methane
content at 0 g-ZVSI/g VSS was quite low and stable in 62.9–
63.2% during 27–52 days, which further rose to 65.1–68.3% and
then to 72.6–75.5% (p < 0.05) as ZVSI elevated to 0.5 and 1.0 g/g VSS.
Methane content achieved at the optimum ZVSI of 1.0 g/g VSS in
this experiment was comparably higher than 60.9 ± 0.8% of raw
sludge, 60.2 ± 2.6% of acid pretreated sludge , 64–70% of alkaline-ultrasound
pretreated sludge , 62–69% of sono-alkalization
pretreated sludge [46] or 68.9% of ZVI-sludge