The effects of digestion temperature and temperature shock on the biogas yields from the mesophilic anaerobic digestion of swine manure

In order to obtain basic design criteria for anaerobic digesters of swine manure, the effects of different digesting temperatures, temperature shocks and feed loads, on the biogas yields and methane content were evaluated. The digester temperatures were set at 25, 30 and 35 °C, with four feed loads of 5%, 10%, 20% and 40% (feed volume/digester volume). At a temperature of 30 °C, the methane yield was reduced by only 3% compared to 35 °C, while a 17.4% reduction was observed when the digestion was performed at 25 °C. Ultimate methane yields of 327, 389 and 403 mL CH4/g VSadded were obtained at 25, 30 and 35 °C, respectively; with moderate feed loads from 5% to 20% (V/V). From the elemental analysis of swine manure, the theoretical biogas and methane yields at standard temperature and pressure were 1.12 L biogas/g VSdestroyed and 0.724 L CH4/g VSdestroyed, respectively. Also, the methane content increased with increasing digestion temperatures, but only to a small degree. Temperature shocks from 35 to 30 °C and again from 30 to 32 °C led to a decrease in the biogas production rate, but it rapidly resumed the value of the control reactor. In addition, no lasting damage was observed for the digestion performance, once it had recovered.