are “fixed” or “attached” to a solid surface.

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The surface to which the bacteria are attached can be rocks and gravel, as

in trickling filter systems; or plastic media. Bacteria that are attached to a surface

are called Biofilm.

Some attached bacteria processes are completely fixed and immobile (e.g.,

trickling filter) while others have media that can move. The key actor that

determines whether bacteria are suspended or not, is whether the bacteria

themselves are freely suspended within a fluid and can move with it, or whether

they are attached to some structure, and have their movements restricted only to

the movements that the attachment structure can make.

Figure 10: Plastic media showing evolution of biofilm growth4

4 https://www.alibaba.com/product-detail/High-efficient-surface-plastic-MBBR-biofilm_60490485181.html

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Figure 11: Attached Growth Processes

Secondary clarifiers

Secondary clarifiers are used to separate the treated wastewater from the

bacterial solids that were used to treat it. Without effective clarification, there

will be a breakthrough of bacteria into the effluent, which can cause a plant to

violate its permits.

By removing solids, clarifies also play an important safety role. The bacteria

used in wastewater treatment can be pathogenic, which means they can cause

illness and diseases if they are present in large amounts and people are exposed

to them. Without good clarification, the lakes and streams that

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Figure 12: Secondary Clarifiers

Tertiary treatment

Tertiary treatment systems refer to all of the additional treatment steps that are

carried out on the clarified effluent from the secondary clarifier.

It includes all of the physical, chemical and sometimes, even biological treatment

steps that are used to remove more nutrients (e.g., ferric dosage for phosphorus

removal)

Figure 13: Tertiary Treatment

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Biosolids treatment

When organic materials is treated in a wastewater plant, the bacteria

convert the organics to two major products. Firstly, as bacteria respire, they

convert the carbon in organic contaminants into CO2. Secondly, the bacteria grow

and form new cells. In addition to the biological solids that are created, there is

also another major organic solids stream coming from the underflow of the

primary clarifiers. This consists of organic solids that have a non-biological origin

such as food wastes and other materials that settle out in the primary clarifier and

are usually in the range of >50 to 100 microns.

Biosolids treatment processes include dissolved air flotation (DAF),

thickeners, anaerobic digesters, aerobic digesters, sludge pre-treatment systems

(e.g., thermal hydrolysis units), dewatering systems, screens, cyclone separators

and struvite precipitation reactor.

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Figure 14: Biosolids Treatment

Uses of Simulators Simulators can be used for a variety of application in the wastewater treatment

process. Examples of the tasks and projects to which they can be applied are:

• Designing new facilities or upgrades for wastewater plants

• Comparing varies designs

• Quantifying process bottlenecks

• Identifying cost saving strategies

• Verifying existing plant capacity

• Determining the impact of operational variables and process changes

• Testing various operational strategies

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• Planning for maintenance shut downs

An example of how a simulator can be used to design a plant is provided below.

Figure 15: Virtual Wastewater Plant designed in a simulator

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Figure 16: Actual Wastewater Plant

When a wastewater simulator is used for developing designs for a wastewater

plant, it is easier to handle the many complex relationships that occur in the

wastewater treatment process.

Flight Simulators in Wastewater Treatment

In the field of aviation, flight simulators are used for teaching and training

pilots. They provide a Virtual Environment for aviation industry professionals to

train and learn about their field in a manner that is as close to the real experience

as possible.

Why are simulators important in Aviation? It is largely because the

consequences of failure or error are so severe that professionals in that field have

to be provided with a way to learn and practice, in a risk free environment. The

virtual environment is one in which failures are not costly events that can lead to

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regulatory violations or loss of life. The worst outcome possible from a failure in

the computer simulator is a loss of points, virtual penalties, point deductions, or

the need to repeat the failed lesson. Failures in a virtual environment leads to a

teachable moment that demonstrate to the professional in a realistic manner,

some ways in which failure could occur under real operations.

Flight simulators and virtual environments in general work well as a training

and testing tool because all of the factors that affect the operations of an aircraft

– aerodynamics, turbulence, air currents, weather, physics, avionics and the like

are very well understood and are incorporated into the working of the simulator.

All of the scientific principles that govern flight are turned into mathematical

equations that allow the simulator to mimic and replicate how an actual airplane

would behave. These mathematical equations that describe all of the different

scientific principles are what we call models. A simulator is just a computer

program that allows those models to be implemented.

Similar mathematical relations have been developed for all of the complex

processes that occur during wastewater treatment, and those models are the

basis of this unique training and testing platform.

The benefits of a simulator based training program is its unique ability to

provide realistic scenarios. It allows for more than just paper and pencil type

learning. It allows users to be presented with realistic scenarios that replicate the

challenges of the actual situation

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Chapter 3: About OpToolPro™

OpToolPro™ is a wastewater treatment process simulator that uses an

interactive and intuitive interface to teach activated sludge concepts through

running a virtual wastewater treatment plant. Users can make changes to

operational parameters such as recycle rates, wastage rates, airflow and chemical

dosage. An underlying mathematical model (driven with the Hydromantis GPS-

X™ simulation engine) predicts the performance of the wastewater treatment

plant (WWTP).

OpToolPro™ uses a series of mathematical process models to describe the

behaviour of the various clarifiers, tanks, dosage pumps, etc., throughout the

WWTP. These models are based on fundamental mass balances of carbon,

nitrogen and phosphorus, and have been developed over many decades. The

biological model for activated sludge growth, for example, originates from the

Learning Objectives

• Understand the operations of OpTool Pro • Review the two simulator modes - Training and

Testing mode • Learn how to use OpTool Pro to address

operational issues in a wastewater plant • Understand the details of the wastewater

treatment plant provided in OpTool Pro

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family of models (ASM1, ASM2d, etc.) originally developed by the International

Water Association (IWA) in the 1980’s (Henze, et al., 1987).

The underlying mathematical models use the influent loading and

operational parameters to predict the effluent flow and concentrations such as

BOD5, COD, TSS, ammonia and nitrate. The effluent flow and concentrations from

one unit are then used as the inputs into the next unit downstream.

OpToolPro™ uses an overhead view of the WWTP as a way to allow users to

easily access different parts of the system, and make desired operational changes.

With the simple click of the Start button, the effect of those operational changes

can be immediately seen in the energy cost, chemical cost, and effluent quality.

Figure 16: Virtual Wastewater Plant in OpToolPro™

This method of interactive simulation transforms a complex system of

chemical, biological and physical processes into a simple-to-use, and simple-to-

understand virtual tool.

Users can play with an airflow rate to understand the consequences of

under-aerating the activated sludge system, or see the effects of overdosing

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chemicals for phosphorus removal. The ability to immediately see the effects of

the change to an operational parameter is a powerful and direct method for

instilling knowledge and understanding of how the activated sludge process

works.

Teaching through simulation is a very efficient and popular method for

students who “learn by doing”, rather than through passive reception of

information. OpToolPro™ is designed to allow teachers to use the tool for

demonstration, instruction and explanation of the effects of various process

changes on the activated sludge system. It also provides a series of challenges for

the students to test their knowledge and understanding, by performing a series of

troubleshooting exercises for different WWTP problems. Wastewater treatment

plant operators, managers and training staff can use OpToolPro™ as a part of

their regular training and certification curriculum.

The Two Modes in OpTool Pro OpToolPro™ has two modes: Training and Testing. Training Mode allows

instructors to freely run any simulations needed to demonstrate and educate

about the activated sludge process, whereas Testing Mode presents challenging

questions to the students to exercise and use their activated sludge problem-

solving skills:

• Training Mode is designed for demonstrating and teaching activated sludge

concepts through running simulations under different conditions – it uses free

and open simulations, without challenge questions, timer or scoring

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• Testing Mode is designed to test the knowledge and understanding of the

concepts presented by using Teaching Mode. Students are presented with 15

challenges and asked to solve as many problems as possible in the time

provided. A final score is calculated by summing up the scores obtained across

all the challenges.

Wastewater Treatment Plant Details

The WWTP used in the OpToolPro™ challenges is shown in Figure 16. The various

unit processes are labelled in the diagram. The plant itself is a conventional

activated sludge plant, with the following unit processes:

• an influent pumping station

• 4 rectangular primary clarifiers

• 2 plug-flow activated sludge aeration tanks

• 4 circular secondary clarifiers

• 2 chemical dosage points for iron addition for chemical phosphorus

precipitation:

o Immediately upstream of the primary clarifiers

o Immediately downstream of the aeration tanks

• a chemical dosage point for methanol addition at the head of the plug-flow

aeration tank

• a recycled activated sludge (RAS) pumping station

• a waste activated sludge (WAS) pumping station

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• 2 gravity sludge thickeners

In the upper left-hand corner of the screen, several process performance

variables are shown. The following table describes how these variables are

calculated.

Variable Calculation MLSS Solids concentration at the effluent from the aeration tank.

SRT SRT (days) = mass of solids in bioreactor (Ibs)

mass flow of waste activated sludge (Ibs/day)

DO in Aeration Tank

The dissolved oxygen (DO) concentration is reported from the end of the aeration tank.

Energy Cost

The total energy cost includes the following energy costs: • Primary sludge pumping • Aeration energy (blower power) • Internal recycle pumping in aeration tank • Recycled activated sludge (RAS) pumping • Waste activated sludge (WAS) pumping

Chemical Cost

The total chemical cost includes the following chemical costs: • Primary ferric dosage for phosphorus removal • Secondary ferric dosage for phosphorus removal • Methanol dosage for enhanced denitrification

Sludge Production

Sludge production is calculated as the solids concentration times the flow for all sludge (combined primary and secondary) being hauled from the gravity sludge thickeners.

Table 2

Starting OpToolPro™

1) Click on the OpTool Pro icon on your desktop:

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2) Clicking on the Icon should open up the program. Once the Splash Screen with pops up, you will be prompted to select either “Training” or “Testing” mode to start the OpToolPro™ software:

Figure 17: Selecting the virtual plant mode in OpTool Pro

You can switch between Training and Testing Modes at any time while using the software by clicking on the Mode menu in the upper-left hand corner of the screen.

3) Select which set of units that you would like to work in (SI or US):

37

Figure 18

US Units – All parameters will be shown in MGD, gpd, MG, ft3, mg/L, °F, etc.

SI Units – All parameters will be shown in MLD, m3/d, m3, mg/L, °C, etc.

Navigating the Interface

The OpTool interface contains several panels of information, plus a simulation toolbar that contains tools for running simulations. Each serves a unique purpose, as shown below:

Figure 19: A Clickable Wastewater Plant

Clickable Wastewater Treatment Plant Unit Processes A diagram of a conventional activated sludge wastewater treatment plant

(WWTP) is shown in the upper half of the OpToolPro™ screen. Several of the

individual unit processes (aeration tanks, clarifiers, pump buildings) can be clicked

on to access input parameter settings in the middle of the three panels below the

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plant diagram. When the mouse pointer is placed over a unit process that has

parameters that can be adjusted, a red box will appear around the unit:

Figure 20

If you place the mouse point over a building in the WWTP and no red box

appears, that unit process cannot be adjusted for simulation.

Clicking on a unit process will change the center panel below the diagram to the

input parameter menu for that process. For example, clicking on a secondary

clarifier will bring up settings take those units off line, and clicking on a chemical

dosage building will bring up the dosage rate setting.

Accessing More Operational Details & Information

Users can right-click on the unit process to bring up additional input and output

menus. These menus will allow the user to access more input and output

variables than are provided below, and can be used for advanced troubleshooting

and analysis.

39

Figure 21

Figure 22

Bringing units In and Out of Service For some units (e.g. clarifiers), right-clicking on the unit will allow the user to take

that unit in or out of service. Any units that are out of service are shown with a

grey box covering the unit.

Right clicking on the aeration tank brings up options to access Input or Output menus

Clicking on Output allows the user to see more details about the internal workings of the process

40

Figure 23

Figure 24

Instructions/Questions Panel

The left-most of the three panels along the bottom of the screen displays

information to the user regarding the current objectives of the simulation mode.

In Training Mode, the panel displays instructions on how to make changes to

model input, run simulations, and observe model output (which is also covered in

this manual). Users can navigate through the instructions by clicking on the

Previous and Next buttons at the bottom of each page.

Image shows two clarifiers are greyed out and are therefore out of service.

The service status of the units can also be confirmed by reviewing the Simulation Input panel.

41

Figure 26

In Testing Mode, the panel displays a series of simulation challenge questions.

Users can record their name and a team name in the initial screen, and then click

on the Start button to start the countdown timer. Users can then choose a

question from the main menu (left), and proceed through the details of the

questions (centre). After running a number of simulations to answer the

question, users can proceed to Submit their answers (right) and return to the

main menu:

Figure 27

Simulation Input Panel

The centre of the three panels at the bottom of the screen provides menus for

simulation input. The content of this panel changes depending on which unit

42

process has been selected in the wastewater plant diagram above. For example,

clicking on the aeration tank in the WWTP diagram (left) switches the Simulation

Input Panel to the aeration tank inputs (right):

Figure 28

Note that there can be multiple tabs across the top of the menu. containing

several different sets of parameters (e.g. in the menu above on the right, DO

controls are on the first tab, tank status is on the second tab, and internal recycle

settings are on the last tab).

Users can change as many settings as desired on these menus before starting a

simulation.

Simulation Output Panel

The panel in the lower-right corner of the screen displays simulation output

– specifically the effluent concentrations, such as TSS, TKN and BOD5. This panel

does not change when different units are clicked on in the WWTP diagram. Note

that the user may have to scroll the window down to see all of the available

output.

Different versions of the effluent simulation output window appear in

Training Mode and Testing Mode. In Training Mode (left), the output menu

43

displays a comprehensive list of effluent concentrations. Each time the simulation

is started, the numbers will be updated. In Testing Mode (right), the list of output

parameters varies from question to question, depending on the focus of the

troubleshooting challenge.

In addition to the effluent concentrations, some outputs will show

challenge targets in blue, such as achieving an effluent ammonia concentration

below 1 mg/L. If the simulation is showing an output that meets the target, it will

be shown in green with a small checkmark. If the target is not met, the output

will be shown in red.

Figure 29: Simulation Output Panel in Training Mode

44

Figure 30: Simulation Output Panel in Testing Mode

Additional Process Performance Variables

In the upper left-hand corner and the bottom right corner of the screen, a

summary of critical plant-wide process variables shows details of overall process

performance, including Solids Retention Time (SRT), energy and chemical costs,

total airflow, clarifier loading, F/M Ratio, % BOD, TN, NH3 and TP removed, as well

as sludge production. These values are updated each time a simulation is

executed, and can be used to troubleshoot and analyse the plant performance.

Figure 31: Operational summary in top left corner of the interface

45

Figure 32: Operational summary in bottom right corner of the interface

Simulation Control Panel

The simulation control panel is found along the bottom of the screen, and is used

to update the simulation, and to control the information displayed on the WWTP

screen.

Figure 33

The Start Button is used to update the results of the simulation at any time. The

Progress Bar indicates the progress of the mathematical solver while computing

the solution to the model. The bar will appear red while the solution is being

calculated, and then switch to green when the final result is determined.

All updates on the Simulation Output Panel and Performance Variables

table will be updated immediately. The pull-down display menu can be used to

Start Button Progress Bar Information on WWTP

Restore Button

46

display values of various parameters right on the image of the WWTP. For

example, selecting TSS will show values for the TSS concentration at various

points around the treatment plant, as shown below.

Figure 34: TSS concentration displayed throughout the WWTP

Running Simulations

Running simulations is done the same way in both Teaching Mode and Testing

Mode. There are 4 steps involved in running a simulation:

1) Click on Unit Process in Plant Diagram

2) Make Parameter Changes (repeat for each unit process as necessary)

3) Click on Start

4) Observe Results

Step 1 - Select a Unit Process

Clicking on any tank or clarifier brings up the operational parameters for that unit:

47

Figure 35

Try clicking on a few different unit processes to see the input parameters for that

process. (Note that not all tanks and buildings in the diagram are active.)

Step 2 - Make Changes to Input Parameters You can adjust operational parameters in the window to the right. For some unit

processes, there are options for turning controllers on/off and taking

tanks/clarifiers offline by clicking on the appropriate buttons.

Figure 36

Step 3 - Start the Simulation Once you have set the input parameters for the simulation, click on the Start

button below this panel.

48

Figure 37

The model will take a moment to converge to an answer. You can view the

progress with the tracking bar at the bottom of the window. Once the bar turns

green, the simulation is complete, and you can view the results.

Step 4 - View the Results

The predictions of effluent concentrations for BOD, COD, Nitrogen and

phosphorus concentrations are shown in the lower right-hand corner of the

screen.

Figure 38

Note that you may have to scroll the window up and down to see all of the model

predictions. Each time the Start button is pushed, the numbers in the windows

will be updated with the latest prediction. If the user is in Testing Mode and

Start Button

49

attempting to meet various targets, the condition of the target will be updated at

the same time.

Switching Between Training and Testing Modes

Users can switch between Training Mode (open-ended simulations) and Testing

Mode (questions with timer and scoring) by selecting from the Mode Menu at the

top of the screen:

Figure 39

Exiting the Simulator Users can exit the simulator by clicking on the File > Exit menu option, or by

clicking on the X in the upper-right-hand corner of the window.

