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

Traditional wastewater treatment plants (WWTPs) are electrical consumers, with a usual high demand in the range of 0.3 to 0.6 kW h per m3 of wastewater treated. Their digesters are commonly oversized, and consequently operated at low organic loading rates (OLRs). This provides a great opportunity for anaerobic co-digestion (AcoD) as an interesting technology to increase methane productivity and electrical self-production in WWTPs. However, there is a quite limited implementation of AcoD in full-scale plants, since the transition from mono- to co-digestion and further AcoD optimisation are crucial and delicate steps 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 (3500 m3) was operated 30 days in advance to anticipate and, if needed, correct any operational destabilization that might occur. 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 the increase of 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 make decisions concerning waste blending and the strategy of increasing the OLR. Besides, the proposed control system provides the steps to ensure a safe transition from anaerobic mono- to co-digestion and further optimisation in full-scale plants.