(756f) Robust Stabilization of a Two-Stage Continuous Anaerobic Bioreactor

Continuous anaerobic digestion processes cover a wide range of applications in biochemical engineering and environmental technology, such as the anaerobic water treatment and biogas production. The biogas produced via this process, which consists of mainly methane and CO₂, serves as an important alternative renewable energy source. Compared with other biochemical production processes from biomass, the anaerobic digestion process with production of biogas has shown its benefits of robustness to a wide variety of substrates, potential incorporation with carbon capture, minimal safety requirements and low cost. Meanwhile, due to the constantly changing microbial environment and the system’s marginally stable property at moderately high dilution rates, there are major challenges in the dynamic operation and control of the process.

Traditional operation of anaerobic digestion involves a single stage, where a large number of microbial populations with varying characteristics grow in the same reactor. To enhance the system stability and product quality, the two-stage process, consisting of two continuous bioreactors connected in series, with different reactions occurring in separate stages, has been proposed in the literature [1]. In this way, it is possible to achieve: 1) higher yield, 2) higher methane concentration in the output gas, and 3) a more flexible gas production [2][3]. However, the stable region around the operating point is still narrow when it is operated at moderately high dilution rates to maximize product yield and purity. Consequently, it is likely to lead to the washout of biomass when disturbances occur. Therefore, a controller is needed to enlarge the size of the stability region around such optimal operating conditions. There is a large body of literature on control algorithms for stabilization of traditional one-stage processes, e.g. [4][5][6]. However, there are currently no research results on the control of two-stage anaerobic processes.

In this work, we study the problem of robust stabilization of a two-stage anaerobic bioreactor. The steady state properties of the system are first investigated to show that the need for control is due to the fact that the optimal steady state is extremely close to an unstable steady state and the stability region is in need of enlargement. The control problem is rigorously tackled based on a three state approximate dynamic model, and later applied to a more detailed seven state model to test its performance. With dilution rate as an input and methane production rate and VFAs concentration as outputs, a constant-yield nonlinear controller first derived based on intuitive considerations. A Lyapunov function approach is used to prove the stability of the closed-loop system over the entire positive orthant. The robustness properties of the control law are also studied. The control law is proved to be robust to both errors in the kinetic model parameters and disturbance in the inlet feed concentration, in the sense of preserving the stability region of the system. Performance of the control system can be even enhanced if a feedforward measurement of the inlet feed concentration is incorporated in the control law.

References:

1. Pohland, F. G., and S. Ghosh. "Developments in anaerobic stabilization of organic wastes-the two-phase concept." Environmental letters 1.4 (1971): 255-266.

2. Parawira, Wilson, et al. "Energy production from agricultural residues: high methane yields in pilot-scale two-stage anaerobic digestion." Biomass and Bioenergy 32.1 (2008): 44-50.

3. Salomoni, C., et al. "Enhanced methane production in a two-phase anaerobic digestion plant, after CO₂ capture and addition to organic wastes." Bioresource technology 102.11 (2011): 6443-6448.

4. Syrou, L., et al. "Robust global stabilization of continuous bioreactors." 7th International Conference on Dynamics and Control of Process Systems (DYCOPS 7), Boston, MA. 2004.

5. Savoglidis, Georgios, and Costas Kravaris. "Constant-yield control of continuous bioreactors." Chemical engineering journal 228 (2013): 1234-1247.

6. Gouze, Jean-Luc, and Gonzalo Robledo. "Feedback control for competition models with mortality in the chemostat." Decision and Control, 2006 45th IEEE Conference on. IEEE, 2006.