energy recovery in wastewater treatment project report rev2

7/29/2019 Energy Recovery in Wastewater Treatment Project Report Rev2

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CITY COLLEGE OF NEW YORK

DEPARTMENT OF CIVIL ENGINEERING

REPORT

CE G7900

ENERGY RECOVERY IN WASTEWATER TREATMENT

Denny Halim

November 24th

, 2012


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Energy Recovery in Wastewater Treatment

ii

City College of New York

Abstract

Wastewater treatment is an important infrastructure for especially major cities in order to improve water

quality in water bodies. Many development has been going on to improve the quality of wastewater treatment,but unfortunately the more advanced the treatment usually the more energy required in order to produce better

effluent quality. While the energy source such as fossil fuel is very limited and could deplete in near future,alternative source of energy is needed to be developed. Municipal Wastewater Treatment Plant (WWTP)commonly use 1% to 4% of city electricity which is one of the largest municipalities energy consumer. The

method to recover energy in WWTP are reviewed and estimated. Currently, the best methods to recover energy

are from utilization of biogas and biosolids which could potentially satisfy more than half of energy requiredfor WWTP. This report calculated that 70% of electricity needed in WWTP (477 GWh/year) could be satisfied

with energy recovered in WWTP (340.95 GWh/year). In term of watt per capita basis, WWTP could providetotal energy of 8.7 Watts per capita. There is also Microbial Fuel Cells (MFCs) technology which could have

big potential in recovering energy in wastewater, but MFC technology still faces major challenges in term of

its efficiency. MFC is also reviewed and estimated based on recent available research.

Keywords energy recovery, biogas, biosolids, sustainable, energy efficient


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Table of Contents

Chapter 1. Introduction.................................................................................................................................. 1

1.1 Introduction .......................................................................................................................................... 1

1.2 Objectives............................................................................................................................................. 2

1.3 Methodology ........................................................................................................................................ 2Chapter 2. Energy Consumption in Wastewater Treatment ........................................................................... 2

2.1 Overview of Wastewater Treatment ..................................................................................................... 2

2.2 Consumption of Energy in Wastewater Treatment ............................................................................... 3

Chapter 3. Energy Recovery in Wastewater Treatment ................................................................................. 5

3.1 Method for Energy Recovery with Applicable Technology ................................................................. 7

3.1.1 Biogas Utilization for Energy Recovery ...................................................................................... 8

3.1.2 Biosolids Utilization for Energy Recovery .................................................................................. 9

3.1.3 Microbial Fuel Cells (MFC) ...................................................................................................... 11

Chapter 4. Analysis of Energy Balance (Study Case New York City) ........................................................ 14

Chapter 5. Conclusion and Recommendation ............................................................................................. 17

Reference ............................................................................................................................................................ 18


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Tables and Figures

Table 2-1 Electricity Consumption for Recycling Units ....................................................................................... 4

Table 3-1 Typical Constituent Concentrations and Energy Content of Untreated Domestic Wastewater ............. 5Table 3-2 Comparison Coal vs. Biosolids ............................................................................................................. 7

Table 3-3 Prime Mover Performance Specification for Use in Technical Potential Model .................................. 9

Table 3-4 Energy Recovery from Biosolids Incineration with Electricity Generation was Calculated based onReported Values .................................................................................................................................................. 10

Table 3-5 Power Outputs in lab-scale MFCs during the Treatment of Several Wastewaters using Pt/C and

Hexacyanoferrate (HCF) as a cathode ................................................................................................................ 13

Figure 2-1 Wastewater Treatment Flow Diagram ................................................................................................. 2Figure 2-2 Distribution of Energy Usage in Wastewater Treatment ..................................................................... 3

Figure 2-3 Energy Input Based on Flow Input and Treatment Process................................................................. 4

Figure 3-1 Composition of Raw Primary (from Primary Treatment) and Waste Activated Sludge (from

Secondary Treatment) ........................................................................................................................................... 6

Figure 3-2 Heat Recovery using Heat Pump in Sewer ......................................................................................... 7Figure 3-3 Anaerobic Digester .............................................................................................................................. 8

Figure 3-4 Cogeneration or CHP System.............................................................................................................. 9

Figure 3-5 Use of Waste Heat from Incineration for Electrical Power Consumption ......................................... 10Figure 3-6 Biosolids Management in New York State ........................................................................................ 11

Figure 3-7 Scheme of the Potential Niches for MFCs ........................................................................................ 13

Figure 4-1 Energy Opportunities from Domestic Wastewater ............................................................................ 16


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Chapter 1. Introduction1.1 IntroductionIt has been realized that since ancient time, human have aware about the sanitation and thinking about how to

treat or dispose their waste. Rome in 800 BCE is known to be the first one, who constructed sewer, but thesewer basically served to wash out human wastes inside the sewer by using storm water and it will end up on

the river or surface water body. After Roman Empire fell, the sanitation system revert back to old system by

using cesspit or by digging a hole to dump the human waste and covered it back. Human waste became avaluable source for agriculture which basically works as fertilizer for the farm. The usage of cesspit could be

dangerous because the cover of the cesspit could crumble and trapped people inside the cesspit. During the

middle ages (500 CE 1500 CE), people in the world, especially Europe basically had a very poor sanitationsystem by tossing their human waste out of the window and just leave it on the street. At that time, there was

no person that really concerned about this system, and as population grows out thus increasing the

accumulation of human waste on the street causing potential for diseases. During this time, very little

development happened in sanitation system. The actual big pandemic was started on the 18th

Century, which

was caused by cholera. It was estimated that more than 15 million people died between 1817 and 1960, another23 million people died between 1865 and 1917. Russian deaths during a similar time period exceeded 2 million

of people (Beardslee, GW. 2000). Cholera is caused by the contamination of water and food. This epidemics,awakened people to develop a better sanitation system by constructing combined sewer to carry human waste

together with storm water to the water body. The development of sewer system in the major cities diminishing

cholera epidemics and in 1866 was the year of the last cholera epidemic in the London Area.

