Effect of pH on anaerobic fermentative products distribution for kitchen waste

Batch tests were carried out to investigate the effect of pH adjustment (control, 5.1, 5.8, 6.5, 7.2 and 7.9) on anaerobic fermentative products distribution for kitchen waste. The experimental results showed that H₂ and CO₂ were main gas products during the acidification stage (first 5 day) for all fermentations. After the acidification stage, fermentation type was dependent on pH. No gas produced due to the inhibition of low pH 4.5 for the control. It was hydrogen-producing fermentations when reactors operated at pH 5.0-5.5, while it was methane-producing fermentations when reactors operated at pH 6.0-7.0. The acidogenesis pathway was ethanol-type with a little volatile fatty acids (VFAs) production when the pH in reactors were lower than 5.5, while the acidogenesis pathway was VFAs-type without alcohols production when the pH in digesters were 6.0-7.0. The optimal pH value for VFAs production was 7.0 and it was about 4.5 for ethanol production. No ammonia inhibition was found in this study. The degradations of kitchen waste were inhibited by the high concentration of VFAs. The inhibitory levels of undissociated acids on acidogenesis were about 1185, 1150, 979, 460 and 178 mg/L at final actual pH 5.2, 5.4, 6.0, 6.5 and 7.0 for five fermentations with pH adjusting respectively.