Batch fermentation assays under mes

Batch fermentation assays under mesophilic conditions (37 ± 1°C) were conducted to evaluate the biogas production from raw and pretreated sludge samples. Sealed 120 mL serum bottles were used as anaerobic reactors. A working volume of 60 mL was kept in all batches. WAS sample (substrate) and inocula were introduced to the reactor with the same proportion keeping a VS substrate/VS inocula ratio equal to 1 [22]. The pH of anaerobic medium was adjusted to 7.2. To supply anaerobic condition, all batches were purged with a gas mixture of 75% N2 and 25% CO2 for 2–3 min. Control batch was conducted only with inocula in order to estimate the biogas production resulting from the seeding material. All anaerobic batch tests were conducted in duplicate.Daily biogas production was measured using a gas displacement device. Biogas yield was calculated in terms of biogas volume per mass of substrate VS introduced at the initial time of fermentation. The fermentation tests were conducted for approximately 30 days until biogas production stopped. The modified Gompertz equation was used to study the cumulative biogas generation from batch digesters and the kinetics of biogas production [23]. It is a function of bacterial growth. The modified Gompertz equation is given by: