The United States Environmental Protection Agency (EPA) and the Water Environment Research Foundation (WERF) recognize the value and importance of implementing combined heat and power (CHP) at wastewater treatment facilities (WWTFs). To promote the benefits of CHP and encourage its implementation, the EPA commissioned the development of a CHP guidance document. The EPA-funded document provides guidance to WWTF staff and consulting engineers on the best technologies to maximize the use of digester gas to produce electrical energy and capture waste heat for beneficial use. The document considers a variety of factors to aid utilities in selecting the most appropriate CHP technology for their WWTF, including plant size, emissions requirements, heat uses, gas treatment requirements, and cost.Also in an effort to encourage more widespread implementation of CHP at WWTFs, WERF commissioned the development the Combined Heat and Power System Evaluation Tool (CHP-SET). The CHP-SET is a spreadsheet-based calculator designed for evaluating CHP system performance and is intended for use by utilities already operating CHP systems. The CHP-SET calculates total system efficiencies (inclusive of appurtenant equipment electrical demands) for the production of electricity and collection of heat. The tool also provides a conversion of exhaust emissions (NOx, CH 4 , CO 2 , CO, and N 2 O) into units of mass per unit of net energy output.The CHP-SET was used to evaluate and document the performance of several existing CHP systems ranging in plant size from 41,600 to 1.14 million m 3 /d (11 to 300 mgd) of capacity. Detailed case studies based on CHP system evaluations using the CHP-SET are included in the EPA document.This paper provides an overview of the EPA and WERF projects, but focuses primarily on the case studies developed using WERF's CHP-SET and subsequently published in "Evaluation of Combined Heat and Power Technologies for Wastewater Facilities". KEYWORDSCombined heat and power, sustainability, energy efficiency, power generation, digester gas.

Organic silicon polymers (siloxanes) are increasing in use and are increasingly being detected at municipal sewage treatment facilities and landfills. Several forms of siloxanes are volatile and have been detected in gas evolved from the anaerobic digestion process at many publicly owned wastewater treatment plants (WWTPs). The presence of siloxane compounds in digester gas used to fuel internal combustion engines that generate electricity and heat power (cogeneration) causes severe engine maintenance problems that are related to silicon deposits in piston cylinder and valve head assemblies. At the Annacis Island WWTP in Vancouver, British Columbia and at the Alvarado WWTP in Union City, CA, siloxane related engine maintenance problems lead to decisions to terminate digester gas fueled engine operations.Both plants were motivated to quickly identify and implement systems to mitigate siloxane related engine maintenance problems. After quick review of available options, both plants implemented digester gas treatment systems consisting of gas dehumidification followed by activated carbon adsorption. Activated carbon systems were simple and relied on replacement of exhausted carbon as opposed to on-site regeneration. Activated carbon was extremely effective at eliminating siloxanes from digester gas and carbon life was as expected or better. Removal of siloxane resulted in reduction of build up of silicon in engine oil and dramatic improvements in engine operation as well as reduction in maintenance costs. Maintenance cost reductions greatly exceeded the cost of operating the siloxane removal system.

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