Experimental methods
Three DPARs were operated simultaneously in a semi-continuous mode, one each at a digestion temperature of 15, 25 and 35 °C. At the beginning of the experiment, 4.5 L of inoculation sludge was added to each DPAR. Then, the mixed swine manure was loaded into each DPAR daily. Sufficient digestate was removed daily to maintain a constant working volume throughout the experiment. The organic loading rate for each DPAR was 3.46 g VS L−1 d−1 and the corresponding retention time of the reactors was 61.7 day.