Supervisory control of an anaerobic digester subject to drastic substrate changes

Abstract

Anaerobic digestion (AD) is a green technology that has been applied for many years. One of the main problems in this process is controlling these bioreactors to maximize methane production. A supervisory control strategy has been proposed to improve the methane production rate in an anaerobic digestion process while minimizing the risk of process failure in the presence of several drastic feedstock changes. The inner loop consisted of a feedback control that manipulated the feed flow rate for achieving the desirable methane production rate. A rule based control strategy was used as supervisory control loop. This controller received the total volatile fatty acids concentration in the reactor and the trends of methane production rate to calculate the set-point of the inner control loop. pH was used as a safety control parameter to prevent the system from acidification. The proposed control scheme was tested in the presence of several disturbances in the feedstock, including glucose overload, ammonia inhibition, substrate dilution and shifting feeding from manure to bio-pulp. The controller prevented VFA accumulation above 2.73, 2.94 and 4.09 g L−1, under glucose overload, ammonia inhibition and change of feedstock to bio-pulp, respectively. Thus, the supervisory control could successfully improve the methane production rate and keep the reactor stable against external severe disturbances.