The health consideration that led to current sanitation system in wastewater was due to the waterborne diseases

that contaminated the water resources from the domestic wastewater which could cause large scale impact to abig city if the wastewater is not managed properly. Since the pandemic, development of wastewater system has

become one of important infrastructure in order to improve sanitation condition especially in big cities anddeveloped countries, but major drawback in wastewater treatment plant (WWTP) is high consumption of

electricity energy for mechanical equipment, such as pumping and aeration system. Bacteria needs oxygen to

remove organic pollutant in the wastewater and oxygen could be supplied by pumping air into the waterthrough air diffuser. Method of pumping air into the wastewater is defined as aeration process and it take

55.6% of total energy consumption in the wastewater (Metcalf & Eddy Inc., 2003). In activated sludgetreatment, 1,200 to 2,500 kWh/day of electricity are required to treat each million gallon (3,790 m

3) of

wastewater. This means that to treat 50 MGD (million gallons per day) of wastewater (equivalent with 450

thousands of population) would require 125 MWh of electricity each day or equal to 45,625 MWh for eachyear.

New York City for example, based on the report from Department of Environmental Protection (DEP) inNew York Citys Wastewater Treatment System, current number of population served by WWTP is

7,753,505 and average water consumption per capita in New York City is 141.16 gallons per capita per day. If

it is assumed that 80% (Metcalf & Eddy Inc., 2003) of supplied water goes to the WWTP, thus wastewatergeneration based on served population was 1.09 billion gallons per day and it would need 1,308 MWh of

electricity per day or equal to 477 GWh to treat wastewater each year. This huge amount of electricity could

include WWTP as one of the largest municipalities consumer which is make up about 1 % of New York Cityelectricity use. In 2010, New York City 5 boroughs, plus Westchester County consumed around 60 thousands

MWh (Zimmerman, 2010).

This 1 % of electricity consumption only served for one purpose which is to treat wastewater to clean water forNew York City. It also reported that nearly 4% of the nations electricity use goes towards moving and treating

water/wastewater (Centre for Sustainable Systems , 2011). There are many researches going on in order to find

out an efficient way to reduce energy consumption and recover some amounts of energy in order to reducesuch huge amount of energy in WWTP. There is a possibility that for Wastewater Treatment Plant to become


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energy producer due to its potential in biogas and biosolids production.

1.2 ObjectivesObjective of this project is to:

Find out several methods and best available technology for energy recovery in WWTP

Find out amount of energy could be recovered from WWTP compared with energy input Estimating net energy produced in WWTP

1.3 MethodologyThe research is conducted by correlational method by researching through secondary data.

The estimation of energy recovery from WWTP will be estimated based on the total amount added from

several available methods for energy recovery. The basis data for the estimation are:

a) Energy consumption in wastewater treatment plantb) Energy produced from available methods of energy recoveryc) Calculating net energy produced and possibility to re-purpose WWTP to become net energy producer

Chapter 2. Energy Consumption in Wastewater Treatment2.1 Overview of Wastewater TreatmentWastewater treatment is a process to remove pollutants in wastewater that comes from human activities, so itcan be safely disposed to water body. Wastewater treatment process may vary depending on the level of

treatment in order to achieve certain degree of effluent quality. Typical wastewater treatment process could be

seen on the following figure.

Source: (Hayes, NA)

Figure 2-1 Wastewater Treatment Flow Diagram

From the figure above, WWTP consists of several stages to treat wastewater into clean water as follows:


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a) Preliminary treatment; screening process to remove big objects (plastic, debris, garbage, etc) thisprocess basically to protect mechanical equipment from such objects which could cause damage.

b) Primary treatment; sedimentation process to settle some suspended solids particle that could be easilyremoved by gravity.

c) Secondary treatment; biological process to remove most of the organic matter in wastewater. It is thecore process in WWTP.

d) Final treatment; disinfection process or further process to improve the effluent quality so it could beused as recycle or reused water.e) Sludge treatment; settled solids from primary treatment and secondary treatment contains large

amount of organic matter and pathogens which need to be removed before disposal. Sludge treatment

process is mainly thickening process, stabilization process (digestion) and dewatering process (to

remove water content and produce dried sludge).

Wastewater treatment generally adopted aerobic biological process for the secondary treatment. Aerobic

biological process utilizes microorganisms by using oxygen to degrade all organic matter in the wastewater but

this process could take days in natural environment due to limited oxygen supply in the environment. In

wastewater treatment plant this biological process is accelerated and controlled by supplying oxygen from

atmosphere using aeration system. This aeration system is the major consumer of electricity in WWTP: it

needs to be operated 24 hours continuously and the system will depend on the wastewater flow rate coming

into WWTP.