50

Chapter 4: Meeting Effluent Targets – Optimizing Plant Operations

• Now that we have learned how to use the simulator, we can start using it

for some detailed evaluations of the effect of various variables on the

performance of a wastewater treatment facility.

• Make sure you are in training mode for this exercise. Click on Mode in the

Menu Bar and Confirm that “Training” is selected.

Figure 40: Click on Mode and Confirm that “Training” is ticked

Learning Objectives

• Evaluate the impact of making operational changes

• Use OpTool pro to troubleshoot operational issues at a plant

• Evaluate the impact of various operational actions on a plant’s performance

51

• If you are not already in training mode, then you are in the Testing Mode.

In that case, move your cursor over to select “Training” and Click.

Figure 41: Testing mode is currently selected.

• The system will prompt you to confirm if you really want to change mode.

Click on “Yes”, and you should now be transferred to Training mode.

• Now that you are in Training Mode, you should see the following influent

conditions in the middle panel.

52

Figure 42

• From this starting point, our goal is to explore what it will take for the plant

to meet its effluent targets of COD – 40 mg/l, TN – 10 mg/l and TP 3 mg/l.

Table 3: Summary Table of Effluent Targets

• Now RUN the Simulation to see what the default conditions are at the

plant. To do this, click on the GREEN arrow pointing to the right in the

bottom left corner of the simulator.

Parameter TargetCOD 40 mg/lTN 10 mg/lTP 3 mg/l

53

Figure 43

• Look at the Effluent Parameters. Are we meeting the effluent targets at the

plant?

Figure 44

• Clearly, we are not meeting all the effluent targets.

• The effluent COD is 25 mg/l, while the effluent TN is 31 mg/l and the

effluent TP is 9.25 mg/l. Both TN and TP are above the effluent targets.

• Now you might have some questions about the targets. The effluent targets

include TN and TP, but the influent panel only included TKN and Soluble

Ortho-P. What can we do to see more details on the influent?

• Once the simulation has been run at least once, we can use the Output

function to see more details about what is happening in each process.

• In order to see more details on any object, RIGHT CLICK on the object and

select Output.

Click RUN button

54

• We want to get a more comprehensive description of the influent

conditions, so let’s check out the details of what’s in the influent stream.

Figure 45: Right Click on the Influent, and Select Outputs

• A Panel will open up with multiple sections. You can scroll down to the

various sections.

Figure 46

• Under Organic Variables we see more details on the BOD and soluble BOD

and COD values.

55

• Scroll down a bit more to the section heads labelled Nitrogen Variables,

Phosphorus Variables and pH. There you will see more details on the

breakdown of the different types of Nitrogen and Phosphorus that are in

the influent. This is where you will find the TN and TP values.

Figure 47

• Now that you know where to find additional details that are not specified in

the summary panels, we will repeat the facility treatment objectives by

specifying both the influent loadings and effluent targets for each variable

Table 4

Parameter Influent TargetCOD 416 mg/l 40 mg/lTN 42 mg/l 10 mg/lTP 13 mg/l 3 mg/l

56

• Currently, the plant is not meeting its effluent targets (see Figure 44).

Figure 48

• The operational summaries indicate that the MLSS is 758 mg/l, while the Solids

Retention Time is just 2 days. While about 99% of the BOD is being removed,

only 14.8% of the NH3 and 33.6% of TP are removed.

• This facility clearly has problems with ammonia removal (nitrification) and

Phosphorus removal). We can also see that the DO in the Aeration Tank is at a

value of 0.5 mg/l.

• As earlier noted, the wastewater facility is a conventional activated sludge

process with a preliminary treatment train, a secondary treatment train and a

solids handling process

Optimizing Plant Operations

Adjusting Dissolved Oxygen levels • Aeration and Dissolved Oxygen (DO) levels are effective tools for ensuring

that a plant is able to meet its organic treatment potential • Now, let’s adjust the DO settings. To do this, click on the aeration tank. The

middle panel should now reflect conditions in the aeration tank

57

Figure 49

• As you can see, under Aeration settings, the DO controller is switched on. With the DO levels all set to 0.5 mg/l in each zone. Note that the facility has four separate zones in the aeration tank.

• Let’s start by testing to see if increasing the aeration levels can help improve operations at the plant.

• Set the DO level to 2 mg/l in all of the four zones.

58

Figure 50

• Now that the DO level has been set to 2 mg/l in all the zones, run the model again to see the impact of the changes in DO on the effluent parameters. To run the model use the Green RUN button at the bottom left corner of the simulator.

• Now check to see the impact of providing increased aeration. To see the results check the Effluent Parameters

59

Figure 51

• As you can see (Figure 51), the COD removal has now improved significantly

and now measures 24.4 mg/l. Ammonia has gone down to 9.8 mg/l, the

combination of Nitrite and Nitrate is now about 15 mg/l and total

phosphorus is now about 9.26 mg/l.

Figure 52

• The process summary also shows that the MLSS is 751 mg/l, COD removal is

99.1%, while TN and TP removal are 40.9% and 33.5% respectively.

60

• Although the plant is doing a better job of treatment, we are still not

meeting treatment goals. The plant can certainly still do better.

Optimizing F/M by Increasing Mixed Liquor Suspended Solids

• A well operated wastewater treatment plant will typically operate within a

specific set of acceptable ranges for the major operational parameters such as

MLSS< Food to Mass (F/M), Solids Retention Time (SRT) and Hydraulic loading.

• Examples of some acceptable design and operational ranges for these

parameters are given in Table 5. Note that these values are not set in stone.

Well operated facilities can still operate below or above these values. Think of

them as guides that can help you determine how your operations compare to

other systems

Parameter Units Low Range High Range

MLSS mg/L 2,000 4,000

Food to Mass Ibs BOD/Ib

SS/day

0.2 0.5

SRT Days 2 10

Primary Clarifier

Hydraulic Loading

Gals/ft2.day 500 1,500

Secondary Clarifier

Hydraulic Loading

Gals/ft2.day 200 800

Table 5

61

• When compared to this table, what do you notice about the current

operations at out virtual plant?

• It is obvious that our plant has a lower hydraulic loading than the average

plant, but the F/M is much higher than the average facility at a value of 0.82

Ibs BOD/Ibs MLSS.day.

• The MLSS in the virtual plant is also lower than the typical range.

Parameter Impacting Factors / Variables

Control Variables

Dissolved Oxygen Aeration rate Aeration rate

Food to Mass • Organic loading (Flow, COD) • Mixed liquor

Concentration (MLSS)

• Flow, COD • WAS

SRT • MLSS • WAS

• WAS

MLSS • Mixed liquor Concentration (MLSS)

• WAS

Hydraulic Loading • Flow • Flow

Table 6

• The current F/M level is quite high. As Table 6 indicates the MLSS value can

have a significant impact on F/M. Our first corrective action will be to attempt

to reduce the F/M, and raise the MLSS.

• Based on the ranges I Table 5, our target will be a MLSS value in the 2,000 to

4,000 mg/l range. Having more solids in the aeration basin is an effective way

to ensure that we have more bacteria cells present in the treatment process so

that treatment can be more robust.

62

• Also, the more solids (MLSS) there is in the system, the more “Mass” is

available. Recall the Food to Mass ratio:

𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 𝑡𝑡𝐹𝐹 𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 = 𝑂𝑂𝑂𝑂𝑂𝑂𝑀𝑀𝑂𝑂𝑂𝑂𝑂𝑂 𝐿𝐿𝐹𝐹𝑀𝑀𝐹𝐹𝑂𝑂𝑂𝑂𝑂𝑂 𝑡𝑡𝐹𝐹 𝑡𝑡ℎ𝑒𝑒 𝑃𝑃𝑃𝑃𝑀𝑀𝑂𝑂𝑡𝑡

𝐵𝐵𝑀𝑀𝑂𝑂𝑡𝑡𝑒𝑒𝑂𝑂𝑂𝑂𝑀𝑀𝑃𝑃 𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 𝑂𝑂𝑂𝑂 𝑡𝑡ℎ𝑒𝑒 𝐴𝐴𝑒𝑒𝑂𝑂𝑀𝑀𝑡𝑡𝑂𝑂𝐹𝐹𝑂𝑂 𝑇𝑇𝑀𝑀𝑂𝑂𝑇𝑇

=𝐹𝐹𝑃𝑃𝐹𝐹𝐹𝐹 (𝑀𝑀𝑀𝑀𝑀𝑀) ∗ 𝐵𝐵𝑂𝑂𝑀𝑀 �𝑚𝑚𝑂𝑂𝑃𝑃 � ∗ 8.34

𝑉𝑉𝐹𝐹𝑃𝑃𝑉𝑉𝑚𝑚𝑒𝑒 𝐹𝐹𝑜𝑜 𝑇𝑇𝑀𝑀𝑂𝑂𝑇𝑇 ∗ 𝑀𝑀𝐿𝐿𝑀𝑀𝑀𝑀 �𝑚𝑚𝑂𝑂𝑃𝑃 � ∗ 8.34

• Because the mass of the biological solids is the denominator in the Food to

Mass equation, any increase in the MLSS should lead to a decrease in the F/M

• As Table 6 shows, the Waste Activated Sludge (WAS) affects both the MLSS

value and by extension the F/M. we will change this variable first.

• As can be seen in the Teeter – Totter diagram, reducing the Wastage Rate of

the Sludge (WAS) will lead to implications for F/M, MLSS and SRT.

• In addition to wanting to reduce the F/M ratio, we would also be interested in

raising the SRT at the facility. Raising the Solids Retention Time (SRT) or the

Sludge Age, as it is also called, can be an effective way to improve the

Nitrification rate in the process, which can certainly assist us in getting more

reliable ammonia removal in the process.

• We know that the sludge age can have a big impact on the effectiveness of

biological nitrogen removal. The bacteria that convert ammonia to nitrates are

called nitrifiers, and they are slow growing organisms (see the Figure 54)

63

Figure 53: Blue – Altered (Independent) Variable; Red – Affected (Dependent)

Variable

• The Y axis (the North pointing arrow), shows the cell divisions. This is a

measure of how quickly the bacterial cells are growing, while the X axis (the

East pointing arrow) shows the amount of time that has passed.

WAS F/M

MLSS SRT

64

Figure 54: Nitrifiers vs Heterotroph growth5

• As you can see from the graph, the carbon (COD or BOD) consuming

organisms called heterotrophs can have as many as 2 cell divisions in about

3 hours, while it would take over 48 hours for Nitrifiers to achieve two cell

divisions. Heterotrophs are very fast growing.

• The implication of this is that in a wastewater plant, there is always a

smaller fraction of nitrifiers compared to the heterotrophs. Another even

more important implication is that if a plant needs to have a good

complement of Nitrifiers in the system, because they are slow growing, a

longer sludge age will be required. Generally plants that nitrify aim for a

sludge age or solids retention time (SRT) of between 5-10 days.

• We can increase the quantity of bacterial cells (MLSS) in the system by

reducing how much sludge we waste. We should therefore take aim at the

5 https://teamaquafix.com/ammonia-removal-restoring-nitrification/

65

waste activated sludge (WAS) rate. Reducing that should decrease the

quantity of solids leaving the plant.

Figure 55

Changing the WAS Rate:

• The WAS rate is a critical operational parameter in wastewater treatment, and as we have earlier noted, it can be used to control variables such as MLSS and SRT.

• We will now explore the impact of WAS control in the optimization of biological wastewater treatment systems.

• Click on the WAS Control unit. The middle panel will now show the value of the WAS discharge rate on the WAS Control Valve. The current WAS rate is 158,500 gpd.

Figure 56

66

• To increase sludge age, we need to reduce the amount of sludge wasted. • Since the current sludge age is 2.1 days, our goal is to raise it to about 5

days. That means we want to increase sludge age by over two times its current value.

• Remember that:

𝑀𝑀𝑃𝑃𝑉𝑉𝐹𝐹𝑂𝑂𝑒𝑒 𝑀𝑀𝑂𝑂𝑒𝑒 =𝑀𝑀𝐹𝐹𝑃𝑃𝑂𝑂𝐹𝐹𝑀𝑀 𝑂𝑂𝑂𝑂 𝐵𝐵𝑀𝑀𝑀𝑀𝑂𝑂𝑂𝑂𝑊𝑊𝑀𝑀𝑀𝑀𝑡𝑡𝑒𝑒𝐹𝐹 𝑀𝑀𝑃𝑃𝑉𝑉𝐹𝐹𝑂𝑂𝑒𝑒

=𝑉𝑉𝐹𝐹𝑃𝑃𝑉𝑉𝑚𝑚𝑒𝑒 𝐹𝐹𝑜𝑜 𝐵𝐵𝑀𝑀𝑀𝑀𝑂𝑂𝑂𝑂 (𝑂𝑂𝑀𝑀𝑃𝑃𝑀𝑀) 𝑋𝑋 𝑀𝑀𝐿𝐿𝑀𝑀𝑀𝑀 �𝑚𝑚𝑂𝑂𝑃𝑃 � ∗ 8.34

𝑊𝑊𝑀𝑀𝑀𝑀𝑡𝑡𝑒𝑒𝐹𝐹 𝑀𝑀𝑃𝑃𝑉𝑉𝐹𝐹𝑂𝑂𝑒𝑒 (𝑂𝑂𝑔𝑔𝐹𝐹) ∗ 8.34

• We will now estimate the amount by which we should change the WAS rate.

• Click on the aeration tank and select the tab labelled Aeration Tank Status

Figure 57

• You will see that the aeration tank volume is 343,420 gals (US) per tank.

Since we have two aeration tanks, the total aeration tank volume is about 2

X 343,420 gals (US) = 686,840 gals.

67

• To determine the Solids Retention Time (SRT), we also need to know how

much solids are contained in the wasted sludge. This requires that we know

the Total Suspended Solids (TSS) in the WAS.

• To find the TSS levels in the WAS, go to the Display menu at the bottom of

the tool, and click on the down arrow. A menu of options will open up.

Select TSS (mg/l). The solids concentrations will now be displayed on the

layout.

Figure 58: Display Menu showing variables that can be displayed on the layout

68

Figure 59: TSS values displayed on the layout

• The numbers that are now displayed on the layout are the TSS values in

mg/L. You will see that the MLSS is 751 mg/l while the WAS TSS is 1560

mg/l

• Putting all the values we have obtained into the SRT (or sludge age)

equation, we obtain:

𝑀𝑀𝑃𝑃𝑉𝑉𝐹𝐹𝑂𝑂𝑒𝑒 𝐴𝐴𝑂𝑂𝑒𝑒 =𝑀𝑀𝐹𝐹𝑃𝑃𝑂𝑂𝐹𝐹𝑀𝑀 𝑂𝑂𝑂𝑂 𝐵𝐵𝑀𝑀𝑀𝑀𝑂𝑂𝑂𝑂𝑊𝑊𝑀𝑀𝑀𝑀𝑡𝑡𝑒𝑒𝐹𝐹 𝑀𝑀𝑃𝑃𝑉𝑉𝐹𝐹𝑂𝑂𝑒𝑒

=𝑉𝑉𝐹𝐹𝑃𝑃𝑉𝑉𝑚𝑚𝑒𝑒 𝐹𝐹𝑜𝑜 𝐵𝐵𝑀𝑀𝑀𝑀𝑂𝑂𝑂𝑂 (𝑂𝑂𝑀𝑀𝑃𝑃𝑀𝑀) 𝑋𝑋 𝑀𝑀𝐿𝐿𝑀𝑀𝑀𝑀 𝑏𝑏𝑀𝑀𝑀𝑀𝑂𝑂𝑂𝑂 �𝑚𝑚𝑂𝑂𝑃𝑃 � ∗ 8.34

𝑊𝑊𝑀𝑀𝑀𝑀𝑡𝑡𝑒𝑒𝐹𝐹 𝑀𝑀𝑃𝑃𝑉𝑉𝐹𝐹𝑂𝑂𝑒𝑒 (𝑂𝑂𝑔𝑔𝐹𝐹) ∗ 𝑇𝑇𝑀𝑀𝑀𝑀 𝑊𝑊𝐴𝐴𝑀𝑀 (𝑚𝑚𝑂𝑂𝑃𝑃 ) ∗ 8.34

=686840 𝑂𝑂𝑀𝑀𝑃𝑃𝑀𝑀 ∗ 751 𝑚𝑚𝑂𝑂/𝑃𝑃158500 𝑂𝑂𝑔𝑔𝐹𝐹 ∗ 1560 𝑚𝑚𝑂𝑂/𝑃𝑃 = 2.1 𝐹𝐹𝑀𝑀𝑑𝑑𝑀𝑀

69

• Now, we can see that a simple way to increase the sludge age is to reduce

the wastage rate. Try reducing the Sludge wastage rate by a factor of 2. This

means the new WAS rate will be 79,250 gpd i.e., 158500/2.

Figure 60

• Now RUN the model and see the results

Figure 61: Simulator showing the impact of changing WAS rate on operational variables such as SRT and MLSS

• The results summary shows that the MLSS has increased to about 1,223

mg/l

• You can now see that the SRT has bumped up to 3.8 days. You can see that

reducing the WAS rate by a factor of 2 led to a 1.8 X increase in sludge age.

It is not exactly a 1: 1 change – but it is close to it.

