&EPA
United States
Environmental Protection
Agency
Wastewater Management Fact Sheet
Energy Conservation
INTRODUCTION
Continual increases in energy costs in the United
States affect wastewater treatment plants
(WWTPs) just as they do other facilities. Energy
costs can account for 30 percent of the total op-
eration and maintenance (O&M) costs of
WWTPs (Cams 2005), and WWTPs account for
approximately 3 percent of the electric load in
the United States. Furthermore, as populations
grow and environmental requirements become
more stringent, demand for electricity at such
plants is expected to grow by approximately 20
percent over the next 15 years (Cams 2005).
Energy conservation is thus an issue of increas-
ing importance to WWTPs. This fact sheet
describes possible practices that can be imple-
mented to conserve energy at a WWTP.
APPLICABILITY
Evaluating a facility for energy efficiencies and
adopting an energy conservation plan often result
in increased treatment efficiency, along with the
potential for increased treatment capacity, an
increased ability to meet effluent limitations,
reduced O&M requirements, and reduced energy
costs.
The main requirement on the part of the
WWTP staff is a commitment to spend the initial
time needed to evaluate the system, to follow
through with the development of an energy con-
servation plan, and to implement the plan's
recommendations.
KEY COMPONENTS OF AN ENERGY
CONSERVATION PLAN
A number of U.S. facilities, including the Wash-
ington Suburban Sanitary Commission (WSSC)
and the East Bay Municipal Utility District
(EBMUD) in the San Francisco Bay area, have
developed and implemented energy conservation
and management plans (Taylor 2005, Cohn 2005).
These plans typically have the goal of reducing
energy costs by a specified percentage.
The key components of an effective energy man-
agement plan are:
Creating a system to track energy usage and
costs
• Performing energy audits of major operations
• Upgrading equipment, systems, and controls,
including facility and collection system im-
provements to increase energy efficiency
• Developing a cost-effective electric supply
purchasing strategy
Optimizing load profiles by shifting opera-
tions where possible
• Developing in-house energy management
training for operators
These components are explained more fully
below.
Tracking and Evaluating Energy Usage and
Costs
The first step in evaluating energy usage and
costs at a treatment facility is gaining an under-
standing of where the energy is being used. This
information allows the WWTP staff to identify
areas for conservation and to determine where
energy is being used inefficiently. At many
WWTPs the facility's energy use is recorded at a
single recording location. The disadvantage of
this method is that it does not allow personnel to
see the energy used by each individual process,
and thus operating inefficiencies in these proc-
esses might be overlooked.
For example, the WSSC commissioned the es-
tablishment of an Energy Information System
(EIS) in fiscal year 2002 (Taylor 2005). A Java
Web application replaced the spreadsheets that
had been used to track energy data. The EIS da-
tabase tracks energy consumption, demand, and
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costs by major processes at the Blue Plains
WWTP in Washington, DC. With this informa-
tion, an energy audit can determine the most
energy-intensive operations.
A facility's energy usage can be compared with
energy usage at similar facilities to identify areas
that should be examined further. Once the effi-
ciencies of different pieces of equipment and
process operations are determined, the facility
can begin to develop energy conservation meas-
ures by answering the following questions for
each piece of equipment and process:
• Does the process/equipment need to run at
all?
• Is it possible to run the process/equipment
for fewer hours?
• Is it possible to shift this activity to off-peak
hours (for some auxiliary functions)?
• Are energy efficiency process modifications
or equipment upgrades practical and possible
while maintaining equipment efficiency?
• What equipment is most energy efficient for
this process?
• Is it possible to run more efficient pumps for
normal base loads or to use lower-efficiency,
larger units for only the peak flows?
The answers to these questions will help deter-
mine what processes can be modified or what
equipment can be operated more efficiently or
replaced to save energy (Cams 2005).
Performing Facility Energy Audits
A comprehensive energy audit allows a facility to
determine the largest, most energy-intensive op-
erations. By determining the energy demands of
the various processes and equipment at a WWTP,
personnel can look at improving the treatment
energy efficiency. The objectives at most facilities
are lower energy consumption, demand, and costs
(Taylor 2005). In some cases, life-cycle cost ana-
lyses can be used to help assess and optimize the
selection of individual components and systems.
For example, the WSSC developed an energy
performance project evaluation process to assist
in determining whether to proceed with different
opportunities to upgrade or replace various sys-
tems (Taylor 2005). Equipment upgrades and
maintenance were then funded from the energy
savings realized. The WSSC's Energy Perform-
ance Project had two phases. Phase I involved
detailed engineering feasibility studies with as-
sociated evaluation and recommended technical
solutions. Preliminary design work was done and
the scope of the project, costs, and financing
were established.