2.2 Consumption of Energy in Wastewater TreatmentWWTP use high consumption of energy in the form of electricity for the operation of pump and other

mechanical equipment. The energy requirement to treat one Mgal of wastewater could range from 1,073 4,630 kWh/Mgal (SBW Consulting, Inc. 2002) depending on the treatment used. The energy used in the

biological treatment and disinfection process such as UV system could use huge amount of energy. One of the

activated sludge process to treat 11.5 MGD, would require average total plant operation of 1,690 kWh/Mgal.Which means that total energy required is 19,435 kWh/day or 7,093 MWh/year.

Source: Hawzen and Sawyer, 2012

Figure 2-2 Distribution of Energy Usage in Wastewater Treatment

Based on the figure above there are three major units in WWTP that consumed most of electricity which are:a) Aeration process, accounted for more than 50% of total energy consumption, due to continuous

operation of aeration system to dissolve oxygen into the wastewater.

b) Wastewater pumping, accounted for more than 10% of total energy consumption, in the inlet point ofWWTP, the depth of sewer end point could be more than 10 meter deeper than WWTP elevation, thus

it need to be pumped up.


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c) Anaerobic digestion, accounted for more than 10% of total energy consumption which is mainly usedfor increase the wastewater temperature in order to ensure bacteria growth in the digestion.

In the past time, the wastewater treatment process only focused on the removal of Biological Oxygen Demand(BOD) which quality could be easily achieved by only using conventional activated sludge. However, in

developed countries and big cities where WWTP has been established, people keep on researching on how to

improve the quality of effluent and thus lead to stringent standards. Nowadays, WWTP is also required toremove nutrient (nitrogen and phosphorus) which could lead to eutrophication on surface water body and this

would require a substantial modification in the biological treatment, thus increasing energy input to make

better quality in the effluent.

The modification in treatment process could cause a quite significant increase in electricity use as shown on

the Figure 2-3 which could range from 1000 kWh/Mgal to 3000 kWh/Mgal of electricity use per day. In

average, conventional activated sludge process required only 1,333 kWh/M gallon, while advanced treatment

with nitrification required 1,920 kWh/M gallon which is 40 % increase from conventional one. Nitrificationprocess needs more oxygen than conventional activated sludge to be supplied into wastewater in order to

promote growth of bacteria for the nitrification process.

Source: (EPRI, 2000)

Figure 2-3 Energy Input Based on Flow Input and Treatment Process

Moreover, people also consider wastewater as important water source especially in country with water scarcity.

Installation of recycle water system in WWTP, such as UV and membrane filtration (microfiltration and

reverse osmosis) system could become a good addition for WWTP, but it also requires more energy to treat the

wastewater. Energy impact from the installations of recycle system could be seen on the table below.

Table 2-1 Electricity Consumption for Recycling Units

Units

Electricity

Consumption

kWh/Mgal

Ultraviolet (UV) disinfection +50 to +200

Membranes Filtration

0

500

1000

1500

2000

2500

3000

3500

1 MGD 5 MGD 10 MGD 20 MGD 50 MGD 100 MGD

kWh/Mgal

Activated Sludge

Advanced Treatment

Plant Without

Nitrification

Advanced Treatment

Plant with Nitrification


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Microfiltration +200 to +400

Reverse osmosis +1000 to +2000

Source: (Metcalf & Eddy Inc., 2003)

Chapter 3. Energy Recovery in Wastewater TreatmentToday, there were many development and research going on in order to recover energy in wastewater treatment.

It was found out that the potential for energy recovery is actually quite high and WWTP could become netenergy producer and help to supply energy for cities. Even though the potential is quite high, current

technology still cannot fully recover the energy in wastewater with high efficiency or the cost to install energyrecovery system was still very high, one of the promising technologies to recover energy is such as microbial

fuel cells and thermoelectric generators.

The energy content of wastewater is in four dominant forms: chemical, thermal, kinetic, and potential energy.

Described briefly below (WERF, 2011):

a) Thermal energyThermal energy is the heat energy contained in the wastewater and is governed by the specific heatcapacity of water, which is approximately 4.2 KJ/kg.K or 4.2 MJ/m

3per

oC of temperature change.

b) Hydraulic (Kinetic and Potential) energyPotential energy is the energy due to the water elevation and is calculated by mass x acceleration dueto gravity x head = 9.8 kJ/m3 per m of head for water. Kinetic energy, or the energy due to the

momentum of the water, is calculated as 1/2mv2

= 0.18 kJ/m3 for a water velocity of 0.6 m/s (2 feetper second). Most of the WWTP is located on the low elevation and very close to the river body, thus

the hydraulic head will not be so significant and will provide a small amount of energy.

c) Chemical (calorific) energyThis is the energy content stored in the various organic chemicals in the wastewater. The organic

strength is typically expressed as a chemical oxygen demand (COD) in mg/L. As shown on table

below, chemical energy content is around 12 - 15 MJ/kg COD (13 MJ/kg COD typical). Electricityrequired to treat wastewater is around 1000 to 3000 kWh/Mgal per day. Typical COD concentration inwastewater is 430 mg/L, therefore if 1 Mgal (3,785 m

3) of wastewater is treated per day, potential

chemical energy that could be recovered is 21,158.15 MJ (5,882 kWh). This means that energy

required to treat wastewater is much less than potential energy could be recovered.