70

Figure 62: Effect of WAS change on Effluent

• The effluent parameter also shows that nitrification has improved

significantly at the plant. Ammonia levels are now at 0.1 mg/l, while Nitrate

levels are at 25.1 mg/l.

• Almost all of the ammonia has been completely nitrified. The phosphorus

levels have not changed much, and are still at a value of about 9.59 mg/l for

TP, and 9.29 mg/l for soluble Phosphorus.

Effect of sludge wastage on solids generation at the plant

• We have already established that changing the sludge age leads to a

number of operational changes at a facility. One obvious change that we

have seen is that reducing WAS can causes an increase in sludge age as well

as MLSS, provided the clarifiers are working well, and solids are not being

lost due to clarifier overflow.

• WAS rates also affect the amount of solids wasted from the plant. To check

the effect on sludge wastage, let us take a look at the solids leaving the

plant. An easy way to do that in the simulator, is to use the Display function

at the bottom of the simulator.

71

• As we did before, go to the Display tab at the bottom of the tool, and click

on the down arrow. A menu of options will open up. Select TSS (Ibs/d). The

solids mass flows will now be displayed on the layout.

Figure 63

• Once the TSS Mass Flow (Ibs/day) option is selected, the mass flows are

now displayed on the layout.

Figure 64

72

• The layout now contains the mass flows. If ever in doubt about what values

are displayed on the layout, check out the bottom of the layout to see what

variable is selected in the Display section. The currently active display will

be shown.

Figure 65

• As can be seen in the display, 1,740 Ibs/day of solids are being wasted to the thickeners from the WAS control valves, while 2,750 Ibs/day of solids are being wasted from the Primary clarifiers.

• Now, let us increase the sludge age by further halving the WAS rate from the current value of 79,250 gpd to 39,625 gpd. RUN the simulation so that the impact of the new change can be determined.

Figure 66

• The MLSS is now 1823 mg/l and the SRT is now 6.8 days. The secondary sludge wastage from the aeration tanks is now at a value of 1,320 Ibs/day,

73

while the wasted sludge from the primary clarifiers remains unchanged at 2750 Ibs/day

• Note that while the level of Ammonia in the effluent is really low at 0.1 mg/l (see Figure 66), the TN level is very high at 28.9 mg/l because of the high level of Nitrates in the process (26.4 mg/l).

• We will make one more change and reduce the WAS rate to 20,000 gpd, and then RUN the simulation. What do you observe? Now the SRT is 10.9 days.

• We can summarize the operational changes we have made so far and the

impact on wasted sludge in the table below.

• As can be seen, the sludge age increases as WAS rate is decreased. Also, the

sludge wastage mass (Ibs/day) decreases as sludge age increases.

Table 7

• The relationship between the solids wasted from the secondary tank and the SRT can be plotted (see Figure 67).

SRT MLSS WAS (gpd) WAS (Ibs/day) DO (mg/l)2 758 158500 2090 0.5

2.1 751 158500 2070 23.8 1223 79250 1740 26.8 1823 39625 1320 210.9 2472 20000 912 2

74

Figure 67

• The chart in Figure 67 is based on the data table generated using the

simulator, and it shows an important operational principle. As the sludge

age increases, the wasted sludge rate decreases. So, a reduced sludge

generation rate is a direct impact of increasing the sludge age.

• An added impact of operating at a longer sludge is the possibility that there

will be a reduction in the solids that are generated at the facility.

• Now, let us return to the virtual facility. Recall that the effluent targets are

COD - 40 mg/l and TN - 10 mg/l and TP - 3 mg/l

• At a WAS rate of 39,625 gpd, the effluent COD is 34 mg/l, TN is 28.9 mg/l

and TP is 10 mg/l while soluble Phosphorus is 9.61mg/l (see Figure 66)

• We have met the COD limit, but TN and TP are still above the effluent

targets

• The high TN value is due to the high amounts of Nitrates (26.4 mg/l). In

order to get the TN to less than 10 mg/l, the Nitrate levels must be

reduced.

0

500

1000

1500

2000

2500

0 2 4 6 8 10 12

WAS

(Ibs

/day

)

Sludge Age

Sludge Age vs Wasted Sludge

75

• Nitrate is reduced mainly by creating conditions in which the dissolved

oxygen level is so low that bacteria will be required to take the oxygen they

need from nitrates.

• Nitrates have a formula NO3, and when the oxygen in nitrates is used up by

bacteria, in a process called denitrification, nitrates are then converted to

nitrogen gas, N2.

Figure 68: Relation between ORP and metabolic processes (Goronsky, 1992)6

• The chart in Figure 68 shows that when conditions turn anoxic, then bacteria are more likely to use Nitrates (NO3) as an electron acceptor.

6 Source: http://onlinembr.info/nutrient-removal/denitrification/

76

• We will create anoxic conditions in the first two zones of the aeration tanks

by changing the DO setpoints in Zone 1 and 2 to 0.5 mg/l each.

• Click on the Aeration Tanks, make sure DO Controller is selected, and then

set the DO levels to 0.5 mg/l, 0.5 mg/l, 2 mg/l and 2 mg/l in each of the four

zones. Next, RUN the simulation.

Figure 69

• The results clearly show that ammonia is almost fully nitrified to Nitrates,

with a TN value of 19.9 mg/l, of which 17.3 mg/l is Nitrates.

• We will now explore whether a further decrease in DO levels

Using External Carbon Sources

• The bacteria that carry out denitrification are called heterotrophs. They

need to have access to readily biodegradable COD in order to do the job of

denitrification effectively

• However, in most wastewater plants, there is usually not much readily

biodegradable organic carbon available for effective nitrification. Most of

the readily biodegradable carbon is used up early in the treatment process,

and is not available for denitrification.

77

• Theoretically, about 2.86 mg/l of COD is required to denitrify 1 mg/l of

Nitrates. So, to denitrify 10 mg/l of Nitrates, we need about 28.6 mg/l of

readily biodegradable COD.

• Notice the emphasis on the word soluble. Some COD is not readily

biodegradable. COD might be soluble, which means it is not visible to the

naked eye and will pass through filter paper, but it might not be readily

biodegradable. For example, the effluent COD in a wastewater plant is

mostly soluble, but it has not been successfully biodegraded within the

treatment process, which means it is not readily biodegradable.

• Simulators can allow us to determine how much soluble carbon is available

in a wastewater treatment facility. To determine how much soluble COD is

available in our virtual facility, right click on the aeration tank and select

outputs.

Figure 70

• Once output is selected, a menu item will pop up with four tabs. Click on

the tab labelled Profiles. This provides the quantity of various variables in

each of the four zones of the aeration tank.

78

Figure 71

• The section labeled Organic Variables contains the soluble COD values in each zone. It can be seen that the soluble COD ranges from about 21.8 mg/l to 28.8 mg/l across the four zones.

• However, what we do not know is whether this soluble COD is readily biodegradable. To check for that we need to look at the soluble BOD5. We do not have soluble BOD5 as part of the profiles Tab of the aeration tank, but we do have soluble BOD5 values specified in the Effluent of the simulator. Let us take a look at how the soluble COD and soluble BOD5 values compare

• Right click on the effluent and click on Outputs

79

Figure 72

• There is only one tab in the effluent Output, and it is labelled – Plant Effluent Outfall. Look under the Organic Variables section as we did for the aeration tank Outputs.

Figure 73

• You can see that the soluble COD has the same value as the soluble COD in

the final zone of the aeration tank i.e., 21.8 mg/l soluble COD

80

• Also, you will see that the soluble BOD5, which is a measure of the readily

biodegradable organic material has a value of just 0.71 mg/l.

• In order to significantly reduce the nitrates, we need to make enough

readily biodegradable COD available to the denitrifiers in the aeration tank

• One way of providing readily biodegradable COD for denitrification is to add

a compound that is highly biodegradable to the anoxic zone of a

wastewater treatment plant

• Such readily biodegradable organic compounds include methanol, ethanol,

acetate and glycerol.

• The simulator allows the addition of methanol. We will now evaluate the

impact of adding methanol to the aeration tank on nitrate reduction at the

facility.

• Click on the object labelled Methanol Dosing. The middle panel should now

open up to show a Carbon Addition tab.

Figure 74

• The methanol dosage is currently zero (0) gpd.

81

Figure 75

• As a reminder, note that at the current value of zero gpd of methanol addition, the effluent nitrates are still quite high at 17.3 mg/l and TN is at 19.9 mg/l (Figure 75).

• Let us add some methanol and see how much of an impact methanol addition will make to effluent nitrate levels. We will increase the methanol dosage fom zero to 100 gpd. After raising the methanol dosage to 100 gpd, RUN the simulation to see the impact

Figure 76

82

• Reviewing the results (Figure 76), we can see that the impact of adding 100

gpd of methanol is a reduction of the nitrates from 17.3 mg/l to 13.7 mg/l,

while TN dropped from 19.9 to 16.5 mg/l. we are moving in the right

direction!

• We will now increase the methanol dosing again to see the impact on

Nitrates and the TN level. This time, we will double the methanol dosage

rate from 100 gpd to 200 gpd.

Figure 77

• The results indicate that with the higher level of methanol dosage, we have now succeeded in reducing the nitrates to zero. However, now we have about 5.7 mg/l of nitrites. TN level is now also at 8.9 mg/l – which satisfies our effluent targets for Total Nitrogen. However the effluent COD is now 41 mg/l, slightly higher than our effluent target of 40 mg/l.

• One strategy we could use to decrease the COD is to reduce the solids loading on the clarifier, so that we can optimize solids removal.

• To see the current RAS rate, click on the object label RAS Pump

83

Figure 78

• The middle panel will now open up to show a tab labelled RAS Pumping Station Building

• The RAS Pump Status provides the value of RAS flow back to the Aeration Tank. The current RAS flow is 2.1134 MGD (see Figure 79)

Figure 79

• We will now change the RAS flow to 1.5 MGD. After making the change, Click RUN

84

Figure 80

• The results of the simulation now show that we meet both the COD and TN targets (Figure 80).

• However, we are nowhere close to the TP reduction targets. The current effluent TP value is 9.47 mg/l, far from our goal of 3 mg/l.

• We will now explore chemical phosphorus removal as a means of achieving our targets.

Chemical Phosphorus Removal

• There are two ways that phosphorus can be removed in wastewater plants

– it is either incorporated into biomass (Biological Phosphorus Removal), or

removed as a chemical precipitate (Chemical Phosphorus Removal) using

chemical aids like Ferric and Alum.

• Chemical Phosphorus removal can occur at three major places in a

wastewater plant – in the primary clarifiers, in the secondary clarifiers or in

a tertiary treatment process.

• The virtual plant simulator allows us to make adjustments to the chemical

dosage at two points in the plant – ahead of the primary clarifiers

85

(Preliminary Ferric Addition) and ahead of the secondary clarifiers

(Secondary Ferric Addition).

• The image below shows the two major dosage points labelled Ferric dosing

Figure 81: Two Ferric Dosage Points

• What are the implications of dosing Ferric in each of the two points? Dosing

Ferric in either dosage point has pros and cons. We will now use the

simulator to evaluate the impact of Ferric dosage at either point.

• Let us investigate the impact of dosing ferric in the primary section of the

treatment system. To do so, click on the Preliminary Ferric addition point in

the front end of the plant.

Figure 82

86

• The middle panel should reflect the dosage of Ferric in the preliminary

treatment dosage point (Figure 83).

Figure 83

• The ferric dosage at the Preliminary Ferric Addition point is currently zero

(0) Ib Me/d, where Me stands for Metal. This basically means we have 0 Ibs

of Fe Metals dosed per day. Let us bump the dosage up to 100 Ib Me/day

and RUN the simulation

Figure 84

87

• What was the effect observed? We would expect that the Ferric would

have reacted with soluble phosphate and precipitated some of the

phosphorus as a phosphate salt. This should lead to a decrease in both the

soluble and total phosphorus levels

• We observe that the phosphorus levels have indeed decreased in the

effluent to 7.21 mg/l for soluble phosphorus and 7.7 mg/l for Total

phosphorus (see Figure 84).

• However, we are still a long way from a total P value of 3 mg/l in the

effluent. Clearly, adding Ferric has helped., so let us add some more Ferric

by doubling the dosage rate to 200 Ib Me/day. RUN the simulation after

maiing the dosage change.

Figure 85

• The TP in the effluent now drops to 5.54 mg/l while the soluble Phosphorus

is now at 4.95 mg/l. We are clearly moving in the right direction. Let us see

how far another round of dosage increase will get us by raising the Ferric

dosage to 300 Ib Me/day. Make the change and RUN the simulation

88

Figure 86

• The results indicate that the effect is still positive (Figure 86). Effluent TP is

now 3.59 mg/l while the soluble P level is 2.92 mg/l

• Now increase the Ferric dosage to 400 Ib Me/day, and RUN the simulation

to observe the effect.

Figure 87

• Raising the ferric dosage to 400 Ib Me/day allows us to reach the effluent

TP target. The plant is now achieving all of its target effluent goal with COD

of 33.1 mg/l, TN of 9.3 mg/l and TP of 2.1 mg/l.

89

• Let’s see what happens if we increase the ferric dosage to 500 Ib Me/day?

Increase the dosage to 500 Ibs Me/day and RUN the simulation to observe

the effect.

• The effluent quality improves (Figure 88). TP is now 1.22 mg/l while soluble

P is now 0.44 mg/l.

Figure 88

• The obvious question is this: will we continue to see an improvement in

effluent quality of we kept dosing Ferric to the preliminary treatment

system?

• To explore what happens if we drastically increase ferric dosage on the

preliminary treatment train, increase the Ferric dosage by 100 Ib Me/day

and RUN the simulation each time, until you get to 1000 Ib Me/day. What

do you observe?

90

Table 8: Impact of Preliminary Fe Dosage on Effluent Water Quality

• Table 8 summarizes the results. The dark green shaded cells represent the

only conditions under which all three goals of COD. TP and TN are met

• An interesting feature can be seen in the results – as the ferric dosage

increases, TP continues to decrease. However we get to a point where

further decreases in TP lead to a significant increase in both COD and TN.

Why is this?

• At these points, we have basically turned the plant into one that is deficient

in phosphorus. We notice this effect starting at a dosage of about 600 Ib

Me/day. You will observe that from this point onwards, there is very little

soluble Phosphorus (OP) available in the effluent.

• Figure 89 shows the data from Table 8 in graphical form. As can be seen,

beyond a dosage of about 600 Ib Me/day, the facility actually loses its

ability to effectively remove treat COD and Nitrogen.

Ib Me/day COD TN TP OP0 35.8 8.3 9.47 9.05

100 35.1 9.1 7.7 7.21200 34.4 9.5 5.54 4.95300 33.8 9.4 3.59 2.92400 33.1 9.3 2.1 1.37500 32.5 8.9 1.22 0.44600 34.5 10.1 0.8 0.01700 70.5 16.8 0.73 0800 94.8 19 0.69 0.01900 110 20.4 0.67 0.01

1000 120 20.8 0.65 0.01

91

Figure 89

• One clue into what is happening can be obtained by using the Output

feature to see what is happening in the effluent form the preliminary Ferric

dosage point.

• Set the Preliminary Ferric Dosage to 600 Ib Me/day. RUN the simulation

and confirm that the effluents are the same ones you observed earlier.

Figure 90

• Now click on output, and then scroll down to the section labelled

Phosphorus to see the soluble phosphorus level. This is at a very low value

of 0.91 mg/l, down from an influent value of 13 mg/l (Figure 91).

92

Figure 91

• This example suggests that one potential risk of dosing ferric in the front

end of the plant is that if the dosage is too high, the facility might face a

phosphorus deficiency which will impact the overall ability of the plant to

meet effluent limits.

• Now we will explore what would happen if all the ferric was dosed ahead of

the secondary clarifier instead.

• For this exercise reset the Ferric dosage on the preliminary section to zero,

and run the simulation. You should see the results in Figure 92.

93

Figure 92

• Now click on the secondary ferric addition point

Figure 93

• The middle panel should now show the dosage levels in the Secondary

Ferric Addition point. The default value should be zero (0) Ibs Me/day.

• Now change the value from 0 to 100. Figure 94 shows the result for a Ferric

dosage of 100 Ibs Me/day in the secondary dosage point.

94

Figure 94

• Now raise the dosage gradually from 100 to 1,000 in increments of 100,

each time. i.e., go to 200, 300, 400, 500, increasing by 100 each time, until

you get to 1,000. After each change, RUN the simulation and record the

effluent values for COD, TN, TP and OP.

• Your result should match the outcome summarized in Table 9. The dark

green cells represent the conditions under which the COD, TN and TP

conditions are met.

Table 9

Ib Me/day COD TN TP OP0 35.8 8.3 9.47 9.05

100 36.3 9.6 7.31 6.54200 37 9.8 5.19 4.02300 37.5 9.7 3.19 1.58400 37.7 9.6 2.12 0.2500 37.8 9.4 2.09 0.07600 37.5 12 2.13 0.04700 37.5 11.9 2.19 0.03800 37.6 11.7 2.26 0.02900 38.7 9.2 2.39 0.02

1000 43.7 10 2.81 0.02

95

Figure 95

• Figure 95 is a graphical representation of the results summarized in Table 9.