Phase II involved more detailed design work,
including construction, commissioning, and
training, along with operation and maintenance.
Phase II also included monitoring and verifica-
tion of the performance of the improved systems
and the savings that resulted (Taylor 2005).
Upgrading Equipment, Systems, and Controls
Numerous processes can be upgraded to improve
the energy efficiency of WWTPs. Some of these
were demonstrated when EBMUD instituted an
aggressive energy management program in 2001
(Cohn 2005). EBMUD serves approximately
600,000 people in the San Francisco Bay area of
California. Its Energy Management (EM) pro-
gram included energy demand reduction, on-site
energy generation, and modifications to the way
electricity was purchased. Energy usage was ex-
amined, and a variety of processes were targeted
for energy demand reductions. EBMUD modified
some traditional processes, and the result was
large savings in energy usage. For example, in the
initial stage of the activated-sludge process, a
100-horsepower surface aerator was replaced with
a 25-horsepower subsurface aerator. In addition,
an aerated grit chamber that used approximately
2,900 megawatts per year was replaced with a
vortex system, resulting in energy savings of
approximately 70 percent per year (Cohn 2005).
EBMUD also implemented additional improve-
ments, including the following:
• Installing high-efficiency influent and efflu-
ent pumps, high-efficiency motors, and
variable-frequency drives
• Discontinuing second-stage activated-sludge
mixing
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• Adding plastic balls to prevent heat loss and
evaporation losses in the oxygen production
vaporizer pit
• Tying in pipes on gas recirculation blowers
to allow one blower to service two mixing
tanks
These energy-efficient strategies and modifica-
tions, along with others, resulted in an estimated
annual savings of $2,796,000 (California Energy
Commission, EBMUD Case Study, 2003).
In addition to the upgrades and modifications
mentioned above, there are numerous other proc-
ess changes that can contribute to energy
savings. High rate diffusers are capable of sup-
plying large quantities of air or oxygen with low
pressure drop and small bubble size (approx. 1-4
mm). Fine bubble diffusion is inherently more
effective than coarse bubble diffusers in improv-
ing oxygen transfer efficiency. Systems can be
purchased that incorporate many of the tech-
nologies mentioned in this fact sheet into an
efficient aeration system. Aeration systems can
incorporate high-efficiency motors, variable-
frequency drives (VFDs), and dissolved oxygen
monitoring. This, in conjunction with energy
efficient aeration systems, can provide energy
savings of 10 to 25 percent over traditional aera-
tion processes (Pacific Gas and Electric
Company, 2006).
VFD motors are becoming increasingly popular.
A VFD is an electronic controller that adjusts the
speed of an electric motor by modulating the
power being delivered (California Energy Com-
mission, Variable Frequency Drive, 2003). For
applications involving varying flow require-
ments, mechanical devices such as valves are
often used to control flow. This process uses
excessive energy and can create less-than-ideal
conditions for the mechanical equipment in-
volved. VFDs enable pumps to accommodate
fluctuating demand, resulting in operating at
lower speeds and conserving energy while still
meeting pumping needs. According to the Cali-
fornia Energy Commission, VFDs can result in
significant energy savings: a VFD can reduce a
pump's energy use by as much as 50 percent.
Because the benefit of a VFD is dependent on
system variables like pump size, static head, fric-
tion, and flow variability, it is imperative to fully
examine each application before specifying a
VFD. For example, the Onondaga County (NY)
Department of Water Environment Protection
retrofitted VFDs on the activated sludge pump
motors. Combined with other savings from re-
ducing aeration basin blowing and improving the
efficiency of some pumps, the plant saved 2.8
million kW-hrs per year, an annual cost savings
of over $200,000. Since the cost for implementa-
tion of the program was just over $230,000, the
project payback period was 13 months for the 80
million gallons per day facility (U.S. DOE,
2005).
Another technology readily available to plants is
the use of high-efficiency motors. Since pump
and blower motors can account for more than 80
percent of a WWTP's energy costs and high-
efficiency motors are up to 8 percent more effi-
cient than standard motors, it is readily apparent
that high-efficiency motors can contribute
greatly to reducing facility energy costs.
Design improvements and more accurate manu-
facturing tolerances are keys to the improved
efficiencies with these motors. In addition, these
motors typically have greater bearing lives,
lower heat output, and less vibration than stan-
dard motors. While high efficiency motors have
a 10-15 percent higher initial cost, with their
lower energy consumption and lower failure
rates, these motors should be considered for all
new purchases and replacements (California En-
ergy Commission, Energy-Efficient Motors,
2003).
