(18b) Model Based Monitoring and Control of a Cstr-Type Anaerobic Digester

The process of anaerobic digestion is commonly used for sludge digestion in sewage treatment plants, for high organic load industrial wastewater treatment and for biogas production out of energy crops. One of the major issues addressed is maximizing the biogas production rate for energy purposes. At maximum biogas production conditions the operation of a CSTR digester becomes very sensitive to disturbances (such as organic overload or underload, entry of an inhibitor etc), leading many times to washout of the sensitive methanogenic biomass.

A mesophilic (35oC) CSTR-type 3 liter digester was constructed and properly equipped with the necessary monitoring and control equipment. The reactor temperature was maintained constant through an external jacket. The digester was started up with an anaerobic mixed culture from a wastewater treatment plant. A synthetic glucose-based feed was pumped to the reactor through a computer-controlled peristaltic pump. For monitoring of the process, a LabView monitoring environment was developed. The biogas production rate was measured in discrete time intervals using a standard volume device which was recording the time needed for a given volume of biogas to be generated. During these intervals the proportion of the methane in the biogas was also measured using an infrared ion detector.

The reactor was operated in a sequence of retention times, and the dynamic responses were followed until steady-state was reached. A two-state model (biomass and the limiting substrate as state variables) was developed for monitoring and control purposes. The model parameters were estimated from the steady-state data.

A nonlinear observer was then developed based on the exact error linearization method for the calculation of the observer gains. The observer was used for estimating the unknown states and parameters that cannot be measured and detection of the potential presence and magnitude of a disturbance. These disturbances could either be a change in the organic feed concentration, a change in the feed flow rate or entry of an inhibitor, which would be responsible for a reduction in the maximum specific growth rate of the bacteria. The ability of the observer to properly identify the type and magnitude of the imposed disturbance was demonstrated experimentally.

In the sequel, a control policy near optimal steady state, was approached through proportional output feedback control with a linear control law previously described in [1], and further examined for discrete time systems in [2] and [3]. The proposed control law globally stabilizes the process around the optimal steady state without leading to washout of the biomass. The robustness of the control law was also studied. The control law was extremely robust to variations of certain parameters of the model used.

References

[1]L. Syrou, I. Karafyllis, K. Stamatelatou, G. Lyberatos and C. Kravaris, Robust Global stabilization of continuous bioreactors, Proceedings 7th International Symposium on Dynamics and Control of Process Systems (DYCOPS 7), Cambridge , MA, July 2004.

[2]I. Karafyllis, G. Savvoglidis, L. Syrou, K. Stamatelatou, C. Kravaris and G. Lyberatos, Global stabilization of continuous bioreactors, Paper 15C08, EA64484, 2006 AIChE Annual Meeting, San Francisco, CA.

[3]I. Karafyllis, C. Kravaris, L. Syrou and G. Lyberatos, A vector Lyapunov function characterization of Input-to-State stability with application to robust global stabilization of the chemostat, European Journal of Control, Vol. 14, 2008, p. 47-61.