• The results show that the effluent conditions are not as dramatically

impacted as was the case with the preliminary dosage case

• Notice also that the soluble phosphorus to total phosphorus values are

closer in the effluent for the case where ferric is dosed in the front end of

the plant (preliminary ferric dosage) than in the back end (secondary ferric

dosage).

• When Ferric is dosed in the preliminary section of the plant, the

precipitated phosphorus is removed with the solids in the primary clarifier.

However, when Ferric dosing occurs ahead of the secondary clarifier, some

of the precipitated Phosphorus remains incorporated with the mixed liquor.

This increases solids concentration. However, because the precipitated P is

96

retained in the solids, soluble P levels go down, but the total phosphorus

value stays somewhat high.

• In both dosage cases, we could meet the TP target with a dosage of 400 Ibs

Me/days (Table 8 and Table 9). The choice of which strategy to use

ultimately comes down to the specific situation at each plant. Factors to be

considered include influent concentrations, the reliability of monitoring and

measurement systems, and the solids handling infrastructure at the facility.

• Preliminary treatment dosing works if there are reliable means of ensuring

that there is little risk of over dosage, since this will reduce the availability

of phosphorus for biological processes. Facilities can use a combination of

both options.

• Table 10 shows results obtained by combining both Preliminary Ferric

Dosage (P) and Secondary Ferric Dosage (S). See if you can replicate the

results. For each condition, set the Preliminary and Secondary Ferric

Dosage levels and RUN the simulation.

Table 10

Ib Me/day (T) Ib Me/day (P) Ib Me/day (S) COD TN TP OP0 0 0 35.8 8.3 9.47 9.05

100 50 50 35.7 8.7 7.73 7.11200 100 100 35.7 9.4 5.52 4.67300 150 150 35.6 9.3 3.36 2.25400 200 200 35.5 9.5 1.67 0.34500 250 250 35.1 9.4 1.37 0.09600 300 300 34.6 8.7 1.26 0.05700 350 350 34.1 9.1 1.17 0.04800 400 400 33.7 9.2 1.1 0.03900 450 450 33 10.9 1.05 0.02

1000 500 500 33.1 9.3 1.02 0.02

97

• The figure shows that the COD, TN and TP conditions are met across a

broader range of dosage values when Ferric dosage is implemented in both

the preliminary and secondary treatment sections of the plant.

• Figure 96 shows the results summarized in Table 10 in graphical form.

Figure 96

• In summary, the conditions that allowed for optimal operations at this virtial plant are:

o DO - 0.5, 0.5, 2, 2 mg/l o WAS - 39.625 gpd o Ferric – 200 Ib Me/day Preliminary Ferric Dosage Point and 200 Ibs

Me/day Secondary Ferric Dosage Point o RAS – 1.5 MGD o Methanol – 200 Ibs MeOH/day

98

Chapter 5: Exploring Seasonal Effects on Wastewater Operations

• All of our activities so far have led us to a set of operating conditions that

are optimal for the virtual facility under average conditions.

• Now we will explore how seasonal variations in flow and organic loading

affect the operations of a wastewater plant. Table 11 summarizes the

conditions that will be reviewed

Table 11

Conditions COD TKN NH3 Ortho-P Temp Flow (MGD)

Summer 430 44 33 14 75 2.4 Winter 405 37 25 9 50 3.0 Wet Weather

400 40 29 11 60 4.0

Average 416 42 32 13 64.4 2.641

Learning Objectives

• Evaluate the impact of seasonality on WWTPs • Review challenges that occur during specific

seasons • Explore methods for optimizing facility

operations during different seasons

99

• Recall that the influent conditions can be found by clicking on the Influent

icon and reviewing the middle panel. The current average influent

conditions are summarized in Figure 97.

Figure 97

• Recall that for the average influent loadings, the following operational

conditions were found to be optimal (see Table 12):

o DO - 0.5, 0.5, 2, 2 mg/l o WAS – 39,625 gpd o Ferric – 200 Ib Me/day Preliminary Ferric Dosage Point and 200 Ibs

Me/day Secondary Ferric Dosage Point o RAS – 1.5 MGD o Methanol – 200 gpd

100

Table 12

• The corresponding plant wide operational summary are provided below:

Table 13

• While the resulting effects of these operational conditions are shown in

Figure 98, which shows the influent vs the effluent conditions.

Figure 98

Units Plantwide Zone 1 Zone 2 Zone 3 Zone 4DO mg/l 0.5 0.5 2 2Methanol gpd 200WAS gpd 39625Prelim P Ib Me/day 200Secondary P Ib Me/day 200

101

Wet weather challenges

• Wet weather poses particular challenges to wastewater facilities. Flow is

usually higher, and the high volumes of water from rain events can lead to

increases in inflow & infiltration (I&I) events.

• However, while utilities might face higher wastewater flows, the

concentrations of the contaminants in the wastewater (e.g., COD, TN, NH3

and TP) also tend to be lower, largely because of dilution effects.

• The assumed wet weather flow and average wastewater flow conditions for

this facility are summarized in Table 14.

Conditions COD TKN NH3 Ortho-P Temp Flow (MGD) Wet Weather 400 40 29 11 60 4.0 Average 416 42 32 13 64.4 2.641

Table 14

• Starting with the current operating conditions, we will see how wet weather flows can impact the ability of the plant to meet effluent targets.

• First, click on the Influent.

Figure 99

• Type in the wet weather flow conditions into the middle panel

102

Figure 100

• Click RUN to activate the simulation and see the effect of the wet weather conditions on the effluent parameters

Figure 101

• The results show that even though we commenced with the optimal operational parameters for the average flow, the plant is now unable to meet the COD, TN, or TP limits. Effluent COD is now at 45.3 mg/l, TN is 12.8 mg/l and TP is 3.91 mg/l.

103

Table 15

• The summary data (Table 15) also shows the key operational parameter at

the plant.

• Our task is to explore how to bring the plant into compliance for wet

weather operations. There are a variety of approaches that can be taken to

try to bring the facility back into compliance. The operators must decide on

the course of action that will be taken, and which parameter to focus on

first.

• We will now compare the effluents conditions for both the average and wet

weather situations (Table 16)

Parameters Average Wet Weather BOD5 8.0 12.3 COD 35.5 45.3 TSS 19.0 26.1 Ammonia 0.1 0.6 Nitrite 6.8 9.0 Nitrate 0.0 1.6 e-05 TN 9.5 12.8 Soluble Phosphorus 0.34 2.43 Total Phosphorus 1.67 3.91

Table 16

• We can observe the following changes between the two operational

periods

104

o The BOD and COD values are higher in the wet weather period

o TSS values are higher in the wet weather period by about 7 mg/l.

o Ammonia increase slightly in the wet weather period by 0.5 mg/l

o Nitrite increases slightly by about 2.2 mg/l

o TN increases slightly by about 3.2 mg/l – due to an increase in

ammonia (reduced nitrification) and higher nitrite value (decreased

denitrification)

o TP and OP increase

• The following approaches could potentially be used to try to optimize the

performance of the facility:

o Increase ferric dosage to the facility to reduce the amount of soluble

phosphorus available without leading to a significant increase in the

solids loading at the plant

o Use a combination of further aeration and methanol dosage

increases to enhance denitrification

o Optimize RAS rates to further enhance secondary clarifier

performance

Step 1: Reduce TP by Increasing Ferric Dosage

o Increase ferric dosage to 300 Ibs Me/day at the preliminary ferric

dosage point and RUN the simulation. What do you observe?

o The effluent TP drops to 2.56 mg/l, meeting effluent targets for Total

Phosphorus (Figure 102).

105

Figure 102

Step 2: Methanol Addition

• We now want to explore whether adding more methanol to the facility can

help to improve operations?

• Click on the Methanol Dosage point and increase the quantity of methanol

to 300 gpd. RUN the simulation and observe the results (Figure 103).

Figure 103

• We observe that after increasing the methanol dosage from 200 to 300 gpd, the TN levels drop a bit (from 12.9 mg/l in the last run, to 12.1 mg/l),

106

but the COD now increases to 51.9 mg/l suggesting some breakthrough in soluble COD provided by the methanol addition, to the effluent.

• Simply adding methanol does not appear to be a viable solution. Is there anything else that we can do to optimize the plant? Are there any other actions that can be taken to optimize operations at this plant?

• There are a number of changes that we can make that will get us close the effluent targets, but do not quite get us there in a robust manner.

• Here is one set of actions that will get us close (Figure 104). o DO - 0.5, 0.5, 0.5, 1 mg/l o WAS – 39,625 gpd o Ferric – 300 Ib Me/day Preliminary Ferric Dosage Point and 200 Ibs

Me/day Secondary Ferric Dosage Point o RAS – 1.0 MGD o Methanol – 300 gpd

Figure 104

• As can be seen, the effluent TN and TP targets are met, but the facility exceeds its effluent COD target.

107

• Can you think of other changes that can be made ti improve the performance of the plant?

Summer conditions

Conditions COD TKN NH3 Ortho-P Temp Flow (MGD) Summer 430 44 33 14 75 2.4 Average 416 42 32 13 64.4 2.641

Table 17

• During summer conditions, temperatures generally tend to increase • Depending on the sources that contribute to the wastewater facility, flow

volumes and composition might be different during the summer months compared to the average conditions.

• The major differences between the average and summer conditions in this example is temperature.

• Click on the Restore button at the bottom of the simulator to restore the simulator to its starting conditions (Figure 105).

Figure 105

• If the Restore action was successful, you should find that the operational variables now reads zero (0) throughout (Table 18).

Table 18

108

• Now that we’ve confirmed that the system has been restored to its default conditions, restore the plant to its optimum conditions during average flow.

o DO - 0.5, 0.5, 2, 2 mg/l o WAS – 39,625 gpd o Ferric – 200 Ib Me/day Preliminary Ferric Dosage Point and 200 Ibs

Me/day Secondary Ferric Dosage Point o RAS – 1.5 MGD o Methanol – 200 gpd

• After making each change, RUN the simulation. If you’ve done everything correctly, you should have the effluent results reported in Figure 98 with COD – 35.5 mg/l, TN – 9.5 mg/l and TP – 1.67 mg/l.

• Now click on the Influent object and replace the average default conditions with Summer conditions from Table 17, then RUN the simulation. What do you observe?

Figure 106

• As can be seen in Figure 106, the facility is still able to meet its effluent targets using the optimal operational conditions used during the average conditions.

• Starting with the optimum conditions under average flow conditions, we will now change the temperature first to see the impact of temperature.

109

Effect of Winter Conditions

Conditions COD TKN NH3 Ortho-P Temp Flow (MGD) Winter 405 37 25 9 50 3.0 Average 416 42 32 13 64.4 2.641

Table 19

• Once again, restore the plant to its default conditions by clicking on the Restore button at the bottom of the simulator.

• Restore the plant to its optimum conditions during average flow o DO - 0.5, 0.5, 2, 2 mg/l o WAS – 39,625 gpd o Ferric – 200 Ib Me/day Preliminary Ferric Dosage Point and 200 Ibs

Me/day Secondary Ferric Dosage Point o RAS – 1.5 MGD o Methanol – 200 gpd

• After making each change, RUN the simulation. If you’ve done everything correctly, you should have the effluent results reported in Figure 98 with COD – 35.5 mg/l, TN – 9.5 mg/l and TP – 1.67 mg/l.

• Now click on the Influent object and replace the average default conditions with Winter conditions from Table 19, the RUN the simulation. What do you observe?

Figure 107

110

• You will notice that the facility is now violating its TN and COD limits.

• Although conditions will vary from facility to facility, this brief example

shows that generally when an increase in flow is combined with a reduction

in temperature, facilities will likely require some modifications to their

normal operations to meet the effluent goals.

• This is one of the reasons why facilities might have seasonal limits with

separate numeric targets for the winter and summer limits.

111

Chapter 6: “What if” Scenario Analysis with Simulators

• At some point in a utility, situations might arise that will require having

some units taken offline. These might include maintenance and upgrade

scenarios that might require having entire process trains taken down.

• In most cases, we cannot plan for such events because doing so can put a

plant at risk of violating its permit.

• Simulators allow us to test the impact of such conditions in a virtual

environment, without putting the plant at risk.

• We will now explore what the impact of taking down various process units

on the overall treatment effectiveness at our virtual facility.

• We will evaluate two major types of upgrade and maintenance events

involving aeration tanks and clarifiers.

• The learning objective will be to evaluate the impact of taking units offline.

Learning Objectives

• Evaluate the impact of seasonality on WWTPs • Review challenges that occur during specific

seasons • Explore methods for optimizing facility

operations during different seasons

112

Taking aeration basins offline

• Aeration tanks can be taken offline for various reasons. This might include

diffuser cleaning and replacement, aeration header repairs, basin upgrades,

insertion of baffles & partitioning of aeration zones, expansion of basin

volume, conversion from suspended to media based treatment, and others.

• In this example, we will evaluate the impact of taking one aeration tank

offline

• We will be starting from the optimum operational conditions under average

flow conditions.

• To get there, use the Restore function to get back to the default conditions

and then input the optimal operational conditions below:

o DO - 0.5, 0.5, 2, 2 mg/l o WAS – 39,625 gpd o Ferric – 200 Ib Me/day Preliminary Ferric Dosage Point and 200 Ibs

Me/day Secondary Ferric Dosage Point o RAS – 1.5 MGD o Methanol – 200 gpd

• After making each change, RUN the simulation. If you’ve done everything correctly, you should have the effluent results reported in Figure 98 with COD – 35.5 mg/l, TN – 9.5 mg/l and TP – 1.67 mg/l.

• In order to alter the status of a process unit, two alternative steps can be

taken.

o Alternative 1 – Left Click on the Unit and the middle panel now

reflects all of the input options for the unit

o Alternative 2 – Right click on the unit and tick the “In Service” label to

place the unit online or Offline.

113

• We will stick with Alternative 1.

• Click on the aeration tanks.

• Go to the middle panel, and select the Aeration Tank Status (Figure 108)

Figure 108

• As can be seen, both aeration tanks are online. • Next, take one of the aeration tanks offline by clicking on “Offline”. Click

Offline for Aeration Tank 1 Status.

Figure 109

114

• You will observe that one of the aeration tanks will now be greyed out

(Figure 109).

• Now RUN the simulation to see what the impact of taking one aeration tank

will be on the effectiveness of treatment.

Figure 110

• The results of the simulation indicate that the facility is now unable to meet

its treatment goals. Effluent COD is 43.3 mg/l, slightly higher than the

effluent target of 40 mg/l (Figure 110)

• The TN value is 30.1, and the ammonia value is at an all-time high of 27.1

mg/l, indicating that the facility has totally lost nitrification capacity

• The facility is however still meeting it effluent phosphorus goals.

Table 20

115

• By reviewing the plant-wide operational summary data (Table 20), we can

see that although the MLSS is now about 3094 mg/l, the SRT is just 2.7

days. By taking one tank offline, we have lost about half of the solids

inventory in the plant.

• One obvious question from an operational perspective is to determine the

maximum flow that the facility can handle when only one aeration tank is in

operation.

• An obvious starting point is to reduce the influent flow handled to just half

of the initial amount.

• Click on the Influent and set the influent flow to 1.32 MGD, then RUN.

What do you observe?

Figure 111

• As would be expected, the facility now meets all of its treatment goals (Figure 111).

• In fact, the results clearly demonstrate that at half the average flow, the facility is doing much better than it was doing at full scale flow with both aeration tanks online.

116

• Why might this be the case? There could be several reasons why. At half

the flow, the organic loading to the plant is now at only half the initial

value, making it easier for the facility to handle the load. Also, with the

lower flow rate, loading to the primary clarifier and secondary are now

much lower. This can have the result of allowing for more effective COD

removal through these treatment systems.

• We will now evaluate the extent to which we can take advantage of the

treatment capacity across other process units which have retained their full

capacity (e.g., primary clarifier and secondary clarifiers) to determine the

maximum amount of flow that the facility can handle, while going through

an upgrade that has forced the plant to operate with one clarifier unit

offline.

• Let us bump up the influent flow to 1.6 MGD and then click RUN to activate

the simulation. What do you observe?

Figure 112

117

• The results show that the facility can robustly meet treatment goals at this

higher flowrate (Figure 112). Total COD is 30.9 mg/l, TN is 7.4 and TP is 0.83

mg/l.

• How high can we go and still meet treatment objectives? To find this out,

increase the flow yet again to 1.8 MGD and RUN the simulation. What do

you observe?

Figure 113

• The facility is still able to meet all of its effluent targets (Figure 113). The

results suggest that there is still potential to increase the flow.

• Now increase the flow to 2 MGD, and then RUN the simulation to observe

the effects. The facility is still able to meet its effluent targets (Figure 114)!

118

Figure 114

• Let’s go one step further, and increase the influent flow to 2.2 MGD. RUN

the simulation and observe the results.

Figure 115

• When the flow is raised to 2.2 MGD, the TN value just exceeds the target.

COD is 37.8 mg/l, TP is 1.29 mg/l, while TN is 10.1 mg/l (Figure 115). It

appears we have just gone a little too far.