Table 3-1 Typical Constituent Concentrations and Energy Content of Untreated Domestic Wastewater

Constituent Unit Value (typical)a)

Constituent concentrations

Total Solids (TS) mg/L 390 - 1230 (720)

Dissolved Solids (TDS) mg/L 270 - 860 (500)Total Suspended Solids (TSS) mg/L 120 - 400 (210)

Biochemical Oxygen Demand (BOD) 5-d,

20oC mg/L 110 - 350 (190)

Total Organic Carbon (TOC) mg/L 80 - 260 (140)

Chemical Oxygen Demand (COD) mg/L 250 - 800 (430)

Oil and Grease mg/L 30 - 90 (60)

Energy Contentb)


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Wastewater, heat basis MJ/10oC.10

3m

341900

Wastewater, COD basis MJ/kg COD 12 - 15 (13)

Primary sludge, dry MJ/kg TSS 15 - 15.9 (15.5)

Secondary biosolids, dry MJ/kg TSS 12.4 - 13.5 (13)

Source: (Metcalf & Eddy Inc., 2003)a)

Typical wastewater composition is based on approximate flow rate of 460 L/capita.day (120gal/capita.day)

b)1 MJ = 0.278 kWh

Chemical, thermal, kinetic, and potential energy in the wastewater mostly contained in the form of solids,

liquid, heat, and hydraulic head which will be explained as follows:

a) LiquidLiquid is the wastewater itself which WWTP treat to obtain free contaminant water. Wastewater

contains many constituents as shown on Table 3-1. As explained before, wastewater contains various

organic chemical and it has chemical energy potential. Wastewater also contains nutrients which have

the potential to grow algae (one of the problem in surface water due to eutrophication caused by

wastewater) which could be used to biofuels.

b) SolidsSolids are usually measured as total dry solids and volatile solids. In wastewater treatment, there is a

by-product called as sludge which contains high concentration of solids (TSS and VSS). Sludge is

produced from primary treatment and secondary treatment (biological treatment) and needs to be

further treated to reduce negative environmental impact. Sludge contains huge amount of organic

matter in the form of solids and pathogens. Typical sludge treatment is stabilization and dewatering.

Stabilization is an important process which reduces organic content and pathogens. From the

stabilization process, methane gas is produced and could be recovered as one source of energy. Sludge

that has been stabilized will go to dewatering process and produce a product named as biosolids.

This biosolids is commonly utilized for land application as fertilizer or compost, but some of the

biosolids go to incinerator or landfill as final disposal. The energy content of biosolids is embedded in

the volatile solids portion and it is typically 80% of total dry solids as shown in figure below.

Source: (NACWA, 2010)

Figure 3-1 Composition of Raw Primary (from Primary Treatment) and Waste Activated Sludge

(from Secondary Treatment)

Unprocessed biosolids typically contain approximately 8,000 BTU/lb (5.07 kWh/kg) which is similar

to the energy content of low-grade coal (NACWA, 2010). Comparison of biosolids vs. coal could be

seen on table below.


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Table 3-2 Comparison Coal vs. Biosolids

Analysis Bituminous Coal Dried Biosolids

Ash 30% 30%

Combustibles 70% 65%

Inorganic Components CalciumSilica

Alumina

Iron

CalciumSilica

Alumina

Iron

Heating Value 10,000 btu/lb 6,0007,000 btu/lb

Source: (Montenegro, 2010)

c) HeatWastewater temperature is usually around 20

oC and sometimes it could reach 25

oC. The warmest

temperatures were found near a commercial laundry facility (Haugen, 2012). This warm wastewater is

technically recoverable in the form of low grade heat but it has major challenge due to: low efficiencyat the low temperatures typical of domestic wastewater and typical distances between the plant

influent and heat recovery sources.

Source: (OSCHNER, 2012)

Figure 3-2 Heat Recovery using Heat Pump in Sewer

d) Hydraulic headRecovery of hydrokinetic energy by installing micro-hydro water turbines in channels and conduits

prior to discharge. Power generated by a micro-hydro turbine is represented by the equation P(kW) =

eHQg, where e = efficiency (%), H=head (m), Q = flow (m3) and g = 9.81 m/s2. Efficiencies are

typically in the range of 75-88%. High flow and/or high head between tertiary treatment and

disinfection are required for this technology to recover significant energy. For example, a flow of 4.38

m3/s (100 MGD) at 3m (9.8 ft.) head produces only approximately 105 kW of electricity (25 kW-

hr/MG). Inline hydro is a well-established technology with efficiencies already close to 90%.

3.1 Method for Energy Recovery with Applicable TechnologyThere are several methods to recover the energy, but not all of the method could be applied because of low

efficiency, such as hydrokinetic energy and heat energy. Potential energy that could be recovered with high

efficiency is only from anaerobic digestion which produces biogas and biosolids with most energy could be

recovered from this process.


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Source: (Monroy, 2012)

Figure 3-3 Anaerobic Digester

One of technology that has promising future is microbial fuel cells which currently undergo extensive research

in order to increase its efficiency. Microbial fuel cell is generally installed in tank with anaerobic condition,therefore it could be potentially installed in anaerobic digestion or secondary treatment that utilize anaerobic

condition.

