Effect of moisture content on anaerobic digestion of dewatered sludge: ammonia inhibition to carbohydrate removal and methane production

The purpose of this study is to investigate the effect of moisture content on anaerobic digestion of dewatered sewage sludge under mesophilic condition. The moisture contents of sludge fed to reactors were 97.0%, 94.6%, 92.9%, 91.1% and 89.0%. The VS removal efficiency changed from 45.6% to 33.8%, as the moisture content of sludge fed to digester decreased from 97.0% to 89.0%. The carbohydrate removal efficiency also decreased from 71.1% to 27.8%. Methane production decreased when the moisture content of sludge was lower than 91.1%. The number of glucose consuming acidogenic bacteria was decreased from 3.1×10⁶ to 3.1×10⁸(MPN/mL) as the moisture content decreased from 91.1% to 89.0%. The numbers of hydrogenotrophic and acetoclastic methanogenic bacteria decreased by one order of magnitude when the moisture content was lower than 91.1%. The decrease in numbers of glucose consuming acidogenic bacteria and methanogenic bacteria was found to correspond to the decrease in the carbohydrate removal efficiency and the accumulation of propionic acid. Batch experiments showed that acetoclastic methanogenic bacteria were acclimated to high ammonia concentration, on the other hand, glucose consuming acidogenic bacteria were inhibited.