• Now, reduce the flow back to 2.1 MGD and RUN the simulation. You will

observe that the facility meets its treatment goals at 2.1 MGD (Figure 116)

119

Figure 116

Optimizing the plant at a lower flow

• It is certainly possible to further optimize the plant to reduce chemical

consumption, or even obtain better COD and TN removal.

• We will now explore the optimization of chemical dosage to reduce

operating expense.

• Recall that because we have made no changes to the operating conditions

that we ran at average conditions, this means we have retained the same

level of chemical dosage that we had for the average flow condition of 2.64

MGD.

• At a reduced flow of 2.1 MGD in one aeration tank, the effluent TP is 1.21

mg/l. At the average flow of 2.64 MGD, the TP was 1.72 mg/l. Given the fact

that the target effluent TP is 3 mg/l, there is clearly some optimization that

can be carried out on the amount of Ferric consumed in the plant.

120

Table 21

• Notice that the chemical cost is $59,600/year. While some of this is for

methanol, the balance is for ferric.

• Let’s shave off some ferric dosage. A good starting point is to reduce the

ferric dosage by the same proportion that the flow has reduced.

• The initial flow was 2.64 MGD. Now, we have settled at a new flow rate of

2.1 MGD when the facility has only one operational aeration tank.

• We will now determine by how much the flow has decreased.

• The decrease in flow is given by the former average flow minus the new

flow.

𝑜𝑜𝑃𝑃𝐹𝐹𝐹𝐹 𝐹𝐹𝑒𝑒𝑂𝑂𝑂𝑂𝑒𝑒𝑀𝑀𝑀𝑀𝑒𝑒 = 𝐼𝐼𝑂𝑂𝑂𝑂𝑡𝑡𝑂𝑂𝑀𝑀𝑃𝑃 𝐴𝐴𝐴𝐴𝑒𝑒𝑂𝑂𝑀𝑀𝑂𝑂𝑒𝑒 𝑜𝑜𝑃𝑃𝐹𝐹𝐹𝐹 − 𝑁𝑁𝑒𝑒𝐹𝐹 𝑜𝑜𝑃𝑃𝐹𝐹𝐹𝐹

𝑜𝑜𝑃𝑃𝐹𝐹𝐹𝐹 𝐹𝐹𝑒𝑒𝑂𝑂𝑂𝑂𝑒𝑒𝑀𝑀𝑀𝑀𝑒𝑒 = 2.641 𝑀𝑀𝑀𝑀𝑀𝑀 − 2.1 𝑀𝑀𝑀𝑀𝑀𝑀 = 0.541 𝑀𝑀𝑀𝑀𝑀𝑀

% 𝑜𝑜𝑃𝑃𝐹𝐹𝐹𝐹 𝐹𝐹𝑒𝑒𝑂𝑂𝑂𝑂𝑒𝑒𝑀𝑀𝑀𝑀𝑒𝑒 =𝐹𝐹𝑃𝑃𝐹𝐹𝐹𝐹 𝐹𝐹𝑒𝑒𝑂𝑂𝑂𝑂𝑒𝑒𝑀𝑀𝑀𝑀𝑒𝑒

𝐼𝐼𝑂𝑂𝑂𝑂𝑡𝑡𝑂𝑂𝑀𝑀𝑃𝑃 𝐴𝐴𝐴𝐴𝑒𝑒𝑂𝑂𝑀𝑀𝑂𝑂𝑒𝑒 𝐹𝐹𝑃𝑃𝐹𝐹𝐹𝐹 =0.541 𝑀𝑀𝑀𝑀𝑀𝑀2.641 𝑀𝑀𝑀𝑀𝑀𝑀

• This represents a 20.4% decrease in flow. We will start with a proportional

decrease of 20% in the total ferric dosed to both the front end and back

end of the treatment facility.

• Since 20% of 200 Ib Me/day is 40 Ib Me/day, this means that the ferric

dosage should be reduced to about 160 Ib Me/day in both the preliminary

and secondary dosage points

121

• Change the ferric dosage values in the preliminary and secondary dosage

points to 160 Ib Me/day and RUN the simulation

Figure 117

• The results clearly show that the effluent targets are still met (Figure 117).

The operational summaries show that the chemical costs have decreased to

$47,700/yr.

Table 22

122

• The chemical costs can be further decreased. Maintaining all of the other operating conditions, how far can the Ferric dosage be decreased while still maintaining effluent targets? Test out some scenarios.

Figure 118

• You will find that you can go as far as about 120 Ibs Me/day dosed to both the preliminary ferric dosage point and the secondary ferric dosage point, respectively.

123

Taking Clarifiers Offline

Figure 1197

• Secondary clarifiers are used for separating the biological solids (bacteria)

from the treated liquid (wastewater)

• The operational principle is based on the separation of solids from the

liquid through the process of sedimentation

• Clarifiers have a variety of parts – and they periodically require

maintenance, upgrade and even replacement (see Figure 119).

• We will now explore what happens to the facility’s treatment capacity as 1,

2 and eventually, 3 clarifiers are taken offline.

• To get there, use the Restore function to get back to the default conditions

and then input the optimal operational conditions below:

7Sourced from https://www.monroeenvironmental.com/water-and-wastewater-treatment/circular-clarifiers-and-thickeners/secondary-clarifiers/

124

o DO - 0.5, 0.5, 2, 2 mg/l

o WAS – 39,625 gpd

o Ferric – 200 Ib Me/day Preliminary Ferric Dosage Point and 200 Ibs Me/day Secondary Ferric Dosage Point

o RAS – 1.5 MGD o Methanol – 200 gpd

• After making each change, RUN the simulation. If you’ve done everything correctly, you should have the effluent results reported in Figure 98 with COD – 35.5 mg/l, TN – 9.5 mg/l and TP – 1.67 mg/l.

• Once the facility has been restored to the optimum operating conditions under average flow conditions, we will now proceed to evaluate the impact of taking clarifiers offline.

Figure 120

• There are four clarifiers in the plant (Figure 120). Click on the clarifiers, to

bring up the Secondary Clarifier Status in the middle data input panel.

• Any clarifiers that are offline are GREYED out. You can also confirm the

status of the clarifiers by clicking on the clarifier

• The middle panel will now show the Secondary Clarifier Status and clarifiers

can be brought online and offline by clicking on the individual clarifiers.

125

• You will notice that all the clarifiers are currently Online. Take Clarifier 1

offline by changing the status to Offline.

Figure 121

• You will now see that one of the clarifiers (secondary clarifier 1) is now

greyed out (Figure 122)

Figure 122

• Next, RUN the simulation to see the effect of taking one clarifier offline. Do

you think the facility will still be able to meet its effluent targets?

126

Figure 123

• The results show that the facility is able to meet the COD and TP targets, but not the TN target, since TN is now 11.6 mg/l.

• We will now attempt to optimize the operations to see whether we can get the facility to meet effluent requirements at full flow, with only 1 clarifier operating.

Optimizing air flowrate

• We will now explore the use of the air flowrate feature in the aeration tank • Since the major reason why the TN level is high is the fact that Nitrate

levels are high, it is clear that we might be able to get the target TN levels by improving denitrification.

• There are two major ways to enhance denitrification. We can either provide more external substrate (e.g., methanol) or increase the anoxic environment in the aeration basin. A combination of both approaches might be needed to reach the optimal conditions for TN removal

• The aeration basin has 4 zones, and it would be immensely challenging to try to find the optimal DO levels in each of the four zones that will allow the facility to meet its effluent goals.

• What we will do instead is use the air flowrate feature in Aeration settings to find the right DO settings

• Click on the aeration tank and turn the DO Controller Off

127

Figure 124

• You will now see that the DO set points are greyed over, and cannot be

changed, while the air flowrate to the aeration tanks is now active and can

be changed.

• The default value of air flow is very low and using it will result in very low

DO values. In order to determine a good starting air flowrate, review the

value in the top operational summary. From Table 23, we can see that the

current air flowrate that corresponds to the DO levels of 0.5 mg/l, 0.5 mg/l,

2 mg/l and 2 mg/l in each of the four aeration zones is 32.8 ft3/sec (1968

ft3/min)

128

Table 23

• Right click on the aeration tank and select Outputs. Next, click on the tab

labelled Aeration.

• We will now need to replace the default value of 245.24 ft3/min with the

total air flowrate that we just determined. However, before making the

change, there is one more set of values that we need to review

Figure 125

• Notice that below the total air flowrate, there are numbers labelled “Fraction to Pass 1”, “Fraction to Pass 2” and “Fraction to Pass 3”. These fractions represent the proportion of the airflow that goes to each of the zones.

• As you can see, the default values indicate that 50% of the airflow is sent to zone 1, 25% to zone 2 and 15% to zone 3. The first three zones add up to 90%. This means that 90% of the airflow goes to the three zones. Since all

129

of the air flowrates must add up to 1, this means that zone 4 has an air flowrate equal to 1 – 0.9 = 0.1.

• This implies that only 10% of the air flowrate goes to the fourth zone. • Different plants might have different set ups, but this type of tapered

aeration profile is quite commonly used in facilities with a rectangular or plug flow profile.

• Why would a tapered profile make sense? This is because in a plug flow system, the highest organic loading tends to be in the front end of the plant. The higher the levels of COD or BOD, the higher will likely be the demand for oxygen by the microorganisms.

• You see now that the default air flowrate is much lower than the actual air flowrate we are currently using.

• Now that the aeration feature in the simulator is being controlled using the air flow feature, the effects that we observe in the treatment process will be based on the air flowrate values.

• To demonstrate the importance of the air flow rate, leave the default value in and RUN the simulation. What do you observe?

Figure 126

• Observe that the plant is now DO deficient (Figure 126). The facility is no

longer able to meet any of the effluent limits. Effluent COD is 245 mg/l, TN

130

is 35.6 mg/l, and the facility is unable to nitrify, even at a SRT of about 5.7

days.

• A clue to what is happening can be seen in the operational summary.

Where DO in the aeration tank is now 0.0 mg/l (Table 24).

Table 24

• Change the air flow rate to 1968 ft3/min and RUN the simulation

Figure 127

• You will observe that the results are now similar to what we had when we

used a DO set point to control aeration (Figure 123), but they are not quite

the same. COD and TN are the same at 39.7 mg/l and 1.94 mg/l

respectively, but the TN, Nitrite and Nitrate values are slightly different.

131

• The slight difference lies in the changes in the Fraction of Air flowing to the

various zones. When we specified DO levels, we “Overrode” the fractional

airflow instruction and essentially allowed the aeration system to provide

whatever level of aeration was needed to meet a DO of 0.5, 0.5, 2 and 2

mg/l in each zone.

• Let us review the aeration system under DO control again so that we can

compare the Fractional Airflows under DO control to non-DO regulated

Aeration control.

• Switch back to DO control and RUN the simulation. You should be back to

the conditions in Figure 123 with effluent conditions of 39.7 mg/l COD, 11.6

mg/l TN and 1.94 mg/l TP.

• Now RIGHT CLICK on the Aeration Tank, Select Outputs and Select the tab

labelled Aeration

• You will see the air flowrates to each zone.

Figure 128

132

• Tabulating the values and calculating the Fractional Airflow rates, we obtain

the results summarized in Table 25

Table 25

• While the DO control option forced the aeration volumes in Table 25 to

result, when we use the non-DO control option, we have to specify an

aeration taper.

• We will leave the current default aeration taper in place. Remember that it

is 50%, 25%, 15% and 10% to Zones 1 through 4 respectively.

• Recall that using we are meeting the COD effluent and TP target but not the TN target, largely because the nitrate level is high. Our goal is to decrease the air flowrate to an extent that allows us to extend the anoxic character of our wastewater treatment system to obtain more denitrification.

• Turn the DO Controller Off, reduce the air flowrate to 1800 ft3/min and RUN the simulation. What do you observe?

Zone ft3/min Fraction to PassAirflow Zone 1 915 47%Airflow Zone 2 449 23%Airflow Zone 3 403 21%Airflow Zone 4 189 10%Total Airflow 1956 100%

133

Figure 129

• As can be seen in Figure 129, the plant now meets all its effluent limits by using an air flowrate of 1800 ft3/min with the default aeration taper.

• We will now review the Aeration Tab under Outputs to see the actual DO level in each of the zones. To get to this information, RIGHT CLICK on the Aeration Tanks, Select Outputs and then click on the aeration Tab.

Figure 130

134

• Figure 130 shows the DO profile in each zone.

Removing 2 Clarifiers from Service

Figure 131

• Take one more clarifier offline so that two clarifiers are now greyed out.

• Remember how to do this? Simply click on the clarifier section. The middle

panel now opens up to show the clarifier status. You can then proceed to

take the clarifier offline by selecting Offline on clarifier 2 status.

• RUN the simulation. What do you observe?

• HINT: Another way to change clarifier status is to move the cursor to the

clarifier in question, RIGHT CLICK and select the first item “In Service”. We

will test this out when removing the 3rd clarifier from service.

135

Figure 132

• The facility is now unable to meet its effluent targets (Figure 132).

• As would be expected, the effluent TSS is now quite high and it appears

that this might be the major reason why we can no longer meet the

effluent COD target.

• Why would there be a relationship between a high TSS value and a high

COD value?

• The relationship between TSS and COD is actually due to the fact that the

volatile fraction of the total solids is made up of bacterial cells.

Understanding the relationship between COD, TSS and VSS in the

effluent

https://micro.magnet.fsu.edu/cells/bacteriacell.html

136

Figure 133: The Bacterial Cell8

• Bacteria cells contains mostly water and chemicals such as proteins,

carbohydrates, lipids, etc. These chemicals contain carbon, and therefore

exert a chemical oxygen demand

• Studies of the chemical oxygen demand of bacterial cells have taken place

over many decades and researchers have established that there is a direct

relationship between the volatile suspended solids and COD.

• Figure 134 shows the relationship between the COD and VSS values for

various bacterial types (or species), while Table 26 summarizes some COD

to VSS values obtained by researchers. All the results indicate that there is a

regular and replicable relationship between bacteria and COD. Generally,

for every mg/l of VSS (bacteria cells), we can obtain between 1.29 to 1.49

mg/l COD.

8 http://biology.about.com/od/cellanatomy/ss/prokaryotes.htm

137

Figure 134: Relationship between COD and VSS for different bacterial species

138

Table 26

• Table 26 shows some results of the COD to VSS ratio for a range of different

types of bacterial species and activated sludge. Activated sludge contains

many species or types of bacteria.

• The VSS is the portion of the solids that represents the microbiological

species in the system – that is what we refer to as “Bugs” in the industry

• As the results in Table 26 show, the COD content of bacteria can range from

about 1.29 to 1.49 times the value of VSS. This means that if we have 10

mg/l VSS, we are likely to have a corresponding COD value due to the VSS

of about 12.9 to 14.9 mg/l.

• Assuming that COD is 1.4 times the VSS, what is the contribution of the

solids to the COD in our facility under the current operating conditions? To

do this, we must first find out what the VSS value is for the effluent.

139

• Note that Effluent Parameters contains TSS information only. To find out

more information about the VSS, we must go to the Effluent object in the

layout.

Figure 135

• Right Click on the Effluent and select Output

• A panel will open up that is labelled Plant Effluent Outfall

Figure 136

140

• Under the section titled solids, you will see both the TSS and VSS values.

The VSS value is 37.8 mg/l.

• What is the VSS/TSS ratio in the effluent?

𝑉𝑉𝑀𝑀𝑀𝑀 𝑡𝑡𝐹𝐹 𝑇𝑇𝑀𝑀𝑀𝑀 𝑂𝑂𝑀𝑀𝑡𝑡𝑂𝑂𝐹𝐹 = 𝑉𝑉𝐹𝐹𝑃𝑃𝑀𝑀𝑡𝑡𝑂𝑂𝑃𝑃𝑒𝑒 𝑀𝑀𝑉𝑉𝑀𝑀𝑔𝑔𝑒𝑒𝑂𝑂𝐹𝐹𝑒𝑒𝐹𝐹 𝑀𝑀𝐹𝐹𝑃𝑃𝑂𝑂𝐹𝐹𝑀𝑀𝑇𝑇𝐹𝐹𝑡𝑡𝑀𝑀𝑃𝑃 𝑀𝑀𝑉𝑉𝑀𝑀𝑔𝑔𝑒𝑒𝑂𝑂𝐹𝐹𝑒𝑒𝐹𝐹 𝑀𝑀𝐹𝐹𝑃𝑃𝑂𝑂𝐹𝐹𝑀𝑀

And by putting in the values for VSS and TSS, we obtain

𝑉𝑉𝑀𝑀𝑀𝑀 𝑡𝑡𝐹𝐹 𝑇𝑇𝑀𝑀𝑀𝑀 𝑂𝑂𝑀𝑀𝑡𝑡𝑂𝑂𝐹𝐹 = 𝑉𝑉𝑀𝑀𝑀𝑀𝑇𝑇𝑀𝑀𝑀𝑀 =

37.8 𝑚𝑚𝑂𝑂/𝑃𝑃59 𝑚𝑚𝑂𝑂/𝑃𝑃 = 0.64

• Now we need to determine the COD contribution of the VSS.

• Recall that the relationship between COD and VSS is:

𝐶𝐶𝑂𝑂𝑀𝑀 = 1.4 ∗ 𝑉𝑉𝑀𝑀𝑀𝑀

• So, 37.8 mg/l of VSS has a corresponding COD value of 1.4 * 37.8 mg/l COD

= 52.92 mg/l COD.

