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Wastewater Management Fact SheetEnergy Conservation

INTRODUCTIONContinual increases in energy costs in the UnitedStates affect wastewater treatment plants(WWTPs) just as they do other facilities. Energycosts can account for 30 percent of the total op-eration and maintenance (O&M) costs ofWWTPs (Carns 2005), and WWTPs account forapproximately 3 percent of the electric load inthe United States. Furthermore, as populationsgrow and environmental requirements becomemore stringent, demand for electricity at suchplants is expected to grow by approximately 20 percent over the next 15 years (Carns 2005).Energy conservation is thus an issue of increas-ing importance to WWTPs. This fact sheetdescribes possible practices that can be imple-mented to conserve energy at a WWTP.

APPLICABILITY

Evaluating a facility for energy efficiencies andadopting an energy conservation plan often result

in increased treatment efficiency, along with the potential for increased treatment capacity, anincreased ability to meet effluent limitations,reduced O&M requirements, and reduced energycosts.

The main requirement on the part of theWWTP staff is a commitment to spend the initialtime needed to evaluate the system, to followthrough with the development of an energy con-servation plan, and to implement the plans

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, havedeveloped and implemented energy conservationand management plans (Taylor 2005, Cohn 2005).

These plans typically have the goal of reducingenergy costs by a specified percentage.

The key components of an effective energy man-agement plan are:

Creating a system to track energy usage andcosts

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 managementtraining for operators

These components are explained more fullybelow.

Tracking and Evaluating Energy Usage and

Costs

The first step in evaluating energy usage andcosts at a treatment facility is gaining an under-standing of where the energy is being used. Thisinformation allows the WWTP staff to identifyareas for conservation and to determine whereenergy is being used inefficiently. At manyWWTPs the facilitys energy use is recorded at asingle recording location. The disadvantage of

this method is that it does not allow personnel tosee the energy used by each individual processand 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 JavaWeb application replaced the spreadsheets thathad 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 PlainsWWTP in Washington, DC. With this informa-tion, an energy audit can determine the mostenergy-intensive operations.

A facilitys energy usage can be compared withenergy 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 facilitycan begin to develop energy conservation meas-ures by answering the following questions foreach piece of equipment and process:

Does the process/equipment need to run atall?

Is it possible to run the process/equipmentfor fewer hours?

Is it possible to shift this activity to off-peakhours (for some auxiliary functions)?

Are energy efficiency process modificationsor equipment upgrades practical and possiblewhile maintaining equipment efficiency?

What equipment is most energy efficient forthis process?

Is it possible to run more efficient pumps fornormal 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 whatequipment can be operated more efficiently orreplaced to save energy (Carns 2005).

Performing Facility Energy Audits

A comprehensive energy audit allows a facility todetermine the largest, most energy-intensive op-erations. By determining the energy demands ofthe various processes and equipment at a WWTP,

personnel can look at improving the treatmentenergy efficiency. The objectives at most facilitiesare lower energy consumption, demand, and costs(Taylor 2005). In some cases, life-cycle cost ana-lyses can be used to help assess and optimize theselection of individual components and systems.

For example, the WSSC developed an energyperformance project evaluation process to assistin determining whether to proceed with different

opportunities to upgrade or replace various sys-tems (Taylor 2005). Equipment upgrades andmaintenance were then funded from the energysavings realized. The WSSCs Energy Perform-ance Project had two phases. Phase I involveddetailed engineering feasibility studies with as-sociated evaluation and recommended technical

solutions. Preliminary design work was done andthe scope of the project, costs, and financingwere established.

Phase II involved more detailed design work,including construction, commissioning, andtraining, along with operation and maintenance.Phase II also included monitoring and verifica-tion of the performance of the improved systemsand the savings that resulted (Taylor 2005).

