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 (CBFT3) 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 CBFT3 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.
CWW has led the development of a Class A digestion process named CBFT3. The CBFT3 process will allow CWW to improve the quality and minimize the amount of biosolids generated from the SCWRF and, consequently, reduce land application, landfill, and disposal costs.
The CBFT3 process is a two-stage, thermophilic process consisting of anaerobic continuousfeed, mix digestion followed by anaerobic plug-flow reactors. The CBFT3 process is used to meet the pathogen reduction requirements of 40 Code of Federal Regulations (CFR), Part 503 under Alternative 6: Qualification as an Equivalent to a Process to Further Reduce Pathogens (PFRP). Mesophilic anaerobic digestion is provided downstream of the CBFT3 process to ensure vector attraction reduction (VAR) requirements.
Extensive bench- and pilot-scale testing has shown the ability of the CBFT3 process to meet the Class A requirements for pathogen reduction. CWW received a patent in June of 2005 for the process and use of a plug-flow reactor in conjunction with a thermophilic digester to produce Class A biosolids. That patent was subsequently presented by CWW to the Water Environment Research Foundation (WERF) in October 2005 to hold in the public domain for the use of all wastewater utilities.
The next step in the development of the CBFT3 process is the advanced demonstration project, which will be used to validate the performance of the full-scale system to meet Class A biosolids standards. Furthermore, the advanced demonstration project will include the green power component that uses digester gas to produce electricity and recover waste heat.