Effect of temperature and pH on the kinetics of methane production, organic nitrogen and phosphorus removal in the batch anaerobic digestion process of cattle manure

A study of the effect of temperature and pH on the kinetics of methane prod uction and organic nitrogen and phosphorus degradation in the anaerobic dig estion process of cattle manure was carried out. Two laboratory-scale batch completely mixed reactors, operating at 35 degrees C (mesophilic temperatu re), and other two, operating at 60 degrees C (thermophilic temperature) we re used. For each temperature selected, the influent pH values were 7.6 (in itial pH of the waste used) and 7.0. The apparent kinetic constants of the biomethanization process increased 2.3 times when the initial pH of the inf luent was increased from 7.0 to 7.6 at mesophilic temperature. The values f ound at thermophilic temperature were similar. The kinetic constants of met hane production decreased 2.6 and 7.2 times when the operating temperature increased from 35 degrees C to 60 degrees C for the experiments carried out at initial pH of 7.0 and 7.6, respectively. The methane yield coefficient (1 CH4 STP/g VS removed) also decreased when the temperature increased from 35 degrees C to 60 degrees C for the two initial pH values studied. This b ehaviour agreed with the major inhibition level observed at thermophilic te mperature as a result of the higher organic nitrogen removal and ammonia ni trogen production observed at 60 degrees C. Specifically, the specific rate constants for organic nitrogen removal and ammonia nitrogen production inc reased 3.6 and 12 times when the temperature was increased from 35 degrees C to 60 degrees C for the experiments carried out at initial pH values of 7 .0 and 7.6, respectively. In the same way, the values of the kinetic consta nt for phosphorus removal were 44% and 80% higher than those obtained at 35 OC for the two initial pH values above-mentioned, respectively. Finally, t he experimental values of organic nitrogen and phosphorus concentrations we re reproduced with deviations equal to or less than 10% and 15% in every ca se, respectively.