Dynamic model reduction for two-stage anaerobic digestion processes

- Modeling of two-stage anaerobic digestion is studied and compared to one-stage.
- Two-stage operation could lead to higher product purity under optimal conditions.
- Two-stage operation could not lead to significant enlargement of stability region.
- Invariant manifold methods and observability analysis are used for model reduction.
- Reduced model is of only 3rd order, but in excellent agreement with original model.

This paper deals with the development, analysis and reduction of a two-stage anaerobic digestion process model. A two-stage model based on a two-step mass-balance model is first developed. The model incorporates two stages as two similar CSTRs, with acidogenesis in the first stage and mainly methanogenesis in the second. Steady state optimization validates the benefits of two-stage operation compared to the traditional one-stage process. Using reaction invariants, the dynamics of the fast modes is neglected by projecting the overall system dynamics on the slow-motion invariant manifold, and the original 7-state model is reduced to a 5th order system. By neglecting the nearly unobservable modes, the model is further reduced to 3rd order. Simulation results show excellent accuracy of the reduced model compared to the detailed one. At the same time, they indicate the necessity of process control for stabilization of the system.