Identification of optimal strategies for energy management and reducing carbon
dioxide emission within the Blue Plains advanced wastewater treatment plant.
Steven A. Gabriel, M.ASCE1; Chalida U-tapao2; Christopher Peot, P.E.3;
and Mark Ramirez4
1
Associate Professor, Project Management Program, Dept. of Civil &Environmental Engineering, Univ. of
Maryland, College Park, MD 20742. Email: sgabriel@eng.umd.edu.
2
Ph.D. Graduate Student, Environmental Engineering Program, Dept. of Civil & Environmental
Engineering, Univ. of Maryland, College Park, MD 20742. Email: cutapao@umd.edu.
3
Biosolids Manager, District of Columbia Water and Sewer Authority, 5000 Overlook Ave. SW,
Washington, D.C. 20032. E-mail:chris_peot@dcwasa.com
4
Biosolids Process Engineer, District of Columbia Water and Sewer Authority, 5000 Overlook Ave. SW,
Washington, D.C. 20032. Email:mark_ramirez@dcwasa.com.
Abstract
Wastewater treatment operation has a high potential to be a net energy producer
by producing significant renewable energy from byproducts; biosolids. The best
performing wastewater treatment plants: optimizing their operation, recovering and
reusing resource, and using new technologies, can produce a high percent of the energy
they need to operate the whole process. Most large wastewater treatment plants can
generate energy by producing biogas from the anaerobic digestion process. The objective
of this research is to find optimal strategies for energy management at the Blue Plains
Advanced Wastewater Treatment Plant (AWTP) and to use energy sources that can
reduce the carbon footprint emitted from the whole wastewater treatment process.
More than 330 million gallons a day (MGD) of raw sewage, wastewater and
storm flow into the Blue Plains Advance Wastewater Treatment Plant (AWTP). This is
expected to rise to 370 MGD by 2010, with flows coming from municipal wastewater in
the Washington, D.C., the D.C. metro area (including areas of Maryland and Virginia),
and to produce over 1,163 metric ton per day of biosolids. The energy consumption of the
wastewater treatment utilities and the operations processes consume an estimated
736,087 kilowatt hours per day, which is bought from other contractors. From the present
data, the Blue Plains AWTP is highly efficient in recovering energy by using biosolids in
order to produce biogas, which is estimated at 95.6x103 cubic meters per day and
generates electricity from methane gas at approximately 534.5x103 kilowatt hours per
day. This data shows the generated electricity is not enough for Blue Plain AWTP energy
consumption. Therefore, other renewable energy sources such as wind or solar energy
might be used together. On the other hand, the electricity that is generated from methane
gas also reduces the total amount of carbon dioxide emission, which is considered a
carbon dioxide credit trading in the global market. Biosolids are used in an anaerobic
digester tank before they are applied for land application, creating a high potential to
reduce the carbon footprint by 930 metric ton carbon dioxide per day.
1
Introduction
Major cities in the world are facing problems from increasing of population to
excessive natural resource consumption. Shortage and pollution can emerge when the
consumption is not cautiously and concisely planned. Management of wastes, e.g.
wastewater, from household, which contribute constantly large amount of quantity, is
thus challenging. Wastewater treatment is currently a drain on the economic health of
many countries. Sustainability requires a suitable measure increasing worthwhile values
to the management.
In a large wastewater treatment system capable of treating a large quantity of
wastewater, a tremendous amount of electricity is consumed during the plant operation.
Various wastewater treatment units must improve their efficiency in order to utilize less
energy while increasing treatment. These include mechanical dewatering equipment,
automated process control systems and aeration systems. However, the end-products are
biosolids or byproducts of wastewater, which must be carefully treated before being
released to natural rivers.
Wastewater treatment plants have a high potential to produce energy from
byproducts of operation processes such as methane gas. The huge wastewater treatment
plant can be produced and the energy can be applied in the treatment plant itself. In
addition, this reprocessing can reduce external energy consumption. The amount of
generated methane gas depends on the quantity and quality of wastewater. This methane
production process does not increase the greenhouse gas in the atmosphere due to the fact
that the process applies the renewable source. The objective of this research is to examine
how the production of renewable energy in the form of methane gas and electricity at the
Blue Plains Advanced Wastewater Treatment Plant (AWTP) can be used as energy
sources that that reduce the carbon footprint emitted from whole wastewater treatment
process.
