(580c) Increased Operational Flexibility and Robustness of Anaerobic Wastewater Processes Via Bioreactor Coupling

Anaerobic wastewater purification processes have been increasingly used in the recent years. Anaerobic digestion is a biological process in which organic matter is transformed by a community of microorganisms into biogas (primarily methane and carbon dioxide) in the absence of oxygen. This process appears to be a promising method to solve some energy and ecological problems in agriculture and industry. Advantages this process has over the other common waste treatment process, the aerobic activated sludge process, include (i) much lower energy requirement; (ii) generation of less sludge, curtailing thereby the cost of energy-intensive post-treatment and disposal; and (iii) generation of biogas, which can be used as an energy source. In the past few years, a number of experimental studies aimed at improving the performance of anerobic digestion and kinetic models for the process have been reported. The process involves a consortium of acid generating bacteria (host population) and methane generating bacteria (commensal population). The initiation of this process requires hydolysis of complex insoluble organics by extracellular enzymes synthesized by the host population to produce simple soluble organics, which can be utilized by the host population. A comprehensive analysis of static and dynamic behavior of a mixed culture of involving acidogens (X) and methanogens (Y) in two coupled bioreactors is presented here. The two-bioreactor system is considered here to make the activated sludge process more robust with respect to wide fluctuations in feed conditions. A single continuous culture may operate at up to seven steady states, including up to four coexistence steady states, with only one coexistence steady state being locally stable. The one-way interaction between X and Y allows for compartmentalization of the system for a stand-alone bioreactor and two coupled bioreactors into two subsystems, which facilitates the analysis of steady state types and stability characteristics of these and classification of dynamic behavior. The bioreactors in the two-reactor system are identical only in terms of feed composition and bioreactor space time. The reactors need not have identical volumes and volumetric feed, effluent, and exchange rates. A two-reactor system may admit up to forty nine steady states, which are comprised of up to forty coexistence steady states, at least at very low interaction rates. The interaction rates between the two bioreactors, R₁ and R₂, are considered to be unequal. The steady states are comprised of symmetric steady states and asymmetric steady states. The symmetric steady states correspond to identical composition in the two bioreactors. The remaining steady states, the asymmetric steady states, correspond to different composition in the two bioreactors and are admissible over a range of interaction rates. The static and dynamic analysis of the two-reactor system is facilitated by appropriate grouping of large number of steady states arising for very low R into nine clusters. Numerical illustrations reveal the rich steady state structure of the bioprocess in coupled bioreactors. While a single bioreactor can operate at only one locally stable coexistence steady state, the coupled bioreactors can operate at up to five locally stable coexistence steady states over certain ranges of R₁ and R₂. The two-reactor system is operationally more flexible and more robust vis-a-vis single reactor as concerns maintenance of mixed culture. Emergence of four additional steady state clusters and additional coexistence and partial washout steady states at intermediate values of interaction rates reveals that the coupled bioreactors are an example of a complex system.