AcoD in WWT models - Arnell and Åmand 2013-10-14

Co-digestion of sludge with external substrates at wastewater treatment plants is a common means to use residual wastes, utilise available capacity in the digester and boost biogas production. The Benchmark Simulation Model No. 2 (BSM2) is a platform for comparing plant-wide control strategies and includes among other things a model of the activated sludge process and the digester at a treatment plant. This report reviews the state of knowledge on possibilities and challenges to include codigestion in the BSM2 model. The report also includes a survey of the hydrolysis coefficient for commonly used substrates at treatment plants in Sweden by using nonlinear parameter estimation fitting models to Biomethane Potential (BMP) test data. Since the ADM1-model is COD based the substrates have to be divided into state variables in terms of COD. All the methods available in literature involve some sort of COD analysis on the substrate, and the way to proceed would be different depending on the solids concentration of the substrate. It is recommended that each substrate is characterised individually and that the hydrolysis step is kept virtually separate for each of the substrates in the BSM2 model. The study has found that the present hydrolysis coefficient in BSM2 (10 d-1) needs to be updated and a more reasonable value is around 0.2 d-1 for mixed sludge. The coefficient for food waste is similar to that of mixed sludge.