The Blue Plains AWTP operation process
The raw sewage, wastewater and storm flow into the Blue Plains AWTP is more
than 330 million gallons a day (MGD), with flows coming from municipal wastewater in
the Washington, D.C. (including areas of Maryland and Virginia) and produces over
1,163 metric ton per day of biosolids. The Blue Plains AWTP operation process is shown
in Fig1. These biosolids are used for land applications in order to improve soil efficiency
for agriculture. The biosolids are stabilized by adding lime in order to reduce pathogens.
Therefore, these products can be classified in the biosolids class B. According to the
methane production process, the total amount of biosolids has to eliminate the lime that
was added. The average amount of biosolids which the Blue Plains AWTP can use for
generate methane gas is 1,139 metric ton per day (Fig2).
2
Figure 1 Overview of the wastewater treatment process ( DCWASA 2003 with
permission) [1]
Preliminary process: Liquid process
Wastewater and storm water initially pass through bar screens for trash removal and are
then treated with iron salts (FeCl3) for phosphorus removal. After dosing with iron salts,
grit is removed (settled) from the wastewater as the flow passes through aeration tanks.
The wastewater continues to flow to the primary sedimentation tanks.
3
Primary process: Liquid process
Organic suspended solids and phosphorus are removed from the wastewater flow by
slowing the flow and allowing gravity settling in primary sedimentation tanks. The scum
that floats to the surface in these tanks is skimmed off and combined with the settle solids
on the bottom. Both scum and settled solids are next processed for additional removal of
detritus and gravity thickened. The thickened solids are pumped to the blend tank. The
primary wastewater effluent flows to secondary reactors.
Secondary process: Liquid process
Primary effluent flows in a step feed mode to the secondary aeration reactors, where the
effluent is mixed with waste pickle liquor FeCl3 for additional phosphorus removal and
with secondary and nitrification return activated sludge (RAS). This mixture (mix liquor)
is aerated in the secondary reactors and aerobic microorganisms are grown at a high rate
to remove suspended and colloidal carbon and phosphorus. The mix liquor from the
aeration reactors flows to the secondary sedimentation tanks and the solids gravity is
settled. A portion of these settled solids (RAS) is returned to the aeration reactor. The
rest of the settled solids is pumped to dissolve air floatation thickening tanks. The
secondary effluent flows to the nitrification/denitrification process.
Nitrification/denitrification: Liquid process
The nitrification/denitrification process removes ammonia from the secondary effluent.
Methanol and RAS from the nitrification sedimentation tanks are mixed with the
secondary effluent (mixed liquor) in the nitrification reactors. This flow is processed
through a series of aerobic and anoxic tanks for the conversion and removal of ammonia
to N2 gas. The mix liquor subsequently flows to sedimentation tanks and the solids are
gravity settled. A portion of this settle solids (RAS) are returned to the nitrification
reactor, and the rest of the settled solids are pumped to either the secondary reactors or to
dissolved air flotation thickening tanks.
Dissolved air floating (DAF): Solid process
Settled solids from the secondary and nitrification/denitrification sedimentation tanks are
pumped to air floatation tanks and mixed with compressed air and polymer to coagulate
and thicken the solids. After thickening, the solids are pumped to the blend tank.
Blend tank: Solid process
Gravity-thickened primary solids and DAF thickened secondary are mixed together in the
blend tank. Primary and secondary solids are first stored in separation tanks and then fed
at a calculated blend ratio into the blend tank.
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Dewatering and lime stabilization: Solid process
The solids from the blend tank are mixed with polymer and then dewatered by high solid
centrifuges and belt presses. The dewater cake is then mixed with lime (CaO) for
pathogen reduction to EPA class B level [1].
