An optimised control system to steer the transition from anaerobic mono- to co-digestion in full-scale plants

Abstract

Traditional wastewater treatment plants (WWTPs) are electrical consumers, with a usual high demand in the range of 0.3 to 0.6 kWh/m3 of wastewater treated. Their digesters are commonly oversized, and consequently operated at low organic loading rates (OLRs). This opens a great opportunity for anaerobic co-digestion (AcoD) as an interesting technology to increase methane productivity and the electrical self-production in WWTPs. However, there is a quite limited implementation of AcoD at full-scale plants, since the transition from mono- to co-digestion and the further AcoD optimisation is a crucial and delicate step that could lead to the inhibition of the process if not thoroughly controlled. In this study, a methodology based on an optimum control strategy is explained in detail and it was applied to safely and optimally steer the transition from mono- to co-digestion and to maximize methane production during AcoD. A lab-scale anaerobic digester of 14 L mimicking the full-scale one (3,500 m3) was operated 30 days in advance to anticipate and if needed correct any operational destabilization that might occurr. As a result, the treatment of sewage sludge with two co-substrates (coming from a pig slaughterhouse and from a frying industry), which accounted for just 11% of the feeding flowrate, at a hydraulic retention time of 20 days allowed to raise the OLR and the methane production by 2-fold and 3-fold, respectively, increasing the self-produced electricity from 25% to 75% of the total demand of the WWTP. The diagnosis indicators proved to be accurate to take decisions concerning wastes blending and the strategy of increasing OLR. Besides, the proposed control system provides the steps to ensure a safe transition from anaerobic mono- to co-digestion and further optimisation at full-scale plants