U.S. Department of Energy - Energy Efficiency and Renewable Energy
Federal Energy Management Program
Wastewater Digester Gas Can Produce High Quality Methane Fuel for Federal Facilities
March 31, 2005
Wastewater treatment plants (WWTPs) with anaerobic digesters can produce high quality, high Btu methane that can be used to fuel a federal facility power plant. There are more than 16,000 wastewater treatment plants in the United States ranging in size from multi-billion dollar complexes to small, single community plants. More than 3,500 of these facilities employ anaerobic digestion. Since methane production is one of the products of digestion, many treatment plants use a portion of the gas to supply heat needed to complete the digestion process. But only 2 percent of these plants utilize the digester gas to produce electricity. Most of these plants could produce power from the gas and still heat their digesters with the waste heat from the generation process.
The average American creates approximately 100 gallons of wastewater every day. It is composed of 99.94 percent water and must be treated and purified before it can be reintroduced to the environment. In larger treatment facilities this process involves anaerobic digestion where, in the absence of oxygen, bacteria digest residual solids and create methane gas as a byproduct. This gas can be converted to significant amounts of energy and with minimal processing can be used as a substitute for natural gas.
Wastewater digester gas can serve as a natural gas fuel substitute in applications such as boilers, hot water heaters, reciprocating engines, turbines and fuel cells. The gas produced by anaerobic digestion is usually more than 60 percent methane and some plants with state-of-the-art facilities have the potential of producing a biogas with concentrations of methane that reach up to 95 percent. This biogas is produced on a continuous basis and contaminants, such as hydrogen sulfide, are removed prior to use. Other processing may include dehydration, filtering or carbon dioxide removal.
The most common use of wastewater treatment methane is for internal process heat used in the wastewater digesting process. This can be provided directly or by converting to steam in a boiler. The most popular technology to convert wastewater treatment gas to electricity employs internal-combustion engines that run a generator to produce electricity. This is most often used to power internal operations with the excess being sold back to the grid. Heat generated by these engines can also be recovered and used to heat digesters and plant facilities thus improving overall system efficiency. Another proven application employs microturbines which also produce electricity. These can be modularized and easily expanded as gas production expands.
New technologies are being employed in the use of biogas and these include fuel cells and Stirling engines. Some fuel cells operating on wastewater digester methane produce up to 2 megawatts of electricity. The Stirling engine is attractive for this application because it is an external combustion engine and does not require the degree of gas cleanup that other technologies require. These can also be modularized.
Potential for Federal WWTP Biogas-to-Energy Projects
A recent study found that there were approximately 140 wastewater treatment plants with anaerobic digesters greater than 3 million gallons per day that were within 5 miles of large federal facilities. (Anaerobic digesters are generally used when wastewater flow is greater than 3 million gallons per day). Data obtained from the EPA's Water Discharge Permit database indicates that over 1,600 wastewater treatment plants and nearly 800 federal facilities are located within 15 miles of each other.
Federal energy managers should be aware of two types of opportunities to undertake WWTP biogas-to-energy projects. For large federal facilities that have their own treatment plants, numerous possibilities to save on energy, water, or related operating costs (including sludge removal) should be considered. In addition to the types of energy generation projects discussed above, other improvements could be financed through FEMP's Biomass Alternative Methane Fuels (BAMF) Super ESPC relating to the processing of wastewater. For federal facilities that are located near (under 15 miles) a municipal WWTP, they should explore whether it is of sufficient size to produce excess biogas, the availability of the biogas, and what end-use application would make economic sense.
Benefits of Wastewater Digester Gas and the BAMF Super ESPC
Under the BAMF Super ESPC, agencies can partner with prequalified, competitively-selected energy services companies (ESCOs) and use an expedited contracting process to implement their projects quickly, avoiding the uncertainty and delay of depending on appropriated funding. The ESCO arranges financing for project development, equipment, and installation, and the debt is paid back over time from the guaranteed cost savings generated by the project. FEMP's experienced project facilitators can guide the agency through the entire process, providing expert consultation and assistance with technical, contractual, and financial aspects of the project. For more information about ESPCs, visit FEMP's web site.
In a typical BAMF WWTP digester gas project, the ESCO builds a pipeline from the treatment plant to the Federal facility and then installs or reconfigures the end use equipment to utilize the resource. WWTP gas-to-energy projects can bring immediate and long-term benefits to Federal facilities:
- Energy cost savings.
- Energy security
- When WWTP gas is piped directly to its end use, it provides security from interruptions in the gas and electric grids.
- For facilities that require back-up or standby electricity generation, WWTP gas systems provide the lowest cost while still accommodating a steady base load.
- Utility cost stabilization—Because the WWTP gas resource is obligated under a long-term contract, WWTP systems provide an excellent hedge against fluctuations in fuel and electricity prices.
- Environmental benefits—Significant reductions in greenhouse gas emissions (The methane from wastewater is 25 times more harmful to the atmosphere than carbon dioxide).
- Progress toward Federal goals for use of renewable energy.
Is There a WWTP Gas-to-Energy Opportunity in Your Backyard?
To find out more about the process for using the BAMF Super ESPC to implement a WWTP gas-to-energy project at your facility, please contact the FEMP representative at the DOE Regional Office for your area, or one of the following: Christopher Abbuehl, National Program Representative for the BAMF Super ESPC, at 215-656-6995; Craig Hustwit, BAMF Technical Lead, at 412-386-4532; or Danette Delmastro, FEMP BAMF Team Lead, at 202-586-7632.