Chapter 15 - Sustainability and Stability

Abstract

Is a sustainable system stable? One of the important aspects of reactor design is to seek desirable stable steady state reaction conditions as the reactor set point. Stable steady state is a subset of sustainable states. A sustainable system can be weakly stable, exhibiting limit cycle or oscillation between bound and nontrivial values. The stability of a reactor operation is usually associated with the existence of multiple steady states (MSS). MSS could exist for nonisothermal exothermic reactions (of any kinetics) and isothermal substrate-inhibited systems in a CSTR. Catalyst instability is another important issue. Catalyst deactivation is affected by the reaction mixture, temperature, and flow conditions. The simplistic root for bioinstability is the difference in specific growth rate (for mixed culture) and mutation. Biostability is of concern for genetically engineered cells. Populations containing multiple species are important in natural ecosystems, well-defined processes, wastewater treatment, and systems using genetically modified cells. Some examples of interactions among these species are competition, neutralism, mutualism, protocooperation, commensalism, amensalism, predation, and parasitism. Neither pure competition nor pure mutualism gives a stable steady state in a chemostat. The predator and prey model shows that while there are no stable steady states, the bioprocess can be sustainable and the final prey and predator populations oscillate in a confined cycle. Continuous culture can be employed to screen organisms of certain traits based on the growth instability. Spatial heterogeneity, dynamic fluctuations, and the addition of other interactions can lead to the sustained coexistence of species with competitive or mutualistic interactions. One of the major process uses of mixed cultures is wastewater treatment. The activated sludge system is commonly employed in treating wastewaters.