Biogenic and fossil carbon in sewage sludge and digester gas determined by isotope investigation

Abstract

Sewage sludge and digester gas are used as fuels in various installations including those participating in the European Emissions Trading System (ETS). Monitoring of the emissions from such fuels shall include all carbon dioxide from fossil sources. We analysed the distribution of biogenic and fossil carbon and potential dependencies from available data on the wastewater input characterisation based on a number of samples of sewage sludge (20) and digester gas (14) from a wide variety of municipal wastewater treatment plants (WWTPs) in Germany. The biogenic carbon content of the sewage sludge and digester gas samples were determined by analysing the carbon-14 (14C) content using a liquid scintillation counter (LSC) after combustion to CO2. CO2 already present in the digester gas samples was included in the analysis as this CO2 also originates from the degradation of the contents of sewage. Based on the results, we suggest the input to municipal WWTPs should be characterised by the share of sewage from industry compared to total water input expressed as inhabitants equivalent. The results of the study show that if the share of sewage from industry is below 45%, sewage sludge contains about 76% biogenic carbon while the respective digester gas contains about 83% biogenic carbon. The sewage from municipal WWTPs with higher percentages of industrial wastewater (>/= 45 %) can show significantly smaller proportions of biogenic carbon. Biogenic carbon content of about 28% to 71% were determined in sewage sludge from such wastewater plants, while the respective digester gas contained about 11% to 88% biogenic carbon. The origin of the respective carbon content was not investigated. Sources like cleaning agents and detergents and other persistent synthetic substances among others, were considered as contributors to the fossil carbon in the sewage. Wastewater from industries processing chemicals etc. and fossil fuels in particular, are responsible for higher proportions of fossil carbon in the sewage sludge and digester gas. Wastewater from food processing, paper, gastronomy and the hotel sector all show the same proportion of biogenic carbon as domestic sewage. Quelle: Forschungsbericht

The aim of the study was to conduct an initial analytical survey of a small number of samples analyze the proportion of fossil carbon in sewage sludge and digester gas and to correlate it with other data collected in a sewage treatment plant.A conservative but representative standard value of 80% biogenic carbon could be derived for sewage treatment plants if they receive less than half of the load from commercial sources – despite continued significant fluctuations. This value, which corresponds to a value derived from Article 31 (1 c) MRR can be used as a standard value for sewage sludge and digester gas from sewage treatment plants with predominantly municipal wastewater.