The C and N contents of solid samples were determined using a CE-440 elemental analyzer (EAI Exeter Analytical). Liquid and solid samples from the digesters were mixed with water (10 volume) at room temperature for 30 min prior to pH, ammonium nitrogen (NH4-N), total phosphorous (TP) and volatile fatty acids (VFAs) measurement. The pH value was measured with Orion-3-Star pH meter