3.1.1 Biogas Utilization for Energy RecoveryTotal gas production is usually estimated from the percentage of volatile solids reduction. Typical values vary

from 0.25 to 1.12 m3/kg (12 to 18 ft3

/lb) of volatile solids destroyed. Gas production can fluctuate over a widerange, depending on the volatile solids content of the sludge feed and the biological activity in the digester.

Gas production can also be estimated crudely on a per capita basis. The normal yield is 15 to 22 m3/10

3

person.d (0.6 to 0.8 ft3/person.d) in primary plants treating normal domestic wastewater. In secondary

treatment plant, the gas production is increased to about 28 m3/103

persons.d (1.0 ft3/person.d) (Metcalf and

Eddy, 2003). The composition of anaerobic digester gas from WWTP is usually 60 to 70 percent methane with

the remainder primarily carbon dioxide (CO2). The lower heating value (LHV) of digester gas ranges from 550

to 650 British thermal units (Btu)/ft3, and the higher heating value (HHV) ranges from 610 to 715 Btu/ft

3, or

about 10 percent greater than the LHV. Use of biogas (EPA-CHP Partnership, 2011):

Digester gas for heat. WWTP can use digester gas in a boiler to provide digester heating and/orprovide space heating for buildings on site.

Digester gas purification to pipeline quality. WWTP can market and sell properly treated andpressurized biogas to the local natural gas utility.

Direct biogas sale to industrial user or electric power producer. WWTP can treat, deliver, and sellbiogas to a local industrial user or power producer where it can be converted to heat and/or power.

Biogas to vehicle fuel. WWTP can treat and compress biogas on site to produce methane of a qualitysuitable for use as fleet vehicle fuel.

One of the popular methods to recover biogas is by using Combined Heat and Power (CHP) system or can be

defined as cogeneration system. Cogeneration or CHP is the simultaneous production of electricity and heat

from a single fuel source. CHP is an energy system that can be modified depending on the needs of the energy

Biogas to Energy Recovery

Sludge to dewatering

unitbiosolids


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end user. CHP equipment: prime mover, generator, heat recovery equipment, and electrical interconnection.

Prime mover for CHP systems: reciprocating engines, combustion turbines, steam turbines, microturbines, and

fuel cells (EPA-CHP Partnership, 2011).

Table 3-3 Prime Mover Performance Specification for Use in Technical Potential Model

Prime MoverSize

(kW)

Thermal

Output(Btu/kWh)

Power to

Heat Ratio

Electric Efficiency

(%) (HHV)

CHP

Efficiency (%)(HHV)

Reciprocating

Engine (Rich-Burn)280 5520 0.62 29.1 76

Reciprocating

Engine (Lean Burn)335 3980 0.86 32.6 71

Microturbine 260 3860 0.88 26 56

Fuel Cell 300 2690 1.26 42.3 76

(EPA-CHP Partnership, 2011)

Source: (King County Wastewater Treatment Division, 2012)

Figure 3-4 Cogeneration or CHP System

3.1.2 Biosolids Utilization for Energy RecoveryBiosolids is a term defined for sludge that has been stabilized and dewatered and basically has some amount ofnutrient content which could be used as fertilizer or soil amendment. In the United States, the quality of

biosolids for land application is determined by Environmental Protection Agency (EPA) which mainly

regulates heavy metal and pathogens content in the biosolids. The most common methods of biosolids disposalare landfilling, land spreading, and composting due to cost effectiveness; incineration is an alternative, more

costly disposal method.


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Biosolids incineration with electricity generation is an effective biosolids disposal operation with potential for

significant energy recovery. Two equipment options are commercially available for biosolids incineration:

multiple hearth furnaces and fluidized bed furnaces. Multiple hearth furnaces burn biosolids in multiple stages,

allowing for hot air recycle to dry incoming biosolids and improve heat generation by reducing incoming

moisture. While multiple hearth furnaces can be operated intermittently, continuous operation is preferred.

Fluidized bed furnaces are a newer technology that is more efficient, stable, and easier to operate than multiplehearth furnaces, but are limited to continuous operation only (Stillwell, Hoppock, & Webber, 2010). Both

incineration technologies require cleaning of exhaust gases to prevent emissions of odor, particulates, nitrogen

oxides, acid gases, hydrocarbons, and heavy metals. Using either multiple heart or fluidized bed furnaces,

biosolids incineration can be used to power a steam cycle power plant, where heat from incineration is

transferred to steam that turns a turbine connected to a generator, producing electricity. Reliable electricitygeneration that does not depend heavily on auxiliary fuels requires large amounts of biosolids, making

incineration suitable for medium to large wastewater treatment plants (Metcalf & Eddy Inc., 2003). On March

21st, 2011 EPA published a final rule in Federal Register that defines incinerated wastewater solids as solid

waste. Combustion of solids waste is regulated under Clean Air Act Section 129, requiring control of 9

specific emissions (Cadmium, Carbon Monoxide, Dioxin/Difurans, Hydrogen Hhloride, Lead, Mercury,Nitrogen Oxides, Particulate Matter, Sulfur Dioxide) (Wilson, 2011).

