With cogeneration you would like to use a fuel that is usally wasted ,as in methane from human waste. While using that fuel to run large generators,  you provide electricity to run the plant as well as use the heat from the engine blocks as well as the exhaust to heat the digestion tanks that provide the methane and break down the solids further. You can also use the heat to provide heat through the buildings around the plant. If you used an accessory drive you could also run a compressor to run plant air supply and or another compressor to run air conditioning for the buildings. All while using normally wasted waste. 

Cogeneration or triphase generation is the optimum use of a primary fuel e.g methane to generate electrical power. Use the waste heat for heating or process from the engine block and recover the heat contained in the exhaust gases to generate steam or medium temperature hot water. The steam or heat from the engine can also be used to generate chilled water using an absorption chiller. The economics of the use of the waste heat depends on the primary cost of the power and gas in this instance and the efficiency of any chilling compressors versus the absorption chiller.