Biogas production from low-organic-content sludge using a high-solids anaerobic digester with improved agitation

Sludge with a low organic content has poor biochemical methane potential. High-solids digestion is a possible method to recover bioenergy economically from this kind of sludge, but the blocked mass transfer is a major obstacle. A pilot scale high-solids anaerobic digester equipped with an enhanced stirring system was designed and operated continuously for 9.5 months to evaluate the feasibility of bioenergy recovery from low-organic-content sludge. The results showed that high-solids anaerobic digestion can evolve successfully from low-solids status. Although the system once suffered slight inhibition derived from ammonia, it then stabilized with volatile fatty acids concentration of 200-400 mg/L and free ammonia concentration less than 250 mg/L, exhibiting similar removal rates of organic solids and biogas yields as achieved using low-solids digestion. Statistical analyses proved that the organic removal rate was almost proportional to the organic content of feed sludge. The organic removal rate was nearly 35% when the organic content of feed sludge was 50%, while the system tended to failure when the organic content of feed sludge was less than 38%. High-solids anaerobic digestion, when combined with improved agitation, is an effective method for bioenergy recovery from sludge with organic content of 40-50%.