An example of an emerging technology with po-
tential application to WWTPs is fuel cells
(Figure 1). Like a conventional battery, a fuel cell
uses two reacting chemicals separated by an elec-
trolyte to produce an electric current. Unlike a
conventional battery, however, a fuel cell is not
charged prior to use. The chemical reactants in a
fuel cell are fed continuously to the cell to provide
constant power output. The reaction involves no
combustion and no moving parts, and it produces
little pollution. Heat generated in the process can
be recovered and used in the facility.
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from air
Fuel
Cathode Catalyst
Figure 1. Fuel Cell Schematic
H20
Exhaust
Although fuel cells are costly to install, they have
distinct advantages over the combustion power
sources at WWTPs, such as diesel generators.
One advantage of the fuel cells is lower harmful
emissions. Using diesel driven generators, espe-
cially for continued use as a supplemental power
source, can lead to air quality problems. Many
states (including California) have established
strict emissions limits on all diesel engines. While
most older diesel engines can not meet the new air
restrictions, newer high-efficient, low emission
engine driven generators are now available.
As a fuel source, fuel cells use hydrogen, which
can be derived from methane, natural gas, or
anaerobic digester gas. Digester gas must be
scrubbed before use to remove compounds that
can be problematic for fuel cells (U.S. EPA
1995). Fuel cell emissions are so clean that they
are exempt from many Clean Air Act permitting
requirements (California Energy Commission,
Fuel Cells, 2003).
Energy conservation might also include the in-
vestment in Auxiliary and Supplemental Power
Sources (ASPS) or energy recovery equipment,
which will allow energy to be produced on-site
(EPA, 2006). This energy could then be used to
run processes or power buildings on-site, par-
tially or fully, or could be sold to other users if
there is an appropriate delivery system to the
electric grid. Possible ASPS include bio-gas-
fueled internal combustion engines, microtur-
bines (Figure 2), wind turbines, fuel cells, and
solar cells. Some ASPS available do not con-
serve energy but replace off-site generation with
on-site generation.
The city of Pacifica, California, recently began
operating 1,800 solar panels to supply a portion
of the Calera Creek Water Recycling Plant's
electric needs. The solar panels provide 10 to 15
percent of the treatment plant's energy needs.
The facility estimates $100,000 per year in en-
ergy savings (Manekin, 2006).
Making improvements to the wastewater treat-
ment plant and the collection system has also
been found to result in energy savings. In par-
ticular, installation of an equalization basin
allows the plant to even out pumping needs, and
so allows for "peak shaving" by running pumps
during off-peak hours (Fuller, 2003). Reducing
infiltration and inflow in the collection system
also can pay for itself in energy savings. By re-
habilitating damaged or deteriorated sewer lines
and eliminating improper connections to the sys-
tem, the overall flow to the WWTP is reduced,
thus reducing the amount of energy required to
treat the flows.
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Turbine Exhaust
System Exhaust
(Heat Recovery)
Figure 2. Microturbine Schematic
Another improvement to a wastewater treatment
plant that can result in large energy savings is a
Supervisory Control and Data Acquisition
(SCADA) system. These systems use computers
to automate process monitoring and operational
control. Because such systems monitor energy
usage, cost savings can be realized, along with
the savings associated with enhanced process
control (Fuller 2003). SCADA systems can
monitor and control the activity of wastewater
systems from a single location. Immediate detec-
tion of problems through diagnostic displays
enables quick intervention for fast resolution.
Operators can easily compensate for seasonal
flow and wet weather by automatically adjusting
set points. Centralized control and monitoring of
distribution and collection systems provides data
for water modeling and energy use optimization,
as well as predictive maintenance of distributed
equipment.
In addition to monitoring treatment processes,
SCADA systems can provide continuous moni-
toring and control of plant operations such as:
Pump stations
Sewer diversion
Wet weather
overflow protection
Wastewater
collection systems
Water distribution
systems
Remote operations
Programmable logic
controllers
Creating the most efficient electric supply
purchasing strategy, optimizing load profiles,
and reducing costs
At many facilities, the administrators are un-
aware of the rate structures of their electric bills.
Electricity is typically billed in two ways: (1) by
the amount of energy used over a specific period,
measured in kilowatt-hours and (2) by demand,
the rate of the flow of energy, measured in kilo-
watts. Electric utilities structure their rates on
the basis of the user's required voltage level,
the electricity usage at different hours of the
day, and the peak demand. A WWTP might be
operating equipment when electricity is at peak
rates, resulting in unnecessary costs. Plant per-
sonnel should become familiar with the energy
rate structure to determine whether they can op-
erate equipment at off-peak hours or reduce
energy consumption during peak-demand hours.
For example, the WSSC revised its power pur-
chasing to optimize energy costs at WWTPs. The
WSSC purchases blocks of power supply (kilo-
watt-hours) at a wholesale, competitive level.
This provides for a predictable baseload cost.
The WSSC purchases its remaining kilowatt-
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hours on the spot market. The WSSC also pur-
chases energy (kilowatt-hours) and capacity
(kilowatts) separately. As market prices shift, the
electric utility shifts the WWTP's load accord-
ingly (Taylor 2005). An example of shifting
loads is the use of system storage to store
wastewater during periods of highest load rather
than operating pumps. The stored wastewater can
then be pumped and treated during periods of
low demand.
Another example, EBMUD has also changed the
way it purchases electricity. EBMUD used to
buy electricity solely from Pacific Gas and Elec-
tric at an average cost of $0.11 per kilowatt-hour.
Now EBMUD purchases electricity from the
Western Area Power Administration, which
markets hydroelectric power, at an average cost
of $0.06 per kilowatt-hour (Cohn 2005). It
should be noted that there are risks associated
with purchasing electricity on the spot market.
Correct market forecasts are essential, and
WWTPs must deal with price volatility in the
market.
A technology often used to supplement energy
usage at WWTPs is cogenerating electricity and
thermal energy on-site, capturing and using an-
aerobic digester gas (or bio-gas). For example,
EBMUD generates enough energy for approxi-
mately 50 percent of its energy needs. EBMUD
is considering a digester cover that would store
gas at night, creating a temporary reserve that
could be used during peak-demand periods. The
Encina Wastewater Authority also uses digester
gas (bio-gas) to generate electricity on-site. En-
cina has also adopted seasonally adjusted time-
of-use rates from its electric company. By shift-
ing treatment process times, Encina has been
able to reduce peak-demand rates. By using the
time-of-use rates and cogeneration, Encina esti-
mates annual savings of $350,000 per year. At
EBMUD, cogeneration of electricity and thermal
energy has resulted in cost savings estimated at
$1.7 million annually (California Energy Com-
mission, Encina Case Study, 2003).
Energy Management Education
Energy conservation includes monitoring and
maintaining each process in the plant. Proper
maintenance and upkeep of the equipment and
processes in a facility are an integral component
of a complete energy conservation plan. Em-
ployee training and awareness of the energy plan
and procedures need to be continually updated
to ensure that the goals and energy savings are
targeted.
Training for plant personnel is essential as is
educating the public on energy, efficiency and
conservation. A good option for conserving energy
at a WWTP is the possibility of reducing flows
to the plant by reducing water use in the com-
munity. As less water flows into the plant, less
volume is treated and thus less energy is con-
sumed. An aggressive Infiltration and Inflow
program can also reduce flows to the plant.
Ideas for promoting water conservation include
• Educating residents about high-efficiency
appliances, plumbing fixtures and water-
saving habits
• Educating residents to reduce peak water
demands to avoid the extra costs associated
with operating additional pumps and equip-
ment during peak-flow periods
COSTS
Many WWTPs are beginning to identify a range
of approaches for setting their rate structures
based on full-cost recognition. Under full-cost
pricing, utilities recognize their actual cost of pro-
viding service over the long term and implement
pricing structures that recover costs and promote
economically efficient and environmentally sound
water use decisions by customers. WWTPs are
encouraged to factor in the full spectrum of capi-
tal and O&M costs, including energy usage
(i.e., life cycle costing), in accordance with full
cost pricing concepts (U.S. EPA 2006).
Energy conservation costs depend on the equip-
ment purchased and the plans implemented.
There are costs associated with tracking energy
usage, equipment efficiency, and with gaining
knowledge about the distribution of energy
usage.
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Cost savings are expected as energy use de-
creases. According to the California Energy
Commission's Electric Load Management study
(2003), the Encina WWTP (36 mgd) altered the
operation of certain processes to off-peak hours
and realized cost savings of $50,000 per year.
The study also found that the Moulton Niguel
Water District, which serves 160,000 people,
eliminated peak operations at several pumping
stations and reduced costs by $320,000 per year.
The study concluded that cost savings from im-
plementing an energy management system to
track energy for a WWTP treating an average
daily flow of 15 million to 30 million gallons per
day is estimated to be up to $25,000 per year.
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&EPA
United States
Environmental Protection
Agency
EPA 832-F-06-024
Office of Water
July 2006
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