Sewage sludge treatment: evaluation of the energy potential and methane emissions with COD balancing

Sewage sludge is an important resource for the generation of electricity and heat within a wastewater treatment plant (WWTP). Taking a holistic approach to such use involves considering the greenhouse gas emissions. In particular, for anaerobic sludge treatment, methane emissions are a matter of concern. Therefore, the utilization of a carbon balance, based on the chemical oxygen demand (COD), will be a focus for evaluating the magnitude of methane losses within the sludge treatment. In addition to biogas production and use in combined heat and power plants (CHPs), dissolved methane in digested sludge, as well as the methane slip, have to be considered. Measurements of dissolved methane concentrations in sewage sludge from digesters of various WWTPs show a concentration of about 17-37 mg CH4/L. The COD balance indicates a ratio of the methane emissions (methane slip, dissolved methane and residual gas) of < 4% of the total methane production during digestion. Considering the electricity generation by CHPs and the higher global warming potential of methane, compared to carbon dioxide, emissions of approximately 300 g CO2-equ./kWh(el) result, which is in a similar range to greenhouse gas emissions caused by electricity generation by fossil fuels.