COMPARATIVE EVALUATION OF THERMOPHILIC & MESOPHILIC ANAEROBIC DIGESTION OF MUNICIPAL SLUDGE

The purpose of this research was to evaluate the relative merits of mesophilic and thermophilic digestion of municipal sludge in terms of polymeric conversion efficiency, process kinetics, gas productivity, and effluent quality. High-rate digestion experiments were conducted with primaryactivated sludge mixtures at 35° and 55°C at HRTs of 15, 7, and 3 days. The thermophilic cultures were developed by raising the temperature of the mesophilic inoculum to 55°C in one step. The thermophilic runs exhibited higher gas production rate, and gas and methane yields at all HRTs and loading rates of 2.1 - 7.3 kg VS/m3-day. Thermophilic effluents had higher pHs (7.5), alkalinities, and ammonia-nitrogen and VFA concentrations than mesophilic effluents under all operating conditions. The thermophilic process effected higher lipid conversion but lower carbohydrate reduction than the mesophilic process at all HRTs. However, total volatile solids reductions under thermophilic temperature were higher than those under mesophilic conditions at all test HRTs. Thermophilic effluents had much higher concentrations of ammonianitrogen, acetate, propionate, isobutyrate, isovalerate, and caproate than mesophilic effluents. These compounds were responsible, in part, for the thermophilic effluent being more odoriferous than the mesophilic effluent. Thermophilic acetogens and methanogens had higher μmax (maximum specific growth rates) and Ks (half-velocity constants) than mesophilic acetogens and methanogens. The higher Ks values explain the occurrence of C3 and higher fatty acids in higher concentrations in thermophilic effluents. Except for the slightly more odor, thermophilic digestion was superior to mesophilic digestion in all respects.