Modelling and Simulation of Anaerobic Digester for High Organic Strength Waste

High organic strength waste such as wastewater generated from various industrial activities and sludge generated from industrial and municipal wastewater treatment plant require treatment prior to disposal in order to reduce environment pollution load and also to comply with disposal norms. Waste containing organic impurities in nature must be treated biologically using life forms such as microorganisms, plants etc. Anaerobic digestion is the most suitable treatment method for treating high organic strength waste as compare to other waste treatment methods, because energy in the form of methane is recovered and also treated sludge has good soil conditioning values. Complete mix anaerobic digester is more suitable for treating high organic strength waste as compare to other because of proper heat transfer, proper mixing for maintaining uniformity throughout the reactor. Complete mix reactors are resistant to shock loading and also withstand high organic loading rates. Anaerobic digester suffers stability problem due to accumulation of volatile fatty acids and drop in pH. Once the failure of digester takes place it requires long time and efforts to restart it again. Dynamic modelling and simulation are useful tools for predicting the process stability by studying the behaviour under transient conditions and are also helpful in understanding the process operations. In the present work attempt has been made to develop simplified dynamic mathematical model for the anaerobic digestion of the sludge. The inhibitory function developed by Andrews (1969) given below was an important investigation considering the inhibitory effect of the unionized volatile fatty acid on methanogen bacteria. The unionized volatile fatty acid act as growth limiting substrate at lower concentration and cause inhibition at higher concentration for microorganism utilizing them as substrate. The inhibition model was represented where μ Specific growth rate (day -1 ) û Maximum specific growth rate in the absence of inhibition (day -1 ) HS Unionized substrate concentration (mg/L) Ks Saturation constant (mg/L) Ki Inhibition constant (mg/L) The model was restricted to fixed pH and only methanogenesis step was considered. Andrews et al. (1971) removed the limitation of fixed pH by considering the interaction between liquid, gas and biological phases and pH variation took place between 6-8. The buffering system was carbon dioxide – bicarbonate. The model was also restricted to methanogenesis step. Hill et al. (1977) developed model for animal waste digestion based on two microbial culture acidogens and methanogens. They also considered the inhibition caused by unionized volatile acid and unionized ammonia on growth kinetics of the methanogens.