Activated Sludge

Introduction “Activated sludge“

wastewater being mixed with air or oxygen for a length of time

develops a brown floc consisting of billions of microorganisms and other material

Process provides the environment to keep these aerobic microorganisms under controlled conditions so they can remove most of the suspended or dissolved organics from the wastewater

Introduction The environment is provided by four basic

systems: Aeration Sedimentation return activated sludge (RAS) waste activated sludge (WAS)

Introduction: Typical Activated Sludge Process

Introduction: Definitions of Terms

ACTIVATED SLUDGE floc of microorganisms that form when wastewater is aerated

MIXED LIQUOR mixture of activated sludge and wastewater in the aeration tank

MIXED LIQUOR SUSPENDED MATTER (MLSS) measure of the amount of suspended solids in the mixed liquor

expressed in mg/l MIXED LIQUOR VOLATILE SUSPENDED MATTER

(MLVSS) proportional to the microorganisms concentration in the

aeration tank

Introduction: Terminology

MEAN CELL RESIDENCE TIME (MCRT) the average time a microorganism spends in the treatment

process FOOD TO MICROORGANISM RATIO (F/M)

ratio of the amount of food expressed as pounds of COD (or BOD) applied per day, to the amount of microorganisms, expressed as the solids inventory in pounds of volatile suspended matter.

RETURN ACTIVATED SLUDGE (RAS) settled mixed liquor collected in the clarifier underflow and

returned to the aeration basin

Introduction: Terminology WASTE ACTIVATED SLUDGE (WAS)

excess growth of microorganisms which must be removed to keep the biological system in balance. Various control techniques have been developed to estimate the amount of WAS that must be removed from the process

COMPLETE MIX ACTIVATED SLUDGE an ideal mixing situation where the contents of the aeration tank

are at a uniform concentration PLUG FLOW ACTIVATED SLUDGE

an ideal situation where the contents of the aeration tank flows along the length of the tank

Introduction: Terminology BACK MIXING

mixing the contents of a tank in the longitudinal or flow oriented direction

TRANSVERSE MIXING (or CROSS ROLL) mixing in a direction across the direction of flow

SLUDGE REAERATION practice of aerating the RAS before it is added to the mixed liquor

PROCESS LOADING organic loading range as measured by the F/M

CONVENTIONAL LOADING process loading of 0.2 to 0.5 lbs BOD applied/lb MLVSS/day

HIGH RATE LOADING process loading of two to three times the conventional loading rate

Introduction: Terminology EXTENDED AERATION LOADING

low rate loading that is one half to one tenth of the conventional loading rate

SETTLEABILITY measure of the volume occupied by the mixed liquor after settling in

a graduated cylinder for 30 minutes generally expressed as a percentage based on the ratio of the

sludge volume to the supernatant volume SOLIDS INVENTORY (VOLATILE SOLIDS)

amount of volatile suspended solids in the treatment system

Solids Inventory Suspended matter (SS) that makes up the mixed

liquor consists of living and nonliving organic matter The living organic matter is referred to as being

“active” and contains microorganisms responsible for treating the wastewater

The more accurately the concentration of active microorganisms is known, the more consistently the activated sludge process can be controlled

Many attempts have been made to accurately measure the "active" concentration of the SS

Solids Inventory

Common means of estimating the microorganism concentration is the measurement of volatile suspended matter (MLVSS)

All the organic material in the SS burns to carbon dioxide and water in the MLVSS determination

Typically, 70-80 percent of the MLSS will be MLVSS

Solids Inventory The MLVSS determination provides an approximation

of the concentration of living biological solids, because the MLVSS also includes a nonliving fraction

Even so, the MLVSS has been found to be an acceptable representation of "active" living microorganisms in activated sludge

Treatment Flow Return Activated Sludge (RAS) from the clarifier

underflow is combined with the influent wastewater in the aeration tank to form the mixed liquor

The mixed liquor is usually aerated for a period of several hours in the aeration tank

Treatment Flow

Treatment Flow During aeration, organic material in the wastewater is

converted into new microorganisms, adsorbed onto floc particles and converted (oxidized) to various other products, including carbon dioxide.

Mixed liquor flows through the aeration tank into the clarifier, where it settles to form the RAS.

Clear liquid remaining above the settled mixed liquor is called the secondary effluent, which is discharged from the process.

Aeration Systems

Aeration provides dissolved oxygen and mixing of the mixed liquor and wastewater in

the aeration tank. Factors influencing aeration requirements:

Organic loading MLSS concentration Temperature Other

Aeration Systems Maintain D.O. concentration between 1.0 mg/l and

3.0 mg/l. If nitrification is required and the concentration falls

below 1.0 mg/l, nitrifying organisms become less active.

Aeration Systems Aeration is provided by either diffused or mechanical

aeration systems. Diffused air systems consist of a blower and a pipe

distribution system that is used to bubble air into the mixed liquor.

