The samples were acidified to pH 2 with HNO3 prior to injection. The composition of biogas (CH4 and CO2) in the gas sampling bags was analyzed using a Gow-Mac 350 thermal conductivity GC (GOW-MAC Instrument, Bound Brook, NJ) equipped with HaySep DB stainless steel packed column (9.1 m 3 mm) operating at 30 C with a detector temperature of 200 C and helium as carrier gas (30 mL/min). Standard curves were prepared from calibration standards of CO2 and CH4 in N2 for quantification.