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Chapter 1 Evaluation of Combined Heat and Power Technologies for Wastewater Treatment Facilities 1.1 Anaerobic Digestion with Combined Heat and Power One proven means for reducing electric power consumption at POTWs is to use anaerobic digestion to produce digester gas and then use the digester gas as a fuel for the combined production and beneficial use of heat and electrical power. While the use of anaerobic digestion reduces power and/or fossil fuel consumption compared to other accepted sludge processing technologies (e.g., aerobic digestion and thermal drying), the particulars of anaerobic digestion optimization will only be covered peripherally in this document. Instead, given its objective, this document was written with the assumption that interested POTWs either already operate an anaerobic digestion system or have made the decision to implement anaerobic digestion in the future. For most POTWs, use of digester-gas-fueled CHP has the potential to offset energy consumption by up to 40 percent. The overall percentage of energy recovery is a combination of factors including the effectiveness of the digestion process, the efficiency of the CHP system, the type of treatment processes, and the efficiency of the POTW’s liquid stream treatment. Other measures can be undertaken to offset energy consumption even further. Some plants have been shown to be completely energy-self-sufficient. The Strass plant in Austria, the Grevesmühlen plant in Germany, and the Himmerfjärdsverket plant in Sweden are examples of plants that are energy-self-sufficient. A few POTWs in the USA are reportedly close to being fully energy- self-sufficient and occasionally produce more power than is needed. These POTWs include the Point Loma Waste Water Treatment Plant in San Diego, California, and the Joint Water Pollution Control Plant in Carson, California. In the past, and as a general rule of thumb, CHP systems have been thought to be cost-effective only at POTWs with treatment capacities in excess of 10 million gallons per day (mgd). The reason for this can be summed up by economies of scale. While CHP projects, including all required ancillary systems such as digester gas treatment and heat recovery, may require less capital investment and have lower installation costs at small POTWs, total project costs do not scale proportionally with systems at larger POTWs. However, more and more POTWs treating less than 10 mgd are evaluating the potential for CHP systems at their facilities. In fact, a few of these smaller POTWs are successfully operating CHP systems and many would argue that the traditional 10-mgd threshold should be revised to about 4 or 5 mgd. For example, the EPA’s Combined Heat and Power Partnership (CHPP) estimates 5 mgd of wastewater is equivalent to about 100 kW of electric power generation capacity. The gradual change in this paradigm is due in large part to the ability of POTWs to receive and condition alternative feed stocks for their anaerobic digestion systems and, to a lesser extent, making process enhancements to pretreatment and digestion systems. Alternate feedstocks include fats, oils, and grease (FOG), food waste, and process waste from beverage industries. The addition of FOG and other highly digestible waste streams to the anaerobic digestion process can dramatically enhance digester gas production, making CHP systems cost-effective at POTWs with wastewater flows less than 10 mgd. Regardless of wastewater treatment capacity, POTWs operating or considering CHP should evaluate the potential to add FOG and/or other digester feed stocks to the anaerobic digestion process. The increased digester gas production translates directly into cost offsets for electric power and greater heat production for use as process heat. Additional process heat offsets a larger portion of supplemental fuel needs and associated costs. Increased gas production from alternative feedstocks can be achieved without using up valuable digestion capacity because FOG and similar materials are so readily degradable. This document provides guidance for options to implement digester-gas-fueled CHP considering a variety of factors, including: Equipment and maintenance costs Electrical/mechanical efficiency and heat recovery efficiency Gas quality and treatment requirements Emissions performance and requirements. 1-2PDF Image | Combined Heat and Power Technologies for Wastewater Facilities
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