Mechanical aeration systems consist of pumps or mixers

Activated Sludge Diagram

Diffused Air System Diffused air systems are the most common types of

aeration systems used in activated sludge plants. Produce fine or coarse bubbles.

Fine Bubble Diffusers Fine bubble diffusers are easily clogged by biological

growth and by dirty air, resulting in high maintenance costs.

The air supply for all fine bubble diffusers should be filtered.

Fine Bubble Diffusers

Coarse Bubble Diffusers Coarse bubble diffusers are usually made by drilling

holes in pipes or by loosely attaching plates or discs to a supporting piece of pipe.

Have lower oxygen transfer efficiencies than the fine bubble diffusers.

Coarse Bubble Diffusers

Surface Aerators Two types of mechanical aerators in common use today Surface

use a rotating propeller that pumps the mixed liquor through the atmosphere above the aeration tank.

Oxygen transfer is achieved by the aerator propeller spraying the mixed liquor through the atmosphere.

Surface Aerators Floating or mounted on supports in the aeration tank Materials such as epoxy coated steel are used in

the construction of surface aerators to reduce corrosion

Oxygen transfer efficiency increases as the submergence of the propeller is increased.

power costs also increase because more water is sprayed

Surface Aerators Oxygen transfer efficiencies are stated in terms of pounds

of oxygen transferred per horsepower per hour (lb O2/hp/hr).

Surface aerators are sometimes equipped with draft tubes to improve their mixing characteristics.

Mechanical Aerators Floating or fixed Brush Other

Sedimentation System As the mixed liquor flows out of the aeration

tank, it is transferred to a sedimentation unit which is commonly called a secondary clarifier.

The secondary clarifier provides a reduction in flow velocity allowing the mixed liquor to settle by gravity to the bottom

The design and construction of secondary clarifiers for activated sludge treatment incorporates several methods for the removal of settled sludge.

These generally include the conventional sludge collection equipment found in rectangular and circular primary sedimentation units which collects to a central hopper, and, in recent years, suction-type collectors as shown in Slide 25.

Wasting & Return Rates Constant MLVSS Constant F:M Ratio Constant Sludge Age Return Rates

Constant Variable Intermittent

Process Control A portion of the activated sludge is purposely

removed by wasting it from the process. The wasting of sludge is necessary to maintain the

desired quantity (numbers) of active microorganisms in the process

A basic idea behind successful operation of an activated sludge system is to keep a balance of microorganisms to the amount of food in the wastewater.

Proper operation makes food the only part of microorganisms' diet that limits their growth.

Process Control If nutrients or oxygen limit the growth of the

microorganisms, they will not settle satisfactorily in the clarifier.

The activated sludge process depends on settling the mixed liquor so that it can be returned to the aeration tank to keep in balance with the organic material in the incoming wastewater

Process Control This balance is generally related to process loading

as expressed by the F/M ratio. Inability to settle the mixed liquor can result in a high

concentration of suspended solids in the clarifier effluent.

Nitrification Cycle The first step in the process, conversion of ammonia

to nitrite and then to nitrate, is called nitrification (NH3-NO2-NO3). The process is summarized in the following equations:

NH4 + 3/2 O2 NO2- + 2H+ + H2O

NO2- + 1/2 O NO3

-

Nitrification Cycle It is important to note that this process requires and

consumes oxygen. This contributes to the BOD or biochemical oxygen demand of the sewage. The process is mediated by the bacteria Nitrosomonas and Nitrobacter which require an aerobic (presence of oxygen) environment for growth and metabolism of nitrogen. Thus, the nitrification process must proceed under aerobic conditions

Nitrification Cycle The second step of the process, the conversion of

nitrate to nitrogen gas, is referred to as denitrification. This process can be summarized as:

NO3- + 5/6 CH3OH 1/2 N2 + 5/6 CO2 + 7/6 H2O + OH-

Nitrification Cycle

Decaying Matter

Organic - N

Industrial Fixation

Nitrogen Gas

Oxides of Nitrogen

Fertilizer (Ammonia,

Nitrate)

Nitrogen Fixing

Bacteria

AssimilationAmmonia

Nitrification

Nitrate Denitrification Ammonia, Organic - N in WWTP Influent

Denitrification in Sediments

Biological Growth in StreamNitrification in

Stream

Ammonia, Nitrate in WWTP Effluent

Nitrification Cycle

Toxicity Ammonia toxicity

Ammonium Nitrate Nitrite

Process Control 30 minute settleability

range 400 to 800 ml/l varies with individual plants

Aeration basin dissolved oxygen 1.0 to 3.0 mg/l

Process Control

Mixed Liquor Volatile Suspended Solids 70% of Mixed Liquor Suspended Solids

pH range 6.0 to 9.0 Standard units Ideal 6.8 to 7.4 Standard Units

Process Control Temperature

Colder water may require longer treatment times Industrial discharges may increase the temperature

