Analytical methods The system performance was tested by measuring
biogas and methane productions, VS and TS reductions, ammonium concentration,
acetic acid concentration and pH. Biogas production of the system was determined
daily by displacement of water in a scaled measuring cylinder. Methane content
(CH4) of biogas was measured according to Orsat Method in which KOH solution of
40% by mass was used to absorb CO2, NH3 and H2S (23). Total solids and volatile
solids analysis were conducted according to standard methods (24). Volatile fatty
acid (as acetic acid) and total ammonia nitrogen were analyzed by Hach Lange
Cadas 200 model spectrophotometer using commercially available testing kits
(Hach-Lange LCK 365 and LCK 302, respectively). pH and temperature of the two
stage system were also measured daily.