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Growing concern over the cost of power and long-term availability of limited fossil fuel resources for the production of electricity have caused electrical utilities and governments to promote "green" or renewable power. Solar, wind, geothermal, biomass, biogas, and low-impact hydroelectricity are current acceptable green-power sources.Digester gas is a renewable, green energy resource that has been used in wastewater treatment plant (WWTP) engines since the 1930s. In the 1980s, many WWTPs added cogeneration with rich-burn engines. In the 1980s and 1990s, utilities either converted their rich-burn engines to lean-burn engines or installed new lean-burn engines to meet air quality requirements.Because digester gas is of finite supply and is dependent on operating parameters such as sludge feed and volatile solids destruction, it is desirable for WWTPs to maximize the efficiency of electricity generation and beneficial reuse of otherwise wasted heat. Recently, a number of projects have used innovative cogeneration technologies, such as fuel cells, gas turbines, microturbines, and Stirling Cycle engines, to harness the energy of digester gas. In addition, advanced reciprocating engine systems (ARES) are currently being developed as another cogeneration technology under an initiative sponsored by the United States Department of Energy (USDOE) and U.S. National Laboratories with three reciprocating engine manufacturers.Columbus Water Works (CWW) is currently evaluating the use of ARES engines for combined heat and power (CHP) generation for its Class A biosolids process named Columbus Biosolids Flow-Through Thermophilic Treatment (CBFT 3 ) at the South Columbus Water Reclamation Facility (SCWRF) that currently treats an average flow between 30 and 35 million gallons per day (mgd). The use of ARES engines as part of the CBFT 3 project would represent one of the lowest capital cost, highest net efficiency CHP technologies. The project is expected to provide a payback between 4 to 7 years, with an even shorter payback period depending on the degree of federal funding secured and avoided capital offsets assumed.Other innovative features of the project include the addition of grease trap waste to the digestion process to increase gas and power production, digester gas pretreatment using multiple unit processes, and heat recovery systems. 3570 WEFTEC®.06

Brown and Caldwell and Columbus Water Works (CWW) located in Columbus, Georgia, have led the development of a new Class A digestion process named Columbus Biosolids Flow-Through Thermophilic Treatment (CBFT 3 ). The new process offers a low-cost option -as compared to other EPA approved processes -for upgrading existing anaerobic digestion facilities from Class B to Class A Biosolids. To achieve Class A pathogen reduction, an existing completely-mixed mesophilic digester was converted to operate at thermophilic temperature and is followed by two plug flow reactors (PFRs) operated in series to achieve a minimum contact time of 30-minutes. Sludge is then further digested in the two existing mesophilic digesters to achieve additional volatile solids destruction.This project also incorporates a combined heat and power (CHP) or cogeneration system that will generate about 40-percent of the plant's electricity demand as renewable green power using digester gas that would otherwise be flared. Heat recovered from the cogeneration system will be used to heat the digestion process in its entirety. Construction of the CBFT 3 and Cogeneration System Project is complete and process start-up activities are underway. This paper presents a summary of project goals and benefits, background information regarding project research and engineering evaluation results, and a description of the construction elements and facility improvements.

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