Power for Purchase

Nov. 8, 2011

Public-private agreement helps a utility group achieve its renewable energy goals

About the author:

Andrew Dale, P.E., is a wastewater project engineer for HDR Inc. Dale can be reached at [email protected] or 714.730.2386.

Regional Plant No. 1 (RP-1) is a 44-million-gal-per-day wastewater treatment facility owned and operated by the Inland Empire Utilities Agency (IEUA). IEUA provides regional wastewater service and imported water deliveries to eight contracting agencies within its 242-sq-mile service area. The service area is located in the southwest corner of San Bernardino County, approximately 35 miles east of Los Angeles. RP-1 is a secondary treatment facility capable of further treating a portion of its secondary effluent to produce recycled water meeting Title 22 requirements, making the water suitable for unrestricted landscape irrigation.

The solids generated by the treatment process are stabilized using two-phase anaerobic digestion, which produces an average of 745,000 cu ft a day (515 standard cu ft per minute [scfm]) of anaerobic digester gas (ADG). The ADG is used to fuel an engine-driven generator, and excess ADG is sent to waste gas burners located on site. Power produced by the engine generators is fed back into the plant’s main switchgear, and waste heat generated by the engines is used to augment the plant’s hot-water loop for heating anaerobic digesters.

RP-1 requires a base load of approximately 3.3 MW of electricity, which is expected to increase as recycled water demand increases, a new centrifugal solids dewatering is completed and influent flow increases. RP-1 has the potential to generate 3.8 MW of renewable energy using a 1-MW solar plant and two engine generator sets rated at 1.4 MW using ADG. The solar plant provides a daily average of 0.2 MW with midday peaks of 0.8 MW. As a result of air emissions requirements imposed by the South Coast Air Quality Management District (SCAQMD), IEUA is able to produce an average of 0.9 MW from the two 1.4-MW engine generators.

The engines consume roughly 315 scfm of ADG and produce a majority of the heat required to heat the anaerobic digesters. A small portion of the ADG is used in hot-water boilers, and the remaining ADG is combusted in an open flare with no beneficial reuse. Uncertainty about future and more restrictive air emission requirements has caused IEUA to implement green alternatives that maximize the beneficial use of excess ADG to produce energy.

Power Purchase Agreement

IEUA recently executed a power purchase agreement (PPA) with Sun Power for the installation and operation of an onsite solar power plant. The same approach was used for a second PPA project awarded to UTS BioEnergy LLC for the design, installation and operation of a biogas treatment system and fuel cell power plant at RP-1. The PPA offers a mechanism for IEUA to contract out the design, construction and operations of the energy project to UTS and secure a predictable and fair rate for power produced for the 20-year term of the agreement with no upfront cost to the agency. The PPA contract with UTS was signed in October 2010, with an anticipated start date of December 2011.

The project is designed, constructed, operated and financed by UTS, which selected HDR Constructors Inc. to provide the design-build services. HDR Eng. provided the design services, while Filanc Construction and Big Sky Electrical are the general and electrical contractors currently constructing the project.

The PPA will meet IEUA’s goal of producing renewable energy while meeting SCAQMD emission requirements. The project provides IEUA with energy independence and maximizes renewable energy production by constructing the 2.8-MW fuel cell. Because the project will be funded fully by UTS through the sale of renewable power to IEUA, there is no initial capital outlay by IEUA.

ADG to Energy

The overall project will produce renewable energy and heat using ADG produced at RP-1. The RP-1 Fuel Cell Project includes a 600-scfm digester gas treatment system, 2.8-MW fuel cell power plant, 4.4-MMBTU heat recovery system, power metering, interconnection with the existing RP-1 electrical distribution system, and tie-ins with other existing plant systems for water, gas and drainage.

The molten carbonate fuel cell, a Fuel Cell Energy DFC3000, uses methane in digester gas as the hydrogen source for power generation. The DFC3000 heat rate is LHV 7260 BTU/kWh, producing 2.8 MW of electricity, which requires 20 MMBTU per hour. The available ADG will supply approximately 18 MMBTU, or 90% of the required fuel. The remaining fuel supply will be augmented by natural gas from an existing RP-1 service.

There are very specific requirements for the ADG quality to fuel the power plant. A robust digester gas treatment system must allow the fuel cell to maintain the specified output without degrading the stack life expectancy. These ADG requirements include extremely low limits for sulfur compounds. ADG
quality can vary greatly depending on the source material for digestion.

The RP-1 ADG has high levels of volatile organic compound (VOCs), requiring a robust gas treatment system. Environmental Services and Composites Inc. supplied the digester gas treatment system. To meet the fuel cell gas requirements, a multistage gas treatment system was specified. The first stage of the digester gas conditioning system uses iron sponge scrubbers for hydrogen sulfide removal. The second stage is a gas compression/refrigerated drying system to remove moisture. The system has two blowers—one duty and one standby. The third stage is the activated carbon system, which has three components: a regenerable activated carbon system, a redundant traditional activated carbon vessel and a third metal impregnated activated carbon vessel to polish the conditioned gas before it feeds the fuel cells. The multiple carbon vessels provide redundancy and reliability to remove siloxanes, VOCs and organic sulfur. All activated carbon treatment vessels have a backup vessel of the same size and operate in a lead-lag configuration.

In addition to providing electricity, the project will tie into the existing engine generation heat recovery system, which limits the changes to the existing heat recovery system. The fuel cell has better electrical efficiency than the existing engine generators and will not generate the same amount of waste heat per BTU input. The installed heat recovery system will generate an average of 4 MMBTU per hour, which is about half of the required heat for the anaerobic digesters.

The project is currently in construction, with an anticipated startup in December 2011.


Using a minimum of 75% renewable fuel defines the project as a renewable energy project by California Self Generation Incentive Program (SGIP) standards. This designation rendered it eligible for rebates. The project will receive SGIP funds under the 2010 program, which provides financial incentives in proportion to the generation capacity installed.

This rebate allowed UTS to secure rebates, which it will receive from Southern California Edison after the project is commissioned. This rebate, along with the Federal Incentive Tax Credit, allows the project to provide power at a competitive rate.

Lessons Learned

This project demonstrates the ability of a public agency to engage private companies to achieve its renewable energy goals at a wastewater treatment plant. It also shows that the PPA model can provide a consistent and reliable power source for a wastewater plant without a large capital expense.

The project allows IEUA to focus on the treatment of wastewater without expending resources to maintain and operate the cogeneration system while still achieving renewable energy goals and gaining energy independence.


Challenge: Air emission regulations prompted IEUA to maximize the use of excess anaerobic digester gas from a wastewater treatment facility to produce energy.

Solution: The utility group entered into power purchase agreements to implement a biogas treatment system and fuel cell power plant as well as an onsite solar power plant.

Conclusion: Power purchase agreements can serve the renewable energy goals of public utilities while providing cost-effective and reliable power for wastewater treatment.

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About the Author

Andrew Dale

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