Source: (Wilson, 2011)

Figure 3-5 Use of Waste Heat from Incineration for Electrical Power Consumption

Biosolids incineration has the advantage of achieving maximum solids reduction with energy recovery, inaddition to producing a stable waste material as ash and requiring small amounts of land. Disadvantages

include high capital investments, potentially high operations costs depending on auxiliary fuel use, operational

difficulty, air emissions from combustion that might limit use in non-attainment areas, and possible public

aversion (Stillwell, Hoppock, & Webber, 2010). Currently, United States incinerated biosolids in 1 to 6 pounds

ratio (Wilson, 2011).

Where:

Erincineration = energy recovery from biosolids incineration (kWh/d)

Q = wastewater flow rate (MGD)

Cs = wastewater dry solids content (kg/Mgal)

HV = biosolids heating value (kJ/kg)HR = steam electric heat rate

Table 3-4 Energy Recovery from Biosolids Incineration with Electricity Generation was Calculated

based on Reported Values

Factor Equation Term Reported Value Units

Wastewater dry solids content Cs680-1020 kg/Mgal

0.180-0.269 kg/m3


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Biosolids heating value (Digested

biosolids)HV 9000-14000 kJ/kg

Steam electric heat rate HR 10550 kJ/kWh

*Source did not specify lower heating value versus higher heating value

*Heat rate similar to that of a coal-fired power plant due to the solid fuel nature of biosolids and

associated air pollution control equipment

Source: (Stillwell, Hoppock, & Webber, 2010)

Based on the report from Department of Environmental Conservation of New York State in 2011, incinerated

biosolids was accounted for 17% of total biosolids. 52% of biosolids was disposed to landfill, while 30% of itused for land application.

*Dry Weight Basis

Source: (NYS-DEC, 2011)

Figure 3-6 Biosolids Management in New York State

3.1.3 Microbial Fuel Cells (MFC)Microbial Fuel Cells (MFC), a device that takes advantage of the bacterial oxidative metabolic process to

generate electricity with over 90% efficiency is surely the most promising renewable energy generatingtechnology of this century. MFC technology has been developed as a novel biotechnology to harvest energy

from dissolved biomass (Pham, et al., 2006), and it can produce electricity from organic waste in a direct way,without the need for gas treatment or cogeneration, and this conversion can occur at temperatures below 20 C

with low substrate concentration. These characteristics of MFC make it both advantageous and complimentary

to conventional Anaerobic Digestive (AD) Methane and Hydrogen production technologies. Mediatorlessbacteria frequently found in Waste Water like Shewanella putrefaciens, Geobacter sulfurreducens, Geobacter

metallireducens and Rhodoferax ferrireducens, are the primary candidate for MFC Technology. These bacteria

preferably in mixed cultures have electrochemically active redox enzymes on their outer membranes, and these

membranes can be used to transfer electrons released in the oxidative metabolic process to external materials

such as electrodes for shuttling. And therefore, unlike other bacterium they do not require exogenous chemicals

as a mediator which interfere in the repertory chain to divert the electron and transport it to the externalmaterial. In this oxidative metabolic process, the biomass is converted to water, carbon dioxide and energy.

The oxidation of glucose the most common form of biomass can be presented as:

C6H12O6 + 6 O2 6 CO2 + 6H2O DG = 2843 kJ/mol

Where a simple sugar molecule is oxidized into carbon dioxide, water and free energy. This energy in theoxidizing process is difficult to harvest as it is captured within the microbial metabolism. However with the

help of MFC Technology, by interfering into the respitory chain and technically separating this oxidation and


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recombination process, electrons can be diverted to produce electrochemical energy to power electrical

equipment. In addition this oxidative metabolic process can occur in combination with the conventional

Anaerobic Digestion to enable methane or hydrogen production to cogenerate additional electricity.

When aeration tank is filled with small MFCs, bacteria from the wastewater are grown in the fuel cell anode

like small compartments and under anaerobic condition they form a biofilm on the anode. A biofilm is a

community of bacteria which is usually a mixture of sugar and protein that forms a thin layer on the surface ofthe anode. These bacteria depend on the anode to transfer their electron for their survival. Bacteria need to get

rid of these electrons just like people need to breathe, and by transferring the electrons to the anode these

bacteria can continue to oxidize more biomass. And to use the anode for transferring the electrons in their

metabolic process, they strategically position themselves on the anode surface forming that bacterial

community called biofilm.

Under this anaerobic condition these bacteria produces carbon dioxide, electrons, and hydrogen ions, and due

to lack of oxygen, the hydrogen acceptor in the anode chamber, the proton, the hydrogen with positive charge

is defused through the water and Proton Exchange Membrane (PEM) to the Cathode to combine with oxygen

and produce water. With the help of PEM, MFC takes advantage of this time gap between oxidation andreduction and captures the electron and transport it through the electrode and external circuit to finally the

cathode, thus creating a voltage difference between the anode and cathode with the help of PEM, generating

current and electricity. This separation by PEM, a Nafion, sulfonated tetrafluoroethylene and the capturingprocess causes the two electrodes to be at two different potentials (about 0.5 V), creating a bio-batter while the

counter electrode (the cathode) is exposed to oxygen. At the cathode the electrons, oxygen and protons

combine to form only water.