• Clearly, there is way too much solids in the effluent. We can see clearly that

just the VSS alone is sufficient to cause us to violate our permit

requirement for 40 mg/l COD in the effluent. Is there any way to try to

meet the effluent targets? We will try two approaches.

Reducing solids loading to the 2 operating clarifiers

• Even though two clarifiers are offline, the hydraulic loading on the clarifiers

is still within a reliable range (Table 27)

141

Table 27

• The typical ranges for the hydraulic loading to the secondary clarifier is 200

– 800 gal (US)/(ft2.day). As the operational summary in Table 27 shows, the

current value of the hydraulic loading is 800 gal (US)/(ft2.day), which is just

at the edge of the design limits.

• We will first attempt to reduce the solids loading to the secondary

clarifiers. We can do this by reducing the RAS rate even further.

Figure 137

• Click on the RAS Pump, and then change the RAS Pump Status value from

1.5 MGD to 1 MGD, and RUN the simulation. What do you observe?

142

• You will see that the TSS value has now reduced significantly to 21.5 mg/l,

and the effluent COD is 38.3 mg/l. The effluent target is now being met,

while the TN target is exceeded. The current TN value is 11.6 mg/l.

Optimizing aeration flow to enhance denitrification with two clarifiers online

• The results in Figure 137 suggest that the facility’s major reason for

violating its effluent permit for TN is due to insufficient denitrification. We

will now evaluate whether increasing the anoxic character of the

wastewater treatment process can enhance the ability to denitrify

• We will now decrease the air flowrate to 1700 ft3/min from the current

value of 1800 ft3/min. to do that, click on the Aeration Tanks object, and

make the necessary changes in the middle panel.

• RUN the simulation. What do you observe?

Figure 138

• The facility can now meet all of its effluent targets (Figure 138).

• Right click on the Aeration Tanks object, select Output and select the

Aeration Tab to see the aeration rates and DO levels in each of the four

zones (Figure 139).

143

Figure 139

Removing 3 Clarifiers from Service

• We will now explore how the removal of one more clarifier will affect the

performance of the facility.

• Click on the aeration tanks and turn the third clarifier offline. Three

clarifiers should now be greyed out (Figure 140).

144

Figure 140

• RUN the simulation with three (3) clarifiers offline. What do you observe?

Figure 141

• The facility is now no longer able to meet its effluent targets. Effluent COD

is now 113 mg/l, TN is 26.3 mg/l and TP is 5.29 mg/l (Figure 141).

• The effluent COD is high largely because of the high levels of TSS (really it is

the VSS that is correlated with COD as we learned earlier) in the effluent

due to the lower ability of the plant to effectively separate the liquids and

solids in the clarifier

145

• The operational summary (Figure 142) shows us clearly that we are well out

of the recommended operating range for the clarifiers. The secondary

clarifiers loading is now 1,600 gal (US)/ft2.day – double the maximum

recommended maximum of 800 gal (US)/ft2.day.

Figure 142

• Although it is possible to attempt to bring the facility into compliance under

these strained conditions, it is advisable to ensure that the facility is

operated under recommended design and operational guidelines. For this

case, the facility will either need to modify its plans so that only a maximum

of two clarifiers are taken offline, or some of the flow to the facility can be

diverted elsewhere.

DISCUSSION QUESTIONS • What strategies could have been used to bring the plant into

compliance when clarifiers have been taken offline? • Has your facility ever taken clarifiers offline? What strategies

were used during the period when the clarifier was offline?

146

Chapter 7: Tapered Aeration

• There are generally two major types of aeration basins – 1) circular or

completely mixed tanks, where all of the basin is uniformly mixed and is at the

same conditions and 2) rectangular or plug flow tanks

• In circular tanks, airflow tends to be uniform across the entire tank. Influent is

distributed uniformly across the basin and the concentrations of contaminants

tends to be uniform across the entire tank.

• In a rectangular wastewater treatment facility, the influent is usually charged

to the front end of the basin, and then flows towards the back end of the

aeration tank

• Generally, the highest organic loading is present at the head of the plant. By

the time the wastewater reaches the back end of the aeration tank, it is now

completely treated, and the soluble COD levels are at the effluent target limit.

Learning Objectives

• Understand the effect of aeration taper on wastewater treatment

• Case study based evaluation of the effect of various aeration taper strategies on treatment effectiveness

• Establish the key variables impacted by aeration strategies in a wastewater process

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• Because of the fact that loading is highest at the front end and lowest in the

back end of a plug flow tank, the aeration demand varies.

• When diffusers and / or aeration flow is set up in a way that allows different

flows to be sent to different zones of the aeration tank, the aeration system is

referred to as being tapered.

• In the treatment plant used in the simulation, the aeration basin is rectangular,

with four (4) zones. There is therefore a need for the aeration to be tapered.

• We will now explore the impact of tapering on aeration demand and process

optimization. To do this, we will have to Restore the plant to its optimized

state

• To restore the plant to the two clarifier case, do the following:

o Click the RESTORE button at the bottom of the simulator

o Verify that all the operational summaries are reading zeros (0).

o Click on the Aeration Tanks object. Select Aeration Settings. Set the DO

controller to Off.

o Next, Change air flowrate from the default value to 1800 ft3/min

o Click on Methanol dosing. Next set methanol dosing to 200 gpd

o Click on the Preliminary Ferric dosing point and set Ferric dosage to 200

Ib Me/day

o Click on the Secondary Ferric dosing point and set Ferric dosage to 200

Ib Me/day

o Click on the WAS Control icon and set WAS rate to 39,625 gpd

o Click on the RAS Pump, and set RAS to 1.5 MGD

o RUN the simulation.

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• We should now have the values reported in Figure 143, in which all the

effluent targets are met. Effluent COD is 35.3 mg/l, TN is 7.8 m/l and TP is 1.66

mg/l.

Figure 143

• Note that the aeration rate and Taper value in this case is given as:

Figure 144

149

• The tapered aeration fractions to be different zones can be seen just below the

Total Air Flow to ATs (Aeration Tanks) in the Aeration Settings Tab, where DO

and Aeration settings are made.

• In order to see the profiles of variables like soluble COD, NH3 and NO3, right

click on the aeration tank, select Output and Profiles.

• We will now explore the impact of the fractional airflow distributions to the

performance of the plant using four (4) case examples, with an air flowrate of

1800 ft3/minute (see Table 27).

Case Zone 1 Zone 2 Zone 3 Zone 4 Case 1 0.50 0.25 0.15 0.10 Case 2 0.25 0.25 0.25 0.25 Case 3 0.10 0.15 0.25 0.50

Table 27

Case 1: 50% (Zone 1), 25% (Zone 2), 15% (Zone 3), 10%

(Zone 4)

• This is the default aeration taper. In order to determine its effect, we will

explore the Aeration Tanks Output.

• Click on Aeration Tanks, Select Outputs and then select the Profiles Tab.

• This tab has several sections, and this is where we can find more details on

the values of different variables in the various zones of the aeration tanks.

• We are specifically interested in the top two sections – Organic Variables

and Dissolved Oxygen.

150

Figure 145

• The first profile we will review is the soluble COD. Notice how the soluble COD

reduces as we move from zone 1 through to zone 4 (Figure 145)

• Also notice the impact of the taper profile on the DO levels in the various

zones?

151

• Next we will review the impact of the taper profile on Nitrogen species. Scroll

down to the Nitrogen Variables section (see Figure 146).

Figure 146

• Review the NH3 profile. Notice how the NH3 value decreases across the tank.

Also notice how the Nitrite and Nitrate values change across the zones

Case 2: 25% (Zone 1), 25% (Zone 2), 25% (Zone 3), 25%

(Zone 4)

• This case involves applying a proportional flow of aeration to the tank

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Figure 147

• At a proportional air flow taper, an air flowrate of 1800 ft3/min is adequate to

meet all the effluent targets at the facility

Figure 148

153

• However, an evaluation of the DO profiles shows that the DO levels are now

much lower in the first three zones and higher in the final zone (Figure 148)

• The COD profile also shows that the soluble COD is much higher in zone 1 in

this aeration taper case scenario than the default, largely due to the lower DO

level

• The DO reaches as high as 4.4 mg/l in zone 4

Case 3: 10% (Zone 1), 15% (Zone 2), 25% (Zone 3), 50%

(Zone 4)

• In this case, we are using a reverse taper to case 1, of 0.1, 0.15, 0.25 and 0.5

• The initial zones have a lower proportion of air flowrate compared to the later

zones

Figure 149

• Change the fractional air flow to zones 1 through 3. The airflow to zone four is then calculated. Once the values have been changed, RUN the simulation (Figure 149)

• The results indicate that the COD and TP targets are met, but the TN target is not met.

154

• To evaluate the profiles of the variables in the aeration tank, Right Click on the aeration tank and select Outputs. Next, select Profiles.

Figure 150

• Notice that the soluble COD values are now much higher (Figure 150)

• Table 28 summarizes soluble COD values across the zones for various

tapered aeration cases

Zone 1 Zone 2 Zone 3 Zone 4 Case 1 (0.5, 0.25, 0.15, 0.1)

49.5 20.5 19.1 18.8

Case 2 (0.25, 0.25, 0.25, 0.25)

100 43.3 19.9 18.9

Case 3 (0.1, 0.15, 0.25, 0.50)

143 111 48.2 19.8

Table 28

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Chapter 8: Evaluate Impact of

Temperature

• Temperature is a critical variable that impacts many wastewater treatment

facilities. Although we touched briefly on temperature earlier, during our

discussion of seasonality, a more detailed overview of the impact of

temperature on the activated sludge process will be reviewed in this chapter.

• Bacterial growth is dependent on metabolic reactions which tend to be very

sensitive to temperature. Generally, as temperatures increase so too does the

rate (or speed) of biological reactions, until an optimum value is reached.

Beyond that optimum value, temperature starts to have a disadvantageous

impact (Figure 153).

Learning Objectives

• Review the impact of Temperature om biological processes

• Case study based exploration of how Temperature impacts treatment efficiency in biological processes

• Generate simulation based relations for the effect of temperature on the removal of COD, TN and TP in the activated sludge process

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• Majority of bacteria in a conventional wastewater plant belong to a class of

microorganisms called mesophilic bacteria. Such bacteria thrive best within a

temperature range of 20-35oC (68-95oF).

• Temperature effects on bacteria:

o Low temperature: This occurs usually during the winter season. Influent

wastewater temperatures tend to be low. The activity of bacteria and

other microorganisms are affected. Generally, biological activity slows

down with temperature. The result is that temperature sensitive

operations such as nitrification can slow down significantly in low

temperatures.

o High temperatures: This tends to occur during the summer months. In

some cases, wastewater from industrial facilities can have hgh discharge

temperatures as well. Generally, the typical bacteria found in

wastewater plants (mesophilic) are affected only when temperatures

begin to exceed about 37-39oC.

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Figure 153: Effect of temperature on biological activity9

• We will now explore the impact of temperature on the performance of our

virtual plant.

• Restore the plant to its default condition. When the facility is operated

from its default conditions, the temperature value is 64.4oF on the influent.

You can find this value by clicking on the influent object and reviewing the

data input panel in the middle of the simulator (Figure 154).

Figure 154

9 http://www.ebsbiowizard.com/2010/12/eight-growth-pressures-of-biological-treatment-in-aerated-stabilization-basins-part-7/

158

• Now run the simulator (see Figure 155). BOD removal achieved is about

98.9%, while TN removal is 31.2% and TP removal is 33.6%, as is observed

in the removal summary found on the simulator (Figure 155)

Figure 155: Treatment performance at a temperature of 64.4 oF

• We will now use the simulator to evaluate the impact of using different

temperature levels on the effectiveness of treatment, while leaving all

other variables unchanged. We will evaluate temperatures from 40oF to

90oF.

• As with the default case, you can track the performance of the plant by

reviewing the performance summary table.

• To make changes to the Temperature, click on Influent and make changes

to the temperature variable in the middle panel.

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• Once the temperature is set to whatever value you want, click RUN and

make sure to record the values in the treatment summary.

• Now we will use a temperature set to 40oF as an example. Click on the

Influent.

• Go to the middle section of the simulator, and modify the temperature

value from the default of 64.4 to 40oF.

• Next, click the RUN button and record the results as summarized in the

performance summary table. Record the values for BOD, TN, NH3 and TP

removal (Figure 156)

Figure 156: Results for 40oF

• Now repeat the simulations at temperatures of 50, 70, 80 and 90oF and

record the BOD and nutrient removal values.

160

Figure 157: Results for 50oF

Figure 158: Results for 70oF

161

Figure 159: Results for 80oF

Figure 160: Results for 90oF

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• When you are done, collate the information from Figures 155 to 160. The

treatment performance results obtained from the various simulation runs

should correspond to the table below:

Temperature (oF)

BOD Rem TN rem (%) NH3 rem (%) TP rem (%)

40 57.1 21 6.7 26.7 50 96.5 31 15.6 33.6

64.4 98.9 31.2 14.8 33.6 70 99.1 31 14.3 33.4 80 99.3 66.9 99.0 32.5 90 99.5 71 99.7 31.8

Table 29

• As can be seen from the table, generally, the percentage removal value for

all the contaminants tends to increase with an increase in temperature

Figure 161

• The graphical representation of the results of the simulation exercise

(Figure 161), shows clearly that generally, performance tends to increase

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with temperature for COD, NH3 and TN removal in wastewater treatment

process.

• You can see clearly that it might be more challenging to meet treatment

requirements in colder months than in warmer months.

• As a result it is common to have seasonal limits specified in wastewater

permits.

• Which seasons do you think will have the most stringent limits? If you

thought that would be the warmer months of the year – then you are right!

• The pasted image below is an example of seasonal limits specified in the

NPDES permit for a wastewater utility, with stricter (lower) BOD and TSS

limits specified in the summer vs the winter months (Table 30) and another

showing lower limits for Ammonia in summer vs Winter (Table 31).

Table 30: Example of NPDES permit with seasonal limits10

10 http://www.deq.state.or.us/wqpr/2072_200912160000bCS05.PDF

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Table 31: Example of seasonal permits showing lower Ammonia Nitrogen limits in winter11

11 http://www.klrsd.org/webper/AppendixA.pdf

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Chapter 9: Effect of Inflow & Infiltration

(I&I)

• Inflow and infiltration (I&I) is a common occurrence in many wastewater treatment systems. It refers to the combination of clear waters with water from a sewer system. These clear waters generally come from storm water or surface water flows.

• What do you think the effect of I&I is on wastewater flows and loads? • I&I tends to be water with very low organic loading. The COD, N and P

values are usually much lower than that of the typical sewer flow to the wastewater plant

Learning Objectives

• Review the sources and impact of I&I on wastewater operations

• Explore the operational changes required to bring a plant into compliance when facing I& I events

• Case study driven evaluation of I&I impact on key operational variables in a wastewater plant

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• The resulting effect of I&I is therefore an increase in the flow and possibly in the organic loadings as well.

Figure 162: Sources of contaminants in storm water12

• I& I can cause a wastewater treatment process to be overloaded.

• As can be seen from the image above, there are several factors that can

potentially lead to a high contaminant loading in surface water ranging

from roadway pollutants like de-icing fluids, to wash off of fertilizer and

debris from soils. We will now consider a case study to illustrate the

challenge.

• Do you think I&I also leads to an increase in the average concentration of

COD, N and P to a wastewater facility?

12 http://www.dot.ca.gov/hq/env/stormwater/pdf/CTSW-RT-03-059.pdf

167

Table 32: Examples of nutrient concentration in various urban land covers13.

• As can be seen in Table 32, urban water runoff can have nutrient loadings

as high as 20 mg/l TN and 4.3 mg/l TP. Because the concentrations of

contaminants might be lower in the clear waters (stormwater) than the

sewer water, the concentration of the combined flow might be lower (on a

mg/l basis).

• The following examples will help us work through the implications of I&I on

treatment effectiveness. We will consider 4 cases where different levels of

storm water inflow into the sewer system. The flows will range from 1 to 4

MGD.

13 https://www3.epa.gov/npdes/pubs/usw_b.pdf

168

• Based on the values in Table 32, we assume that the stormwater has

composition of 200 mg/l COD, 5 mg/l TKN, and 1 mg/l soluble Phosphorus.

These concentrations fall within the ranges stated in Table 32.

Table 33: I&I cases and the resulting influent composition.

• The following table is made up of the combined flow based on the sewer

and stormwater inflows.

• We will examine the impact of these cases on the overall performance of

the facility.

• We will restore the facility to optimum operations and set an effluent

target of COD < 40 mg/l, TN – 10 mg/l and TP – 3 mg/L

• Restore the virtual plant to its default positions. To do this click on Restore

and confirm that all the operational variables in the two summary tables in

the layout are all at a value of zero.

