Mathematical modeling of the competition between sulfate reducing, acetoclastic and methanogenic bacteria within multispecies biofilms

Increasing anthropogenic activity has contributed to local imbalances in the natural sulfur cycle, leading to serious environmental problems. Industrial wastewater containing sulfate has contributed to this sulfur imbalance. Biological sulfate reducing processes that involve a bacterial biomass attached to media (biofilm), represent an attractive solution to the problem. The advantage of bacteria disposing in a biofilm is very important in an environmental industrial application, as the bacteria in the biofilm, different from suspended bacteria, cannot be washed out with the water flow. This allows to retain the biomass within the reactor and therefore to operate at shorter hydraulic retention time (HRT),and higher biomass concentration . Biological sulfate reduction in anaerobic fixed growth reactors has been investigated extensively at lab-scale. Under anaerobic conditions dissimilatory sulfate reducing bacteria use sulfate as a terminal electron acceptor for the degradation of organic compounds. In this anaerobic process, sulfate is reduced to sulfide by the action of sulfate reducing bacteria (SRB), which have the ability of coupling the oxidation of organic matter (electron donor) to the reduction of sulfate (electron acceptor) and depend on hydrolytic and fermentative bacteria that degrade complex organic matter. A major problem of sulfate-reducing fixed-growth reactors is the formation of undesired bacteria species which compete for space and substrate in the biofilm with SRB. This work presents a mathematical model able to simulate the physical, chemical and biological processes prevailing in a sulfate reducing biofilm under dynamic conditions. The proposed model includes sulfate reduction by complete and incomplete SRB; COD (lactate) removal by sulfate reduction and by acetogenic bacteria; acetate consumption via methanogenesis. The method of characteristics is used for the numerical resolution of the model equations. In particular the effect of the COD/SO42- ratio and the effect of different simulation times on the reactor performances in terms of bacterial species distribution and substrate diffusion trends in the biofilm have been assessed.