Source: (PennState College of Engineering, NA)

In a practical case study an anaerobic digestion converted 1 kg of COD to roughly 1 kWh of energy, and on

average, the power density (the amount of power, rate of energy transfer per unit volume) obtained is about400 W/m3. And in the case of MFCs, theoretically, 1 kg of COD can be converted to 4 kWh of electricity;

however the current generated by MFCs was 0.1 Amp with the average power density of 40 W/m 3. Recently

another stacked configuration of MFCs was able to increase this power density to 250 W/m3

(Pham, et al.,2006). This 250 W/m

3power density was obtained for every cubic meter of substrate that was utilized, this

amount of energy production specifies that for the given concentration of substrate in the Waste Water and theamount of bacteria for the metabolic process (the cross sectional area of the anode, the biofilm) is the amount

of energy that can be produced for every cubic meter of the anode area.

Actual power density of MFCs using actual wastewater in lab-scale could be seen on the table below which is

based on several research. MFC which is fed from effluent of anaerobic digester showed the highest result with


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Power Density at 42 W/m3

of reactor and showed the highest substrate removal at 2.99 kg COD/m3/day. The

experiment also indicated there is a decline of power after 12 hr and decrease of pH (Aelterman, Rabaey,

Clauwaert, & Verstraete, 2006).

Table 3-5 Power Outputs in lab-scale MFCs during the Treatment of Several Wastewaters using Pt/C

and Hexacyanoferrate (HCF) as a cathode

Substrate Power Density(W/m

3)*

Substrate removal(kg COD/m

3/day)

CE (%) Cathode

Domestic

wastewater1.7 0.430.6 312 Pt/C

Domestic

wastewater3.7 0.2 - 20 Pt/C

Hospital

wastewater8 5 0.71 0.06 22 HCF

Hospital

wastewater14 1 0.67 13 HCF

Influent from

AD25 2 1.23 20 HCF

Effluent from

AD 42 8 2.99 29 HCF

CE: coulombic efficiency; AD: anaerobic digester; *Expressed as NAC: netto anode compartmentSource: (Aelterman, Rabaey, Clauwaert, & Verstraete, 2006)

Configuration of MFC in WWTP could be seen on the following figure.a) MFC stacks provide sustainable energy as a standalone power source.b) The production of high-quality ICE (internal combustion engine) fuel by combining an Anaerobic

digester and an MFC unit followed by WWTP

c) The MFC as a polishing step and energy recovery technology during anaerobic sludge fermentation

Source: (Aelterman, Rabaey, Clauwaert, & Verstraete, 2006)

Figure 3-7 Scheme of the Potential Niches for MFCs


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Chapter 4. Analysis of Energy Balance (Study Case New York City)In 2011, New York City has total population of 8,244,910 (Robert, 2012) and DEP-NYC reported that current

number of population served by WWTP is 7,753,505 with reported average water consumption per capita in

New York City is 141.16 gallons per capita per day (average from 2000 to 2009). Metcalf & Eddy Inc, 2003

stated that 80% of supplied water goes to WWTP. It could be estimated that 1.09 billion gallons per day (4.13

million m3/day) of wastewater. From this wastewater generation value and also wastewater concentrationbased on Table 3-1, energy required and energy produced will be calculated.

1) Energy RequiredBased on the average of 1,333 kWh/Mgal for energy consumption, the energy required to treat 1.09

billion gallons/day is around 1,308 MWh/day (477 GWh/year)

2) Energy RecoveredThe calculation of energy recovery will be calculated based on two reliable energy recoveries which

are biogas and biosolids. Calculation also will be conducted for MFC which will be installed to

Anaerobic Digester.

a)

BiogasBiogas was calculated based on energy generation from CHP system using reciprocating engine

(Rich-Burn) with thermal output 5,520 Btu/kWh and Electric Efficiency (%) HHV = 29.1%.

Wastewater generation = 1.09 billion gallons per day (4.13 million m3/day)

Population served = 7,753,505 persons

Biogas generation unit per capita = 1 ft3/day/person (0.028 m

3/day/person)

Gas Generation (Biogas generation unit per capita x population served) = 7,753,505 ft3/day

(219,554 m3/day)

Gas Heat Content (HHV) = 650 Btu/ft3

(6722.95 W/m3)

Heat Potential of Gas = 5,039,778,250 Btu/day (1.48 GWh/day)

Electric Production = 1,461,535,692 Btu/day (0.428 GWh/day)

Heat Recovery = 2,363,829,367 Btu/day (0.69 GWh/day)

b) BiosolidsBased on Table 3-4, 850 kg of biosolids produced from 1 million gallons of wastewater (3,221.5

tons/m3) (mean value) is selected, Heating Value (HV) = 11,500 kJ/kg, and steam electric heat

rate (HR) = 10,550 kJ/kWh.

Wastewater Generation = 1.09 billion gallons per day

Thus energy recovery biosolids could be recovered is 1,009,928.9 kWh/day (1 GWh/day)

c) Microbial Fuel CellIn MFC case example, MFC will be installed after Anaerobic Digester tank which means that

MFC will be fed with effluent from anaerobic digester. Therefore, volume of tank for MFC need

to be calculated based on reference from anaerobic digester tank volume.

