The TS content was determined by drying at 105 C for 24 h, and
the VS content was based on the weight loss of TS at 550 C till constant
weight was obtained. 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. The NH4-N and TP were
determined by Nessler method 4500-NH3 and ascorbic acid
method 4500-P, respectively using a Beckman D640 spectrophotometer
(Fullerton, CA) (APHA et al., 1998). VFAs were quantified
by gas chromatography (GC) using an Agilent 6890 instrument
(Palo Alto, CA) with an Alltech-Heliflex-ATTM Wax capillary column
(U 0.32 mm 30 m, Deerfield, IL) at 150 C and flame ionization
detection (210 C). 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.