• Now restore the plant to optimum

• To obtain the starting optimum operating conditions

o Set DO Controller Off and Use an airflow of 1800 ft3/min

Flow (MGD) COD (mg/l) TKN (mg/l) NH3 (mg/l) SP (mg/l)Sewage 2.641 416 42 32 13I&I (Case 1) 1 200 5 3.81 1I&I (Case 2) 2 200 5 3.81 1I&I (Case 3) 3 200 5 3.81 1I&I (Case 4) 4 200 5 3.81 1

Combined Flow MGD COD (mg/l) TKN (mg/l) NH3 (mg/l) SP (mg/l)Case 1 3.641 356.68 31.84 24.26 9.70 Case 2 4.641 322.92 26.06 19.85 7.83 Case 3 5.641 301.13 22.32 17.01 6.62 Case 4 6.641 285.90 19.71 15.02 5.77

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o Set WAS to 39,625 gpd

o Set Ferric dosage in Preliminary Ferric system to 200 Ib Me/day

o Set Ferric dosage in Secondary Ferric system to 200 Ib Me/day

o Set RAS to 1.5 MGD

o Set Methanol dosage to 200 gpd

o RUN the model

• Based on these conditions, the following performance features and

operational characteristics will be obtained:

Table 34

• The corresponding effluent conditions ae COD 35.3 mg/l, TN – 7.8 mg/L and TP – 1.66 mg/L.

Case 1: 1 MGD I&I

• In this case, an I&I flow of 1 MGD combines with the regular sewage flow of

2.64 MGD. As previously stated, the I&I has concentration of 200 mg/l COD,

5 mg/l TKN, and 1 mg/l soluble Phosphorus

• The effective influent concentration is given in Table 35.

Table 35: 1 MGD I&I

Combined Flow MGD COD (mg/l) TKN (mg/l) NH3 (mg/l) SP (mg/l)Case 1 3.641 356.68 31.84 24.26 9.70

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• Input these values into the influent. To do so, Click on the Influent and

modify the variables to reflect these conditions

Figure 163

• Once these conditions have been input, click RUN to activate the

simulation. What do you observe? Can we still maintain effective treatment

under these conditions?

• The answer is No (Figure 164). We meet the COD and TN effluent

conditions, however, TP is now slightly higher than 3 mg/l

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Figure 164

• A simple solution might be to just dose a little more Ferric in the influent to remove more phosphorus.

• Increase the Ferric dosage in the preliminary system from 200 to 250 mg/L and click RUN. Now we meet all the effluent targets, with TP now being 2.33 mg/L.

Figure 165

• The optimum conditions under Case 1 I&I are: o Set aeration to 1800 ft3/min o Set WAS to 39,625 gpd o Set Ferric dosage in Preliminary Ferric system to 250 Ib Me/day o Set Ferric dosage in Secondary Ferric system to 200 Ib Me/day o Set RAS to 1.5 MGD o Set Methanol to 200 gpd

Case 2: 2 MGD I&I

• In this case, an I&I flow of 2 MGD combines with the regular sewage flow of

2.64 MGD. As previously stated, the I&I has concentration of 200 mg/l COD,

5 mg/l TKN, and 1 mg/l soluble Phosphorus

• The effective influent concentration is given in Table 36

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Table 36

• Maintain all the other operational conditions set in Case 1. Change the influent concentration to reflect the values in Table 36 by Clicking on the Influent and modify the variables accordingly. Once these conditions have been input, click RUN.

• What do you observe? Can we still maintain effective treatment under

these conditions? Remember we are starting from the optimum conditions

from Case 1.

• The answer is No (see Figure 166). We meet the COD and TN effluent

conditions, however, TP is now higher than 3 mg/l, at a value of 3.97 mg/l.

Figure 166

• Again, a simple solution might be to just dose a little more Ferric in the

preliminary treatment system to remove more phosphorus.

• Increase the Ferric dosage in the preliminary system from 250 to 350 mg/L

and click RUN. Now we meet all the effluent targets, with TP now being

2.33 mg/L.

Combined Flow MGD COD (mg/l) TKN (mg/l) NH3 (mg/l) SP (mg/l)Case 2 4.641 322.92 26.06 19.85 7.83

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Figure 167

• The optimum conditions under Case 2 I&I are: o Set aeration to 1800 ft3/min o Set WAS to 39,625 gpd o Set Ferric dosage in Preliminary Ferric system to 350 Ib Me/day o Set Ferric dosage in Secondary Ferric system to 200 Ib Me/day o Set RAS to 1.5 MGD o Set Methanol to 200 gpd

Case 3: 3 MGD I&I

• In this case, an I&I flow of 3 MGD combines with the regular sewage flow of

2.64 MGD. As previously stated, the I&I has concentration of 200 mg/l COD,

5 mg/l TKN, and 1 mg/l soluble Phosphorus

• The effective influent concentration is given in Table 37

Table 37

• Maintain all the other operational conditions set in Case 2. Change the

influent concentration to reflect the values in Table 37 by Clicking on the

Combined Flow MGD COD (mg/l) TKN (mg/l) NH3 (mg/l) SP (mg/l)Case 3 5.641 301.13 22.32 17.01 6.62

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Influent and modify the variables accordingly. Once these conditions have

been input, click RUN.

• What do you observe? Can we still maintain effective treatment under

these conditions? Remember we are starting from the optimum conditions

from Case 2.

Figure 168

• The answer is No (Figure 168). We cannot meet the effluent COD and TP

targets. COD is now 42.4 mg/L, while TP is 4.31 mg/L.

• Note also that the secondary clarifier loading is now slightly higher than the

maximum recommended value of about 800 gal (US)/(ft2.day).

175

Figure 168

• We will now explore what options we might have for meeting the effluent

targets.

o Reduce RAS: The RAS rate impacts the solids loading to the clarifier.

Reduce RAS to 1 MGD from the current value of 1.5 MGD

o Ferric dosage: the higher P loading obtained due to the 3 MGD of I&I

and the 1 mg/l of soluble P that it contains effectively leads to an

additional value of about 25 Ibs/day of Phosphorus. We will add

some additional Ferric to the preliminary treatment system to try to

precipitate out some of the influent P. Ferric dosage is raised from

the current value of 350 Ib Me/day to 500 Ib Me/day

• Input these new optimum conditions for Case 3

o Set Aeration rate to 1,800 ft3/min

o Set WAS to 30,000 gpd

o Set Ferric dosage in Preliminary Ferric system to 500 Ib Me/day

o Set Ferric dosage in Secondary Ferric system to 200 Ib Me/day

o Set RAS to 1.0 MGD

• RUN the simulation. What do you observe (Figure 169)?

176

Figure 169

• The results indicate that by optimizing ferric dosage, sludge wastage and RAS rates, it is possible to meet the added challenge that is posed by I&I of 3 MGD.

• We must however note that the secondary clarifier loading is at its limits.

Case 4 I&I

• In this case, an I&I flow of 4 MGD combines with the regular sewage flow of 2.64 MGD. As previously stated, the I&I has concentration of 200 mg/l COD, 5 mg/l TKN, and 1 mg/l soluble Phosphorus

• The effective influent concentration is given in Table 38

Table 38

• Input these values in the Influent data input pane. To do this, click on the Influent and modify the variables to reflect the conditions in Table 38.

• Maintain all the optimal conditions established in Case 3. To recap, these are:

o Set Aeration rate to 1,800 ft3/min

o Set WAS to 30,000 gpd

Combined Flow MGD COD (mg/l) TKN (mg/l) NH3 (mg/l) SP (mg/l)Case 4 6.641 285.90 19.71 15.02 5.77

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o Set Ferric dosage in Preliminary Ferric system to 500 Ib Me/day

o Set Ferric dosage in Secondary Ferric system to 200 Ib Me/day

o Set RAS to 1.0 MGD

• RUN the simulation and observe the results.

Figure 170

• What do you observe? Can we still maintain effective treatment under these conditions? Remember we are starting from the optimum conditions from case 3.

• The answer is No (Figure 170). We cannot meet any of the effluent targets. Effluent COD, TN and TP are now 43.1 mg/L, 11.6 mg/L and 3.98 mg/L respectively.

Figure 171

178

• Reviewing the operational variables (Figure 171), we see that the secondary

clarifier loading is now well beyond its recommended range.

• We will now explore what options we might have for meeting the effluent

targets.

• Because the process fails to meet effluent targets for all three variables –

COD, TN and TP – the steps needed to be taken to bring the plant back in

compliance will require multiple levels of operational changes.

o Improve aeration: Set the air flowrate to 2,000 ft3/min

o Decrease RAS: The RAS rate can impact the solids loading to the

clarifier. Reduce RAS flow from 1.0 MGD to 0.75 MGD.

o Decrease WAS: Reduce the WAS from 30,000 to 25,000 gpd.

o Ferric dosage: The higher P loading obtained due to the 4 MGD of I&I

and the 1 mg/l of soluble P that it contains effectively leads to an

additional value of about 33.4 Ibs/day of Phosphorus. We will add

some additional Ferric to the preliminary treatment system to try to

precipitate out some of the influent P. Preliminary Ferric dosage is

raised from the current value of 500 Ib Me/day to 650 Ib Me/day

• After making all these changes, RUN the simulation. What do you observe?

• The plant now meets its effluent targets. It can reliably meet its COD and TP

targets, but just barely meets the TN targets.

• New Optimum for Case 4

o Set air flowrate to 2,000 ft3/min.

o Set WAS to 25,000 gpd

o Set Ferric dosage in Preliminary Ferric system to 650 Ib Me/day

o Set Ferric dosage in Secondary Ferric system to 200 Ib Me/day

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o Set RAS to 0.75 MGD.

o Set Methanol to 200 gpd

Figure 172

• The results indicate that by optimizing aeration, ferric dosage, sludge

wastage, and RAS, it is possible to meet the COD, TN and TP limits at the

facility.

• However, the facility just barely meets its TN targets. The results also

indicate that the aeration rate required to meet the targets under this high

I&I condition is much higher. More chemicals are also needed. The end

result is that higher energy and chemical costs are also incurred. The solids

generation also increases to 4.29 tons/day.

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Questions Chapter 1 Questions (Overview of the wastewater

treatment process)

1. How long ago was the activated sludge process invented? a. 0ver 100 years old b. 10 years old c. 25 years old d. 50 years old

2. Who developed the process? a. Ardern & Lockett b. Smith & Loveless c. Metcalf & Eddy d. Johnson & Johnson

3. What are the major microorganisms used in the activated sludge process? a. Viruses b. Bacteria c. Algae

4. Which of these is not part of the conventional activated sludge process a. Return sludge b. Waste sludge c. Pasteurization

5. Which of these issues can result if wastewater is not well treated prior to discharge?

a. High bacteria growth in rivers, lakes and streams

b. High quality source water for drinking water plants

6. What is the maximum allowable headworks loading when there is only one aeration tank in service

a. 10 MGD b. 8 MGD c. 2.64 MGD

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d. 2.3 MGD 7. Can the facility save on chemical cost by reducing the usage of Ferric when the

flow is 2.3 MGD? a. Yes b. No

8. How low can the Ferric dosage go at a flow of 2.3 MGD to still meet the effluent requirement, if only the dosage in the preliminary treatment system is changed

a. 200 Ib Me/day (39.1, 8.8, 1.38) b. 100 Ib Me/day (40.3, 9.0. 2.12) c. 150 Ib Me/day (39.7, 9.0, 1.59)

9. How low can the Ferric dosage go at a flow of 2.3 MGD to still meet the effluent requirement, if only the dosage in the secondary treatment system is changed (HINT – first return the preliminary treatment dosage to 200 Ib Me/day)

a. 200 Ib Me/day (39.1, 8.8, 1.38) b. 100 Ib Me/day (38.8, 8.6, 1.99) c. 150 Ib Me/day (39.0, 8.7, 1.46) d. 60 Ib Me/day (38.6, 8.6, 2.97)

Chapter 2 Questions (Use of simulators in wastewater

treatment)

1. What is a simulator?

a. A computer program that provides a realistic replication of the operations of a system stems

b. A tool used to stimulate bacteria in wastewater 2. What are some examples of fields where simulators are used?

a. Aviation

b. Wastewater treatment

c. Water treatment

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d. All of the Above

3. What process are found in wastewater simulators?

a. Primary treatment b. Secondary treatment c. Biological treatment d. Chemical treatment e. All of the above

4. Describe what processes are represented in the following model layouts

Plant A:

Plant B:

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Plant C:

Chapter 3 Questions (Introduction to OpTool Pro) 1. How many modes are available in OpToolPro (Select all that apply)?

a. Training

Case 1: This layout is a model of a conventional activated sludge process with four tanks in series. The model also incorporates sludge thickening and anaerobic digestion. Carbon and nitrogen removal are modelled.

Case 2: This layout demonstrates COD removal and nitrification. The layout is set up with one plug flow aerobic tank and one settling tank. The mantis2 model was used for biological reactions, and the Simple1d model for the settling tank.

Case 3: This is a conventional nitrifying activated sludge process. The layout is set up with one plug flow aerobic tank and one settling tank.

Case 4: This is a conventional plant with no primary clarification system. It has a disinfection system after the secondary clarifier

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b. Testing c. Trialing

2. Open up a session of OpTool, and select Training mode and US units. You will see the influent conditions in the middle panel. Select all the influent conditions that are correct.

a. Influent Flow – 2.641 MGD b. Influent Flow – 5 MGD c. Temperature – 70 F d. Temperature – 64.4 F e. COD - 416 mg/l f. COD – 500 mg/l g. TKN – 42 mg/l h. TKN – 30 mg/l i. Soluble Ortho-P – 13 mg/l j. Soluble Ortho-P – 10 mg/l

3. Click on the Aeration Tanks (shown in the image below). The middle panel will now show three tabs. What are their names (select all that apply)?

a. Aeration Settings b. Aeration Tank Status c. Internal Recycle d. Methanol dosing

4. The tab Aeration Settings has a section labelled DO Controller. Is the DO controller Off or On?

185

a. On b. Of

5. The Aeration Tank has four (4) zones). How many DO setpoints are there? a. 1 b. 2 c. 4 d. 8

6. What are the default DO settings used in the simulator? a. 0.5, 0.5, 0.5, 2 mg/l b. 0.5, 0.5, 1, 2 mg/l c. 0.5, 0.5, 0.5, 0.5 mg/l d. 2, 2, 2, 2 mg/l

RUN the simulation using the start button (see image below). The next few questions will be about the results

7. What is the MLSS (see the operational summary table in the top left corner of

the simulator)? a. 2000 mg/l b. 3000 mg/l c. 758 mg/l d. 900 mg/l

8. What is the amount of sludge generated? a. 1 ton/day b. 2 ton/day c. 2.18 ton/day d. 3.11 ton/day

9. What is the total airflow to the system

Start Button Progress Bar Information on WWTP

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a. 10 ft3/s b. 2 ft3/s c. 11.1 ft3/s d. 15.2 ft3/s

10. What is the F/M ratio a. 0.81 Ib BOD/Ib MLSS/day b. 1.81 Ib BOD/Ib MLSS/day c. 2.81 Ib BOD/Ib MLSS/day d. 3.81 Ib BOD/Ib MLSS/day

11. What is the % BOD removed? a. 95% b. 97.1% c. 98.9% d. 100%

12. What is the primary clarifier loading? a. 100 gal (US)/ft2.day b. 200 gal (US)/ft2.day c. 300 gal (US)/ft2.day d. 169 gal (US)/ft2.day e. 251 gal (US)/ft2.day

13. What is the secondary clarifier loading a. 200 gal (US)/ft2.day b. 300 gal (US)/ft2.day c. 365 gal (US)/ft2.day d. 381 gal (US)/ft2.day e. 451 gal (US)/ft2.day

Click on the Primary Clarifiers (see image). The middle panel will now contain some information about the Primary Clarifier. Please confirm which information is consistent with what you can observe

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14. What is the primary clarifier sludge flow a. 19813 gpm b. 19813 gpd c. 19813 MGD

15. What is the primary clarifier solids capture rate? a. 10% b. 20% c. 40% d. 50%

Chapter 4 Questions (Using OpTool Pro to optimize facility

operations)

Restore the plant to its default conditions by clicking the Restore button at bottom of the simulator.

Your task is to make the following operational changes

Starting from the default conditions, RUN the simulation. Now make the following adjustments:

1. Set WAS Flow Rate to 100,000 gpd and RUN the simulation. Based on the results you obtained, which of the following statements is true

a. MLSS is greater than 1,000 mg/L b. The F/M is 0.58 Ib BOD/Ib MLSS/day c. Less than 10% of Total Phosphorus (TP) is removed

Start Button Progress Bar Information on WWTP

Restore Button

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d. Effluent BOD is less than 3 mg/l 2. Set WAS flow to 50,000 gpd and RUN the simulation. Based on the results you

obtained, which of the following statements is true (select all that apply) a. SRT is greater than 5 days b. MLSS is 1577 mg/L c. Sludge production is 1.89 tons/day d. F/M ratio is 0.5 Ib BOD/Ib MLSS/day

3. Set WAS flow to 40,000 gpd and RUN the simulation. Based on the results you obtained, which of the following statements is true (select all that apply)

a. SRT is 6.8 days b. Effluent TN is 15.4 mg/l c. Effluent Total Phosphorus is 10 mg/l d. Total Airflow is 25 ft3/s

4. Set RAS Flow Rate to 2 MGD and RUN the simulation. Based on the results you obtained, which of the following statements is true (select all that apply)

a. SRT is 6.7 days b. Effluent TN is 15.4 mg/l c. Effluent Total Phosphorus is 10 mg/l d. F/M ratio is 0.36 Ib BOD/Ib MLSS/day

5. Set RAS rate to 1.5 MGD and RUN the simulation. Based on the results you obtained, which of the following statements is true (select all that apply)

a. MLSS is less than 1600 mg/L b. % NH3 removed is < 95% c. % TP removed is > 20% d. % BOD removed is > 90%

6. Set Preliminary Ferric Dosage to 100 Ibs Me/day and RUN the simulation. Based on the results you obtained, which of the following statements is true (select all that apply)

a. Total Phosphorus in the effluent is < 5 mg/l b. Sludge production is 2.0 tons/day c. MLSS is greater than 1700 mg/l

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d. DO is 0.5 mg/l 7. Set Preliminary Ferric Dosage to 200 Ibs Me/day and RUN the simulation.