Upgrading Equipment, Systems, and ControlsNumerous processes can be upgraded to improvethe energy efficiency of WWTPs. Some of thesewere demonstrated when EBMUD instituted anaggressive energy management program in 2001(Cohn 2005). EBMUD serves approximately600,000 people in the San Francisco Bay area ofCalifornia. Its Energy Management (EM) pro-gram included energy demand reduction, on-siteenergy generation, and modifications to the wayelectricity was purchased. Energy usage was ex-

amined, and a variety of processes were targetedfor energy demand reductions. EBMUD modifiedsome traditional processes, and the result waslarge savings in energy usage. For example, in theinitial stage of the activated-sludge process, a100-horsepower surface aerator was replaced witha 25-horsepower subsurface aerator. In addition,an aerated grit chamber that used approximately2,900 megawatts per year was replaced with avortex system, resulting in energy savings ofapproximately 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, andvariable-frequency drives

Discontinuing second-stage activated-sludgemixing


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Adding plastic balls to prevent heat loss and

evaporation losses in the oxygen productionvaporizer pit

Tying in pipes on gas recirculation blowersto allow one blower to service two mixingtanks

These energy-efficient strategies and modifica-tions, along with others, resulted in an estimatedannual savings of $2,796,000 (California EnergyCommission, EBMUD Case Study, 2003).

In addition to the upgrades and modificationsmentioned above, there are numerous other proc-ess changes that can contribute to energysavings. High rate diffusers are capable of sup-plying large quantities of air or oxygen with lowpressure drop and small bubble size (approx. 1-4

mm). Fine bubble diffusion is inherently moreeffective 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 anefficient aeration system. Aeration systems canincorporate high-efficiency motors, variable-frequency drives (VFDs), and dissolved oxygenmonitoring. This, in conjunction with energyefficient aeration systems, can provide energysavings of 10 to 25 percent over traditional aera-

tion processes (Pacific Gas and ElectricCompany, 2006).

VFD motors are becoming increasingly popular.A VFD is an electronic controller that adjusts thespeed of an electric motor by modulating thepower being delivered (California Energy Com-mission, Variable Frequency Drive, 2003). Forapplications involving varying flow require-ments, mechanical devices such as valves areoften used to control flow. This process usesexcessive energy and can create less-than-ideal

conditions for the mechanical equipment in-volved. VFDs enable pumps to accommodatefluctuating demand, resulting in operating atlower speeds and conserving energy while stillmeeting pumping needs. According to the Cali-fornia Energy Commission, VFDs can result insignificant energy savings: a VFD can reduce a pumps 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 fullyexamine each application before specifying aVFD. For example, the Onondaga County (NY)Department of Water Environment Protectionretrofitted VFDs on the activated sludge pumpmotors. Combined with other savings from re-

ducing aeration basin blowing and improving theefficiency of some pumps, the plant saved 2.8million kW-hrs per year, an annual cost savingsof over $200,000. Since the cost for implementa-tion of the program was just over $230,000, theproject payback period was 13 months for the 80million gallons per day facility (U.S. DOE2005).

Another technology readily available to plants isthe use of high-efficiency motors. Since pump

and blower motors can account for more than 80 percent of a WWTPs energy costs and highefficiency motors are up to 8 percent more effi-cient than standard motors, it is readily apparentthat high-efficiency motors can contributegreatly to reducing facility energy costs.

Design improvements and more accurate manu-facturing tolerances are keys to the improvedefficiencies with these motors. In addition, thesemotors typically have greater bearing liveslower heat output, and less vibration than stan-

dard motors. While high efficiency motors havea 10-15 percent higher initial cost, with theirlower energy consumption and lower failurerates, these motors should be considered for alnew purchases and replacements (California En-ergy Commission, Energy-Efficient Motors2003).

An example of an emerging technology with po-tential application to WWTPs is fuel cells(Figure 1). Like a conventional battery, a fuel cel

uses two reacting chemicals separated by an elec-trolyte to produce an electric current. Unlike aconventional battery, however, a fuel cell is notcharged prior to use. The chemical reactants in afuel cell are fed continuously to the cell to provideconstant power output. The reaction involves nocombustion and no moving parts, and it produceslittle pollution. Heat generated in the process canbe recovered and used in the facility.


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Although fuel cells are costly to install, they havedistinct advantages over the combustion powersources at WWTPs, such as diesel generators.One advantage of the fuel cells is lower harmfulemissions. Using diesel driven generators, espe-cially for continued use as a supplemental powersource, can lead to air quality problems. Manystates (including California) have establishedstrict emissions limits on all diesel engines. Whilemost older diesel engines can not meet the new air

restrictions, newer high-efficient, low emissionengine driven generators are now available.

