Modeling and control of WRRF biogas production

Abstract

Wastewater treatment sector uses about 1 percent of total energy consumption in European Union, hence development of energy-efficient digital technologies is an urgent challenge. The aim of this article is to develop energy-efficient control strategies for biogas production from sewage sludge at water resource recovery facilities (WRRF). The case study is developed in collaboration Veas WRRF, Norway. The Veas biogas plant is operated semi-continuously in mesophilic conditions. The process includes inlet sludge pumps, four anaerobic digesters, heat exchangers for sludge heating, pumps for sludge recirculation and a compressor for gas recirculation. The process has two controlled variables, biogas flowrate and digester temperature, the main disturbance is the inlet substrate composition. The manipulated variables are flowrates of the inlet sludge, heating medium, and sludge recirculation. The real semicontinuous operation approximated as continuous operation with two hour moving averaging. Transfer functions were identified from the pre-processed data. The accuracy of the models was sufficient 14−60%. The transfer functions were used to design control strategies with PID-controllers and model predictive controller (MPC). The results show that both control strategies can increase biogas production and decrease variability in controlled and manipulated variables compared to the plant operation. MPC gave the best results, increasing biogas production up to 10 % and decreasing variability in controlled variables by 50−80% and by 92−99% in manipulated variables. These results indicate that implementation of advanced control technologies can improve the energy efficiency of biogas production.