DYNAMICS AND MULTIVARIABLE CONTROL ANALYSIS FOR ANAEROBIC-DIGESTION

The stabilization of wastewater sludge by anaerobic digestion has been analysed by developing a multivariable control strategy. The dynamic model of the process was solved by digital simulation. The process variables have been classified into controlled, manipulative, disturbance and inventory variables. The dynamic model was linearized into a state space model, from which a frequency response model was obtained. This model was analysed in terms of interaction of variables and integral controllability. A control system has been developed which manipulates heat addition and inflow so as to control digestion temperature and total organic carbon respectively. It has been found that the stability of the system and its integrals are controllable. Furthermore the control system satisfies the major objectives of digestion, namely sludge stabilization and steady biogas production. The dynamic characteristics of thermophilic digestion are compared with those of mesophilic operation. While the stability and interaction properties were found to be comparable for the two processes, thermophilic operation has a shorter recovery time after upsets. © 1990.