Determining MFC volumeWastewater generation = 1.09 billion gal/day (4.13 million m

3/day)


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Assumed: quantity of dry volatile solids and biodegradable COD removed is 0.15 kg/m3

and

0.14 kg/m3

(Metcalf & Eddy Inc., 2003)

Sludge contains about 95 percent moisture and specific gravity of 1.02, thus sludge volume

could be calculated:

Retention time of wastewater in MFC tank = 12 hr

Thus volume of MFC = (12,147 m3/d) (12/24 d) = 6,073.5 m

3

Electric generationBased on Table 3-5, Power density for MFC which is fed by anaerobic digester influent was

42 W/m3. Therefore electricity generated from MFC = (6,073.5 m

3) x (42 W/m

3) = 255 kW.

Electricity generation per day = 255 kW x 24 hr = 6.1 MWh/day

Based on the calculation above it could be summarized as follows: Potential energy recovered from biogas = 0.428 GWh/day as electricity (30% of energy input in

WWTP) and 0.69 GWh/day as heat

Potential energy recovered from biosolids = 1 GWh/day, but it should be noted that based on Figure3-6, in New York States only 17% of biosolids were incinerated which is basically the only amount of

biosolids available for energy recovery (0.17 GWh/day). 52% of biosolids went to landfill and it could

be potentially reduced by designate biosolids to be incinerated thus recovering the energy. If it is

assumed that 50% of biosolids were incinerated (33% of biosolids were changed from landfill to

incinerator) thus potential energy recovered from biosolids = 0.5 GWh/day

Potential energy recovered from MFC = 6.1 MWh/day which is very small amount compared with theenergy consumption in WWTP. This is due to MFC technology still undergoing development with

many scientist try to look and improve MFC technology. There is a possibility to increase the powerdensity of MFC in the future thus increasing the energy to be recovered.

Therefore, total potential energy could be recovered in New York City WWTP = 0.9341 GWh/day (340.95GWh/year) as electricity and 0.69 GWh/day (251.85 GWh/year) as recovered heat which could be used to

provide heating to WWTP itself and nearby building.

It was calculated that energy input for WWTP is 477 GWh/year thus there is 70% of reduction for WWTP by

considering that all 340.95 GWh/year produced will be used to operate WWTP, while all of the heat could beused for heating for nearby building. Potential reduction is equivalent with electricity demand for 45,661

persons in New York City. Per capita electricity consumption in New York City = 7,467 kWh/capita/year (The

California Energy Commission, 2010).

It was reported that 29% of residential energy in residential was used for heating (MyEnergySolution, NA),New York City will require 2,165 kWh/year/capita for heating only (29% of 7,467 kWh/year/capita). Thus heat

energy reduced from WWTP could be used to heat space for 116,327 persons in New York City.

In term of watt per capita basis, WWTP could provide total energy (electricity + heat) of 8.7 Watts per capita,

while United States energy use is 12,000 Watts and global energy use is 2,000 Watts (Dindo, Cheng, &

Schwartz, 2012). The produced energy from WWTP is relatively small compared with U.S. and global energyuse.


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Source: (WERF, 2011)

Figure 4-1 Energy Opportunities from Domestic Wastewater


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Chapter 5. Conclusion and RecommendationThere are several ways to recover energy in WWTP which potentially could be developed and produce big

amount energy which could make WWTP as net energy producer. But currently, due to limitation of

technology and efficiency in recovering energy, there are only two methods that could potentially be applied

which are:

Recovery of energy from biogas in WWTP which could satisfy 30% of energy input in WWTP andmoreover heat could be produced from CHP or cogeneration thus producing energy as heat.

Recovery of energy from biosolids in WWTP which could potentially satisfy 75% of energy input inWWTP, but biosolids is also very useful for land application thus not all of biosolids could be

recovered as energy.

Biosolids is a good source of energy to replace coal, but it should be noted that biosolids also contains toxic

material and heavy metal. It is important to strictly supervise the biosolids processing into energy. Incinerationprocess of biosolids also require energy input which needs to be calculated in order to estimate the actual net

energy could be recovered by incinerating biosolids.

Estimation of energy could be recovered in New York City WWTP was 0.9341 GWh/day (340.95 GWh/year)

as electricity and 0.69 GWh/day (251.85 GWh/year) as heat, thus reducing electricity demand in WWTP asmuch as 70% of energy input. This could help to reduce energy demand as equal as electricity demand for

45,661 persons in New York City and energy reduced from WWTP could be used to heat space for 116,327

persons in New York City.

Estimation of energy could be recovered in New York City using MFC with current technology was 6.1MWh/year.

MFC technology could become one of the potential candidates to produce large amount of energy, but due tolimitation in technology that could have high efficiency, MFC still cannot be fully utilized for large scale

WWTP. MFC technology extracts wasted energy from the waste water. It is that part of the energy that has

been neglected, and in addition MFC technology is a complimentary technology that can become anotherviable means of extracting energy in combination with other existing energy generating technologies in the

Waste Water Treatment industry. Moreover, with the rising nonrenewable energy prices, and more strict federalregulations being imposed on the waste water treatment industry, the cost of maintaining a WWTP is on the

incline. Therefore it is a good idea to invest more in MFC research and development to find more efficient

alternative materials to lower the cost of MFC production. And with better materials and cheaper installationcost the future of MFC is still promising.


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