Fig xA Effect of urea

2.5. Analytical methods
TS, VS, total Kjeldahl nitrogen (TKN), ammonia nitrogen (NH3-N), and ITF 2357 of samples were measured based on the Standard Methods for the Examination of Water and Wastewater (APHA, 2005). A 5 g sample was suspended in 50 mL of DI water prior to pH measurement. The carbon to nitrogen (C/N) ratio was calculated based on total carbon (TC) and total nitrogen (TN) contents which were determined using an elemental analyzer (Elementar Vario Max CNS, Elementar Americas, Mt. Laurel, NJ, USA). Crude protein content was calculated by determining total organic nitrogen (TKN minus NH3-N) and multiplying by a factor of 6.25 (Hattingh et al., 1967).
Biogas composition (methane and carbon dioxide) was analyzed by a gas chromatograph (Agilent, HP 6890, Wilmington, DE, USA) equipped with a 30 m × 0.53 mm × 10 μm Rt®-Alumina Bond/KCl deactivation column and a thermal conductivity detector (TCD). Helium gas was used as a carrier gas at a flow rate of 5.2 mL/min. The column oven was initially set at 40 °C for 4 min and then increased to 60 °C with an increment of 20 °C/min and held at 60 °C for 5 min. The temperatures of the injector and detector were set at 150 and 200 °C, respectively. Volume of biogas in the Tedlar gas bag was determined by using a diaphragm pump to draw the biogas out from the bag and inject it into a drum-type gas meter (Ritter, Bochum, Germany).