As a fuel source, fuel cells use hydrogen, whichcan be derived from methane, natural gas, oranaerobic digester gas. Digester gas must bescrubbed before use to remove compounds thatcan be problematic for fuel cells (U.S. EPA1995). Fuel cell emissions are so clean that theyare exempt from many Clean Air Act permittingrequirements (California Energy Commission,Fuel Cells, 2003).

Energy conservation might also include the in-vestment in Auxiliary and Supplemental PowerSources (ASPS) or energy recovery equipment,which will allow energy to be produced on-site(EPA, 2005). This energy could then be used torun 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 theelectric grid. Possible ASPS include bio-gas-fueled internal combustion engines, microtur- bines (Figure 2), wind turbines, fuel cells, andsolar cells. Some ASPS available do not con-serve energy but replace off-site generation withon-site generation.

The city of Pacifica, California, recently beganoperating 1,800 solar panels to supply a portionof the Calera Creek Water Recycling Plantselectric needs. The solar panels provide 10 to 15 percent of the treatment plants 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 basinallows the plant to even out pumping needs, andso allows for peak shaving by running pumps

during off-peak hours (Fuller, 2003). Reducinginfiltration and inflow in the collection systemalso can pay for itself in energy savings. By re-habilitating damaged or deteriorated sewer linesand eliminating improper connections to the sys-tem, the overall flow to the WWTP is reduced,thus reducing the amount of energy required totreat the flows.

Figure 1. Fuel Cell Schematic


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Another improvement to a wastewater treatmentplant that can result in large energy savings is aSupervisory Control and Data Acquisition(SCADA) system. These systems use computersto automate process monitoring and operationalcontrol. Because such systems monitor energyusage, cost savings can be realized, along withthe savings associated with enhanced processcontrol (Fuller 2003). SCADA systems can

monitor and control the activity of wastewatersystems from a single location. Immediate detec-tion of problems through diagnostic displaysenables quick intervention for fast resolution.Operators can easily compensate for seasonalflow and wet weather by automatically adjustingset points. Centralized control and monitoring ofdistribution and collection systems provides datafor water modeling and energy use optimization,as well as predictive maintenance of distributedequipment.

In addition to monitoring treatment processes,SCADA systems can provide continuous moni-toring and control of plant operations such as:

Wastewatercollection systems

Water distributionsystems

Remote operations Programmable logic

controllers

Pump stations Sewer diversion

Wet weatheroverflow protection

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) bythe amount of energy used over a specific periodmeasured in kilowatt-hours and (2) by demandthe rate of the flow of energy, measured in kilo-watts. Electric utilities structure their rates onthe basis of the users required voltage levelthe electricity usage at different hours of theday, and the peak demand. A WWTP might beoperating equipment when electricity is at peakrates, 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 reduceenergy consumption during peak-demand hours.

For example, the WSSC revised its power pur-chasing to optimize energy costs at WWTPs. TheWSSC purchases blocks of power supply (kilo-watt-hours) at a wholesale, competitive levelThis provides for a predictable baseload costThe WSSC purchases its remaining kilowatt-

Figure 2. Microturbine Schematic


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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, theelectric utility shifts the WWTPs load accord-ingly (Taylor 2005). An example of shiftingloads is the use of system storage to storewastewater during periods of highest load rather

than operating pumps. The stored wastewater canthen be pumped and treated during periods oflow demand.

Another example, EBMUD has also changed theway it purchases electricity. EBMUD used tobuy electricity solely from Pacific Gas and Elec-tric at an average cost of $0.11 per kilowatt-hour. Now EBMUD purchases electricity from theWestern Area Power Administration, whichmarkets hydroelectric power, at an average cost

of $0.06 per kilowatt-hour (Cohn 2005). Itshould be noted that there are risks associatedwith purchasing electricity on the spot market.Correct market forecasts are essential, andWWTPs must deal with price volatility in themarket.

