Kinetic analysis of anaerobic digestion of pig manure and its design implications

Abstract The kinetic relationships for the design of anaerobic digesters of pig manure were reviewed. Ultimate methane yield and kinetic constanis for the kinetic equatior proposed by Chen & Hashimoto (1978) were estimated from reported data on the anaerobic digestion of pig manure. The effects of loading rate, influent solids concentration and retention time on methane and net thermal energy production were discussed. The model shows that gross and net thermal energy productions from pig manure are maximized at influent Volatile Solids concentrations of 50 and 55 kg m −3 , respectively. Longer retention times produce higher net thermal energy. A digestion temperature of 55°C produces more net thermal energy than one of 35°C only at short retention times. There is no economic advantage in operating a system at 55°C over 35°C at long retention times. The model also shows that the optimal retention times for net thermal energy production per unit cost for a plant processing 1000 kg Total Solids (TS) of pig manure per day occur at 8·2 and 5·0 days for digestion temperatures of 35°C and 55°C, respectively. Few studies on anaerobic digestion of pig manure have been at retention times shorter than 10 days. Future pilot-scale studies at a retention time less than 10 days are needed.