The amount of biosolids per day
amount of biosolids per day(tons)
1,400
1,200
1,000
800
600
400
200
Jun-08
Jul-08
Sep-08
Oct-08
Dec-08
Feb-09
Mar-09
May-09
Jul-09
data in day (2008-2009)
Figure 2 the average amount of biosolids that the Blue Plains AWTP products per day
Energy consumption at Blue Plains AWTP
The Blue Plains AWTP Operation process uses electricity in order to generate
energy for wastewater treatment process. This plant does not generate the electricity by
itself, but uses electricity that it buys from other contractors. The energy consumption,
electricity for operation process data, fluctuates greatly in each day and month.
Nevertheless, it has increased in each year. The average data are 736,087 kilo watt hours
per day (Fig 3).
5
kWH
25,000,000
Blue Plains Electrical Consumption( generation part) 2006-2008
24,000,000
23,000,000
22,000,000
21,000,000
20,000,000
2008
2007
19,000,000
2006
18,000,000
Jan
Feb
Mar
Apil
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Figure 3 the Blue Plains AWTP electricity consumption for operation process
in 2006 -2008
Biogas production by anaerobic digestion
Biosolids are a byproduct of the wastewater treatment process that causes
significant problems such as odor and residues. Many plants spend much of their budget
in order to manage this problem. In the case of the Blue Plains AWTP, biosolids are
classified in class B, and all of them are used for land applications because the chemical
properties (N P K; plant nutrients) can be applied instead of biological fertilizers.
However, these chemical properties can be used in case of biodegradation for renewable
energy. Byproducts of anaerobic biodegradation are not only methane gas that is high
potential energy source but also residual that is effluent of anaerobic digestion can be
used for land applications.
The anaerobic biodegradation can be considered for an anaerobic digester at the
Blue Plains AWTP because it plays a crucial rule in the reduction of the high amount of
methane in biogas composition. Anaerobic digestion produces biogas that is a
composition of CH4 and CO2 and that stabilizes biomass in the following the reaction[2]:
Active biomass + C-substance
CH4 + CO2 + stabilized biomass + H2O+ traces gas
Moreover, the biogas composition is 55-65% of methane gas, 30-40% of carbon
dioxide, and 0-5% of Water vapor, traces of H2S and H2 [3].
6
Figure 4 Subsequent steps in the anaerobic digestion process [3]
The anaerobic biodegradation plays significant role in the transformation of
organic matter into biogas: 55-65 vol.% of methane gas and it also reduces the amount of
final residual for disposal as the same time as destroying almost of all the pathogens
present in the biosolids and limiting odor problems associated with residual matter.
In this research, anaerobic digestion is also used in order to estimate the amount
of biogas. The biogas that generates from anaerobic the digester tank is affected by many
factors such as pH, temperature, solid retention time, and type of digester. The amount of
biogas is predicted by fixing the other parameters and thinking about how solid retention
time can be used in the relation between biogas productions (as shown in fig 5). Time is a
significant factor which anaerobic bacteria need to spend in order to transform the
organic matter to biogas. For instance, a 20-day BOD is considered sufficient time for
measurement aerobically, due to the lower growth rate of anaerobic to 30, 60 or even in
some cases 90 days is accommodated acclimation of the biomass to toxic and/or unusual
pollutants in the industrial wastewater[4]. Nevertheless, this research is focused on the
maximum potential to produce the methane gas from biosolids, and the Blue Plains
AWTP is able to supply the biosolids continuously. Therefore, it is not necessary to spent
time more than the high potential rate that means solid retention time is supposed to be 40
days.
According to Fig 5, the biogas amounts, which are generated from anaerobic
digester and related to the solid retention time, can be estimated; however, the important
property of biosolids that affect the biogas amount is the fraction of organic matter in
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biosolids. The average fraction of dry organic matter in the Blue Plains AWTP biosolids
is 21.01 % of biosolids, so the average organic dry matter in the Blue Plains AWTP
biosolids is 239.08 metric ton per day (Fig 6). From this correlation, the approximate
amount of the Blue Plains biogas is 95.6x103 cubic meters per day, and the methane gas
that is 60% of biogas is 57.4x103 cubic meters per day.
