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

Two anaerobic digestion schemes, namely mesophilic and thermophilic pr ocesses, 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 di sturbance variables. The manipulated variables are sewage sludge influ ent rate Q and specific heat addition rate G(u). The controlled variab les are effluent substrate concentration S and digestion temperature T . A control system including a proportional-integral (PI) controller a nd variable groups is proposed. Multivariable steady state methods suc h as relative gain array, Niederlinski stability criterion, singular v alue decomposition and Morari integral controllability are employed. S everal 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 m ultivariable analysis show that thermophilic anaerobic digestion is mo re favourable in terms of speed of response and disinfection capabilit y. It also maintains dynamic and steady state stability if controlled properly.