Based on the results you obtained, which of the following statements is true (select all that apply)

a. Total Phosphorus in the effluent is 5.94 mg/l b. Total Phosphorus in the effluent is 4.94 mg/l c. Total Phosphorus in the effluent is 3.94 mg/l d. Total Phosphorus in the effluent is 6.94 mg/l e. None of the above

8. Set Secondary Ferric Dosage to 100 Ibs Me/day and RUN the simulation. Based on the results you obtained, which of the following statements is true (select all that apply)

a. Total Phosphorus in the effluent is 5.71 mg/l b. Total Phosphorus in the effluent is 4.71 mg/l c. Total Phosphorus in the effluent is 3.71 mg/l d. Total Phosphorus in the effluent is 2.71 mg/l e. None of the above

9. Set Secondary Ferric Dosage to 150 Ibs Me/day and RUN the simulation. Based on the results you obtained, which of the following statements is true (select all that apply)

a. Total Phosphorus in the effluent is 5.62 mg/l b. Total Phosphorus in the effluent is 4.62 mg/l c. Total Phosphorus in the effluent is 3.71 mg/l d. Total Phosphorus in the effluent is 2.71 mg/l e. None of the above

10. Set Secondary Ferric Dosage to 200 Ibs Me/day and RUN the simulation. Based on the results you obtained, which of the following statements is true (select all that apply)

a. Soluble Phosphorus in the effluent is 0.16 mg/l b. Soluble Phosphorus in the effluent is 0.56 mg/l c. Soluble Phosphorus in the effluent is 1.71 mg/l

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d. Soluble Phosphorus in the effluent is 0.90 mg/l e. None of the above

11. Set Methanol dosage to 100 gpd and RUN the simulation. Based on the results you obtained, which of the following statements is true (select all that apply)

a. Effluent COD is 32.1 mg/l b. Effluent TN is 8.9 mg/l c. Effluent TP is 1.66 mg/l d. Energy cost is $66,900 /year

12. Set Methanol dosage to 200 gpd and RUN the simulation. Based on the results you obtained, which of the following statements is true (select all that apply)

a. Effluent COD is 40 mg/l b. Effluent TN is > 6 mg/l c. Effluent TP is > 2 mg/l d. Energy cost is > $70,000 /year

Chapter 5 Questions (Exploring Seasonal Effects)

Restore the plant to its default conditions by clicking the Restore button at bottom of the simulator.

Next set the following operational variables:

a) WAS = 39,000 gpd; b) RAS = 1.5 MGD c) Aeration = 1850 ft3/min d) Preliminary Ferric Dosage = 200 Ib Me/day d) Secondary Ferric dosage = 200 Ib Me/day e) Methanol dosage = 200 gpd

After inputting these values, RUN the simulation and confirm that you have the following effluent values:

Start Button Progress Bar Information on WWTP

Restore Button

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Now answer the following questions by using the simulator to evaluate your responses

1. During the summer, the wastewater facility has the same flow, but temperatures increased from the default value of 64.4oF to 75oF. Which of these statements is true

a. Effluent TN at 75oF > Effluent TN at 64.4oF b. MLSS at 75oF = 2603 mg/l c. SRT at 64.4oF is 5.6 days d. F/M at 64.4oF is 0.30 Ibs BOD/Ib MLSS/day e. Total sludge production at 75oF > Total Sludge production at 64.4oF

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Technical Support If you require technical assistance for OpToolPro™, you can email Hydromantis at support@hydromantis.com or call our office at (905) 522-0012 during Eastern Time (ET) weekday business hours.

References Henze, M., Grady, C. P. L., Gujer, W., Marais, G. v. R. and Matsuo, T. (1987). Activated Sludge Model No. 1. IAWPRC Scientific and Technical Report No. 1, IAWPRC, London.

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OpToolPro™ Testing Mode Challenges Solutions to the Testing Mode challenges are shown in the following pages. Note

that for each challenge, the listed solution is one of many possible solutions to the

challenge that would meet all the targets.

Challenge 1: High Effluent COD (50 Points)

Parameter Target

Effluent COD < 20.0 mg/L

Dissolved Oxygen (DO) In Reactor 1.0 - 3.5 mg/L

The WWTP starts with a very low MLSS, but the SRT is reasonable at 5.4 days.

There are 3 of the 4 secondary clarifiers off-line. The influent flow is 2.2 MGD

(8,300 m3/d), and the temperature is an average 72°F (22°C).

The targets can be met by making the following changes:

1) Put the 2nd aeration tank and the 3 off-line secondary clarifiers all in service.

2) Turn on the DO controller in the aeration tank, and set all the setpoints to

2.0 mg/L

Challenge 2: High Effluent BOD5 and NH4 (75 Points)

Parameter Target

Effluent BOD5 < 5.0 mg/L

Effluent NH4 < 1.0 mg/L

MLSS > 2000 mg/L

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In this case, the WWTP starts with a very low MLSS, but the SRT is a bit low at 4 days. There are 3 of the 4 secondary clarifiers off-line. The influent loading is 4 MGD (15,100 m3/d).

The targets can be met by making the following changes:

1) Put the 2nd aeration tank and the 3 off-line secondary clarifiers all in service 2) Turn on the DO controller in the aeration tank, and set all the setpoints to

2.0 mg/L 3) Set the WAS pump rate to 25000 gpd (95 m3/d)

Challenge 3: High TKN with Energy Limit (100 Points)

Parameter Target

DO in Bioreactor 1.0 - 2.5 mg/L

MLSS > 2000 mg/L

Total Energy Usage 130000 $/yr

Effluent TKN < 5.0 mg/L

The influent flow has a higher flow rate and higher COD concentration than Challenges 1 and 2. The WWTP plant starts off with a very low MLSS and SRT, but the DO concentration is very high at 7.7 mg/L. Note that the energy consumption is also exceeding the target of $130,000/yr.

The targets can be met by making the following changes:

1) Put the 3 off-line secondary clarifiers all in service (leave the 2nd aeration basin off-line)

2) Set the airflow rate to 3000 ft3/min (122,000 m3/d) 3) Set the WAS pump rate to 50,000 gpd (190 m3/d)

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Challenge 4: High Effluent TN in Cold Weather (50 Points)

Parameter Target

Effluent BOD5 < 5.0 mg/L

Effluent Total Nitrogen (TN) < 6.0 mg/L

The influent flow and loading is similar to Challenge 4, but the liquid temperature

at the WWTP is now 53°F (12°C). Note that one of the targets is now total

nitrogen (TN) requiring a setup for denitrification.

The targets can be met by making the following changes:

1) Put the 2nd aeration tank and the 3 off-line secondary clarifiers all in service.

2) Set the WAS pump rate to 30,000 gpd (115 m3/d)

3) Turn on the DO controller in the aeration tank, and set the setpoint in Pass

1 to 0.0 mg/L (anoxic zone), and 2.0 mg/L in Passes 2, 3, and 4 (aerobic

zones).

4) Set the internal recycle rate (from Pass 4 to Pass 1) to 4 MGD (15,100 m3/d)

Challenge 5: High Effluent TP (75 Points)

Parameter Target

Effluent TSS < 8.0 mg/L

Effluent Total Phosphorus (TP) < 1.0 mg/L

Chemical Dosage Cost < $65,000 /yr

The influent flow and loading is similar to Challenge 5, including the fact that the

liquid temperature at the WWTP is 53°F (12°C). The effluent targets now include

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Total Phosphorus (TP), which will require dosing ferric chloride for chemical

phosphorus removal. One of the targets, however, is chemical dosage cost, which

must be under $65,000/yr.

The targets can be met by making the following changes:

1) Put the 2nd aeration tank and the 3 off-line secondary clarifiers all in service

2) Turn off the methanol dosing, as it is contributing to chemical cost, and not

required for improving phosphorus removal

3) Set the primary ferric dosage to 150 lb Me/d (68 kg Me/d), and the

secondary ferric dosage to 250 lb Me/d (113 kg Me/d).

Challenge 6: High BOD5, NH4 and Cold Temperature (50

Points)

Parameter Target

Effluent BOD5 < 10 mg/L

Effluent NH4 < 1.0 mg/L

Similar to Challenge 5, the influent flow is 4 MGD (15,100 m3/d), with a slightly

lower COD concentration and temperature of 53°F (12°C). Note that the effluent

targets of BOD5 and NH4 are not being met when the simulation starts.

The targets can be met by making the following changes:

1) Put the 2nd aeration tank and the 3 off-line secondary clarifiers all in service

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2) Turn off the ferric dosing in both the primary and secondary dosage points,

as it is causing the soluble phosphorus to be so low as to cause nutrient

limitation

3) Turn on the DO controller in the aeration tank, and set all the setpoints to

2.0 mg/L

Challenge 7: High Total Nitrogen (TN) (75 Points)

Parameter Target

Total Nitrogen (TN) < 8.0 mg/L

Chemical Dosage Cost < 100,000 $/yr

The influent is 2.2 MGD (8,300 m3/d) at a temperature of 75°F (24°C) in this

challenge, with a relatively low influent COD of 300 mg/L, but a high influent

ammonia of 37 mg/L. The target is for Total Nitrogen removal, so denitrification

will be required. The WWTP plant starts off with a very low MLSS and SRT.

The targets can be met by making the following changes:

1) Put the 2nd aeration tank and the 3 off-line secondary clarifiers all in service

2) Turn on the DO controller in the aeration tank, and set the setpoint in Pass

1 to 0.0 mg/L (anoxic zone), and 2.0 mg/L in Passes 2, 3, and 4 (aerobic

zones)

3) Set the internal recycle rate (from Pass 4 to Pass 1) to 5 MGD (18,900 m3/d)

4) Set the methanol dosage rate to 300 gpd (1.13 m3/d)

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Challenge 8: Low SRT and MLSS (75 Points)

Parameter Target

Mixed Liquor Suspended Solids (MLSS) > 2500 mg/L

SRT 6 - 12 days

Effluent TSS < 10.0 mg/L

In this challenge, the influent flow is 5 MGD (18,900 m3/d). The concentrations

and temperature have been reset to their defaults. The WWTP starts out with a

reasonable MLSS of 2000 mg/L and SRT of 5.4 days.

The targets can be met by making the following changes:

1) Put the 2nd aeration tank and the 3 off-line secondary clarifiers all in service

2) Turn on the DO controller in the aeration tank, and set all the setpoints to

2.0 mg/L

3) Set the wastage rate to 40,000 gpd (150 m3/d)

Challenge 9: Energy and Chemical Cost Management (125

Points)

Parameter Target

Effluent TSS < 10.0 mg/L

Effluent TKN < 5.0 mg/L

Effluent Total Phosphorus (TP) < 1.0 mg/L

Energy Cost < $175,000

Chemical Dosage Cost < $65,000

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The influent flow is the same as Challenge 8, however there are now 5 target

conditions, including limits on both energy and chemical cost. The WWTP start

with low MLSS and SRT, and several units off-line. The DO in the aeration basin is

higher than normal at 7.5 mg/L.

The targets can be met by making the following changes:

1) Put the 2nd aeration tank and the 3 off-line secondary clarifiers all in service

2) Set the ferric dosage in both the primary and secondary dosage points to

200 lb Me/d (91 kg Me/d), to remove nutrient limitation on biological

growth, but still remove soluble phosphorus to meet the effluent target

3) Reduce the airflow to the aeration tank to 5000 ft3/min (203,900 m3/d), to

meet energy requirements but still achieve complete nitrification

Challenge 10: High-Strength Wastewater Treatment (50

Points)

Parameter Target

Effluent COD < 130.0 mg/L

Airflow to Aeration Tanks < 3500 ft3/min (2375 m3/hr)

In this question, the influent COD concentration is 750 mg/L, reflecting some high-

strength waste in the influent. The influent flow rate is 5 MGD (18,900 m3/d), as

in Challenge 8. All available reactors and clarifiers are already on-line. The

targets include both the effluent COD as well as the airflow rate in the aeration

tank.

The targets can be met by making the following changes:

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1) Turn off the ferric dosage in both the primary and secondary dosage points

to remove the nutrient limitation on biological growth

2) Set the airflow rate to 3500 ft3/min (142,700 m3/d) to meet the airflow

requirement

3) Set the wastage flow to 75000 gpd (284 m3/d) to bring the MLSS and

effluent solids down

Challenge 11: SRT Control (75 Points)

Parameter Target

BOD5 < 7.0 mg/L

SRT 6 - 8 days

Effluent Soluble Ortho-Phosphorus < 0.1 mg/L

The influent to this WWTP is 4 MGD (15,100 m3/d), with a COD concentration of

520 mg/L at 72°F (22°C). The plant starts with a very low MLSS. The goal is to

achieve a SRT of 6 to 8 days, while also meeting the BOD5 and soluble phosphorus

targets.

The targets can be met by making the following changes:

1) Put the 3 off-line secondary clarifiers in service

2) Turn on the DO controller and set all the setpoints to 2.0 mg/L

3) Increase the wastage rate to 75,000 gpd (284 m3/d) to bring down the SRT

4) Set the ferric dosage rates to 300 and 400 lb Me/d (136 kg Me/d and 181 kg

Me/d), respectively, for the primary and secondary dosage points

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Challenge 12: Energy Management (75 Points)

Parameter Target

BOD5 < 10.0 mg/L

TKN < 3.0 mg/L

Total Energy Cost < $190,000/yr

The influent loading is very high in this challenge question (flow = 6 MGD, COD =

520 mg/L), and the temperature is 53°F (12°C). The simulation starts with very

low MLSS and SRT, with a high DO of 4.7 mg/L in the aeration basin. The targets

require BOD5 removal and full nitrification, while keeping the total energy

expenditure below $190,000.

The targets can be met by making the following changes:

1) Put the 2nd aeration tank and the 3 off-line secondary clarifiers all in service

2) Reduce the airflow to the aeration tanks to 4450 ft3/min (181,500 m3/d) to

meet energy requirement

3) Reduce the RAS flow to 5 MGD (18,930 m3/d) to meet energy requirement

4) Set wastage flow to 180,000 gpd (681 m3/d) to bring MLSS and SRT to

normal levels, to meet nitrification and effluent solids requirement

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Challenge 13: Total Nitrogen Removal (50 Points)

Parameter Target

Total Nitrogen (TN) < 10.0 mg/L

Total Chemical Cost < $75,000/yr

In this question, the influent flow is 8 MGD, with an ammonia concentration of 40

mg/L. The Total Nitrogen effluent target of 10 mg/L requires full nitrification and

denitrification. The simulation starts with a low MLSS and SRT.

The targets can be met by making the following changes:

1) Put the 2nd aeration tank and the 3 off-line secondary clarifiers all in service

2) Increase the RAS flow to 8 MGD (30,280 m3/d)

3) Set the first zone DO setpoint to zero to create an anoxic zone

4) Set the internal recycle from zone 4 to zone 1 to 25 MGD (94,600 m3/d)

5) Set the methanol dosage to 240 gpd (0.91 m3/d) to meet the chemical

dosage target

Challenge 14: Clarifier Maintenance (50 Points)

Parameter Target

BOD5 < 7.0 mg/L

MLSS > 3000 mg/L

Note that in this question, two of the secondary clarifiers are out of service for maintenance. They cannot be put back in service. The influent flow is 2.2 MGD,

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with default influent characterizations. The simulation starts with a very low MLSS of 704 mg/L.

The targets can be met by making the following changes:

1) Put the 1 available off-line secondary clarifier in service (leave the 2nd aeration tank off-line)

2) Increase the RAS flow to 1.4 MGD (5300 m3/d) 3) Turn on the DO Controller 4) Set wastage flow to 5,500 gpd (20.8 m3/d) to achieve the MLSS target.

Challenge 15: Cold Weather, No DO Controller (50 points)

Parameter Target

SRT 10 - 12 days

Dissolved Oxygen (DO) in Aeration Tank 1.0 - 3.0 mg/L

The influent flow is this challenge is 4 MGD (15,100 m3/d) with a relatively high

COD concentration of 520 mg/L, and temperature of 53°F (12°C). Note that in

this question, the DO Controller option is not available. The simulation starts

with a low MLSS and DO concentration in the aeration tank.

The targets can be met by making the following changes:

1) Put the 3 off-line secondary clarifiers in service

2) Increase the airflow to the aeration tanks to 4,000 ft3/min (163,100 m3/d)

to increase the DO concentration

3) Reduce the wastage flow to 45,000 gpd (170 m3/d) to bring up the MLSS

and SRT to the target levels

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