Developing Process Models to Accurately Assess Global and Energy Performances of a WWTP Sludge Line: A Case Study in France

Mathematical models for the sludge line of a full-scale WWTP were developed with the aim of being integrated in a whole-plant model. A model describing the primary clarifier was also included. Reconciled data from plant routine sampling over a 2-year period together with data from dedicated campaigns were used to develop and calibrate the models. Different empirical correlations were identified to describe solids removal in primary settler, primary and activated sludge thickening, dewatering and drying, avoiding the assumption of constant removal efficiency. Analyses showed that characteristic particulate ratios (CODP/TSS, P-org/TSS, N-org/TSS, VSS/TSS) remain constant during thickening, dewatering and drying processes. Therefore, in the implemented model these ratios were used for predicting concentrations of particulate compound in the process outputs. Mass balances of COD, N and P were completed by assuming that concentrations of soluble compounds are conserved. An extended version of ADM1 was implemented to simulate the full-scale digester fed with primary and activated sludge. A complete influent characterisation including Biochemical methane potential (BMP) tests was performed to provide an ADM1 compatible substrate fractionation Additionally, energy models for each unit process were determined through regression analyses to fit energy consumption with operating parameters. An energy audit of the plant was conducted to assess the energy consumption of each process. Electric energy, the consumed biogas as well as other energy sources were considered in the analyses. Single models were assembled to build the sludge line model that will be used to obtain reliable predictions for reject water composition, biogas production, dry sludge and energy consumption.