COMPARISON BETWEEN DYNAMICS AND CONTROL PERFORMANCE OF MESOPHILIC AND THERMOPHILIC ANAEROBIC SLUDGE DIGESTERS

Two anaerobic digestion schemes, namely mesophilic and thermophilic processes, have been analysed for process stability and controllability. As in any control problem, three types of variable are distinguished in this study, i.e. disturbance, manipulated and controlled variables. Substrate concentration S(i) and influent temperature T(i) are the disturbance variables. The manipulated variables are sewage sludge influent rate Q and specific heat addition rate G(u). The controlled variables are effluent substrate concentration S and digestion temperature T. A control system including a proportional-integral (PI) controller and variable groups is proposed. Multivariable steady state methods such as relative gain array, Niederlinski stability criterion, singular value decomposition and Morari integral controllability are employed. Several dynamic analyses such as biggest log-modulus tuning, robustness analysis and Tyreus load rejection criterion are carried out. Steady state analysis results are used for variable pairing. The results of multivariable analysis show that thermophilic anaerobic digestion is more favourable in terms of speed of response and disinfection capability. It also maintains dynamic and steady state stability if controlled properly.