Figure 5 biogas productions VS SRT [3]
ODS: organic dry solids of the sludge (wt %)
The average amount of organic dry matter of biosolids per day
organic dry matter of biosolids
per day(tons)
300
250
200
150
100
50
Jun-08
Jul-08
Sep-08
Oct-08
Dec-08
Feb-09
Mar-09
May-09
Jul-09
data in day (2008-2009)
Figure 6 the average amount of organic dry matter of the Blue Plains AWTP
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Energy recovers from methane gas
The Blue Plain AWTP usually buys electricity for operation process from out side
contractor. The energy consumption of the whole plant normally present in electricity
consumption, so the methane gas that generate from anaerobic digester have to transform
to electricity by a kind of generator. However, the approximate amount of electricity can
be estimated by using the heat value capacity of methane gas; methane heat value is
3,412 BTU can generate electricity 1 kilowatt hour (http://tonto.eia.doe.gov). Therefore,
the Blue plains AWTP has high ability to generate 534.5x103 kilowatt hour per day.
Carbon dioxide emission
According to the Blue plains wastewater treatment process, liquids phase,
preliminary, primary and secondary treatment, has the carbon dioxide (CO2) emission
from endogenous respiration and oxygen requirement. The objective of this research does
not need to change any process of liquids phase, so the carbon dioxide emission are
negligible. On the other hands, the total amount of carbon footprint from wastewater
treatment operation process might consider from solids phase of operation process. The
important factor is which one is biosolids handling process.
A lot of research articles show many biosolids treatment processes have a lot of
result of carbon dioxide emission. In case, anaerobic digestion process that generate
methane gas in order to generate electricity gain a huge carbon dioxide credit more than
land application process. Furthermore, the wastewater treatment plant will gain higher
carbon dioxide credit from biosolids management by including anaerobic digester and
land application process.
Carbon dioxide gas release from anaerobic digestion is about 30-40% by vol. of
biosolids, and methane gas is about 55-60% by vol. of biosolids. After electricity is
generated by methane, carbon dioxide gas will be released again by this process. The
approximate ratio of carbon dioxide credit per weight of biosolids is 0.8 Mg CO2 credit
per dry Mg biosolids[5] (tons CO2 credit per dry tons biosolids).The total amount of
carbon dioxide gas that the Blue plains AWTP can keep by using anaerobic digestion is
930 metric ton carbon dioxide per day.
Conclusion
The main objective of wastewater treatment plant is treating wastewater by using
highest potential, so the result of this purpose is the operation process normally spends a
lot of energy especially the huge wastewater treatment plant like the Blue Plains AWTP.
If this plant can not decrease the efficiency of operation process by decreasing energy
consumption, it has to generate energy from byproduct in order to use on site. The Blue
Plains AWTP has a lot of biosolids from production process that have high potential in
order to transform to electricity by using anaerobic digester. This process can handle the
significant problems like how to disposal them and where is the landfill area. In addition,
the carbon emission from this process also decreases. This is a good opportunity to
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change from a crisis to a chance. From this research, the Blue Plains AWTP has
significant energy source such as biosolids, which has high efficiency in order to generate
methane and electricity respectively. In addition, the carbon emission from anaerobic
digestion decreases when compare with using for land application. The Blue plains
AWTP will gain carbon dioxide credit 930 metric ton carbon dioxide per day.
Nevertheless, the electricity that generate from biosolids is not enough for the Blue Plains
AWTP in order to use on site. This plant needs 736x103 kilowatt hours per day, but it can
generate only 534.5x103 kilowatt hour per day. The Blue plains have to find the other
energy sources to fulfill this requirement in order to complete highest efficiency of plant.
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Gabriel, S.A., et al., Statistical Modeling to Forecast Odor Levels of Biosolids
Applied to Reuse Sites. Journal of Environmental Engineering, 2006. 132(5): p.
479-488.
Rosso, D., M.K. Stenstrom, and et al, The carbon-sequestration potential of
municipal wastewater treatment. Chemosphere, 2008. 70: p. 1468-1475.
Appels, L., et al., Principles and potential of the anaerobic digestion of wasteactivated sludge. Progress in Energy and Combustion Science, 2008. 34: p. 755781.
Speece, R.E., Anaerobic Biotechnology for Industrial Wastewater. 1996.
Brown, S., H. Gough, and et al., Green Aspects of Biosolids Processing and Use.
2009.
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