Nitrogen Content Phosphorus Content

Side Stream Impacts

Return activated sludge (RAS) Supernatant from digester Filtrate from solids processing

Troubleshooting

If conditions in the aeration system deteriorate, undesirable microorganisms may thrive

Filamentous microorganisms long, thread-like organisms with increased

surface area more competitive for growth in conditions of

low DO or low nutrient concentrations

Troubleshooting Filamentous organisms hinder settling

cause excessive bridging and matting of the floc result in a mixed liquor which does not settle

well Bulking sludge

Poor settleability associated with the presence of too many filamentous organisms

Settles poorly Operation in a bulking sludge condition may result in the

loss of the mixed liquor into the effluent

Troubleshooting Approaches to cure bulking

treat the return activated sludge with oxidizing agents, such as chlorine or hydrogen peroxide

improve the treatment conditions so environment is less favorable to the growth of filamentous organisms

Settling problems and solids losses can also occur during high flow periods

Activated Sludge Process Variations conventional tapered aeration complete mix extended aeration step aeration contact stabilization high rate Sequential batch reactors Fixed film suspended growth

Activated Sludge Process Variations Variations involve changes in loading rates or a

physical rearrangement of the process The various levels of process loading are described

by the F/M ratio and MCRT “Physical arrangement“

structural arrangement of the aeration tank various arrangements of the process streams that are used

to provide flexibility

Process Loading Ranges The three basic ranges of process loading are for a

plant operating on a typical domestic wastewater at a temperature of about 20 degrees centigrade

high rate

conventional rate

extended aeration

High Rate Loading Takes advantage of the settleability of sludge

when the treatment system is loaded at a fairly high rate

Level of treatment which results is comparable to a typical high-rate trickling filter plant

Conventional Rate Conventional process

MCRT values of 5 to 15 days F/M ratios of 0.2 to 0.5 lbs BOD applied/lb MLVSS/day

Plants operating in the middle of this range produce an excellent effluent quality and do not (in theory) nitrify

Most large municipal treatment plants operate in the conventional activated sludge zone

At the lower end of this loading range, better effluent is sometimes produced, though problems can occur if the plant slips slightly or goes completely into nitrification

Conventional Rate Filamentous growth and poor sludge settleability have

been associated with the conventional process at the upper loading ranges

Dispersed growth and cloudy effluent are also quite common

Usually the operator can see this sort of condition coming by plotting a trend of the organic loading in his treatment process (either the F/M ratio or the actual MCRT)

Another sign of an "overloaded" condition White foam may indicate a “young” sludge

Extended Aeration Rate Extended aeration rate

The lowest range of process loading where successful operation may be accomplished

Plants operating in this range generally small in size do not receive 24 hour supervision Such plants are very conservative in design MCRT of 20-40 days F/M ratio of 0.05 to 0.15 lbs BOD applied/lb MLVSS/day.

Extended Aeration Rate Sometimes referred to as the "total oxidation

process" Term is derived from the fact that these plants are

designed with such low loadings that theory predicts that all influent BOD will be converted to CO2

Extended Aeration Rate Effluent of the extended aeration process often contains

small pinpoint floc, which may be observed passing over the weirs of the secondary clarifier

In the higher end of the loading range, a number of operating problems may occur

Because the entire aeration range is in the nitrification zone, denitrification and rising sludge problems may result

brown, greasy foam Filaments poor settleability

Activated Sludge With Re-aeration Re-aeration processes are variations in the

arrangement of the process streams All sludge re-aeration processes involve stabilization

by aeration of the return sludge prior to its contact with the untreated wastewater

Most examples require different ratios between the amount of return sludge under aeration and the amount of sludge in the contact section of the process

Activated SludgeWith Re-aeration

Contact stabilization and step aeration (step feed) are two variations of sludge re-aeration

Both processes represent the extremes of the contact/stabilization ratio; however, both have established a successful record of performance

Activated SludgeWith Re-aeration Successful process installations with

contact/stabilization ratios over the whole range between contact stabilization and step feed can be found in great numbers

Most have been shown to greatly increase the capacity of the activated sludge process to handle high organic loadings in smaller aeration tank volumes

Contact Stabilization

Step Feed

Complete Mix

References Kerri, Kenneth D., et al. Operation Of Wastewater

Treatment Plants. Third Edition, 1990, Vol. I, California State University, Sacramento.

EPA Process Control Manual. Aerobic Biological Wastewater Treatment Facilities. March 1977. U.S. Environmental Protection Agency, Office of Water Operations, Washington D.C.

Water Environment Federation. Design of Municipal Wastewater Treatment Plants. Fourth Edition, 1998, Volume II.