Advantages and limits of co-digestion in wastewater treatment plants – comparison to co-digestion in agriculture

The Master Project offers a survey on several aspects of co-digestion in wastewater treatment plants (WWTP) and in agriculture. The description of the legal context and the technical processes present the basis for the subsequent analysis of the co-digestion at the WWTP in Bern and in four agricultural installations. For the ecological evaluation a material flow analysis was made to determine the nutrient loss due to co-digestion in WWTP. In addition, the energy and CO2 balances were established to understand environmental impacts. For the economical evaluation financial indicators were calculated in order to judge profitability and efficiency of the installations. The social impacts analyzed include worker security, financial risk, employments and regional economy. Considering the loss of nutrient, a threshold for co-digestion in WWTP is proposed: if energy loss by the nutrients exceeds the net energy production of a co-substrate, it should not be digested in WWTP. The study shows that sustainability of co-digestion does not depend on the sector but on the modalities of each plant. The main factors determining sustainability are size and degree of capacity utilization of the plant, the degree of utilization of produced energy, transport distance of co-substrates, and, in agriculture, the storage and deploying of the digestion end product.