A technology often used to supplement energyusage at WWTPs is cogenerating electricity andthermal 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. EBMUDis considering a digester cover that would storegas at night, creating a temporary reserve thatcould be used during peak-demand periods. TheEncina Wastewater Authority also uses digestergas (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 beenable to reduce peak-demand rates. By using thetime-of-use rates and cogeneration, Encina esti-

mates annual savings of $350,000 per year. AtEBMUD, cogeneration of electricity and thermalenergy 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 andmaintaining each process in the plant. Proper

maintenance and upkeep of the equipment andprocesses in a facility are an integral componentof a complete energy conservation plan. Em-ployee training and awareness of the energy planand procedures need to be continually updatedto ensure that the goals and energy savings aretargeted.

Training for plant personnel is essential as iseducating the public on energy, efficiency andconservation. A good option for conserving energyat a WWTP is the possibility of reducing flowsto the plant by reducing water use in the com-munity. As less water flows into the plant, lessvolume is treated and thus less energy is con-sumed. An aggressive Infiltration and Inflowprogram can also reduce flows to the plant.

Ideas for promoting water conservation include

Educating residents about high-efficiencyappliances, plumbing fixtures and water-saving habits

Educating residents to reduce peak waterdemands to avoid the extra costs associatedwith operating additional pumps and equip-ment during peak-flow periods

COSTS

Many WWTPs are beginning to identify a rangeof approaches for setting their rate structures based on full-cost recognition. Under full-costpricing, utilities recognize their actual cost of pro-viding service over the long term and implementpricing structures that recover costs and promoteeconomically efficient and environmentally soundwater use decisions by customers. WWTPs areencouraged 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 energyusage, equipment efficiency, and with gainingknowledge about the distribution of energyusage.


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Cost savings are expected as energy use de-creases. According to the California EnergyCommissions Electric Load Management study(2003), the Encina WWTP (36 mgd) altered theoperation of certain processes to off-peak hoursand 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 pumpingstations and reduced costs by $320,000 per year.The study concluded that cost savings from im- plementing an energy management system totrack energy for a WWTP treating an averagedaily flow of 15 million to 30 million gallons perday is estimated to be up to $25,000 per year.

REFERENCES

California Energy Commission, 2003.Water/Wastewater Process Energy, CaseStudies: East Bay Municipal Utility District.

California Energy Commission, 2003.Water/Wastewater Process Energy, CaseStudies: Encina Wastewater Authority.

California Energy Commission, 2003.Water/Wastewater Process Energy, ElectricLoad Management.

California Energy Commission, 2003.Water/Wastewater Process Energy, Energy-Efficient Motors.

California Energy Commission, 2003. Water/Wastewater Process Energy, Fuel Cells.

California Energy Commission, 2003.Water/Wastewater Process Energy,Variable-Frequency Drive.

Carns, K., 2005. Bringing Energy Efficiency tothe Water & Wastewater Industry: How Do

We Get There? In WEFTEC 2005Proceedings.

Cohn, A., 2005. East Bay Municipal UtilityDistrict: Energy Management Tools andPractices at a Municipal WastewaterTreatment Plant. In WEFTEC 2005Proceedings.

Fuller, J., 2003. Energy Efficient Alternatives forthe Fortuna, CA Wastewater TreatmentFacility, Community Clean Water Institute.

Manekin, M., 2006. Green Sewage Plant GrowsGreener, InsideBayArea.com.

Pacific Gas and Electric Company, 2006. Energy

Reduction Action Plan for WastewaterTreatment Customers.

Taylor, R., 2005. A Comprehensive EnergyManagement Program at the WashingtonSuburban Sanitary Commission. InWEFTEC 2005 Proceedings.

U.S. Department of Energy, 2005. PerformanceSpotlight, Onondaga County Departmentof Water Environment Protection.Available at: www.eere.energy.gov.

U.S. Environmental Protection Agency, 1995.Office of Environmental Engineering andTechnology Demonstration, EPA-600/R-95-034.

U.S. Environmental Protection Agency, 2006.Office of Water, Office of WastewaterManagement. Sustainable WaterInfrastructure for the 21st Century.Available at http://www.epa.gov/water/infrastructure/index.htm.

U.S. Environmental Protection Agency, 2006.Office of Water, Office of WastewaterManagement. Auxillary and SupplementalPower Fact Sheet: Viable Sources. EPA832-F-05-009.

EPA 832-F-06-024Office of WaterJuly 2006

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