Characterization of methanogenic activity during high-solids anaerobic digestion of sewage sludge

High-solids anaerobic digestion of sewage sludge has advantages of smaller digesters and a lower energy requirement for heating than conventional digestion, but its efficiency is negatively influenced by high solids concentration. To discover the mechanism causing this deteriorative performance, the specific methanogenic activity (SMA) of high-solids digestate was measured using typical substrates including acetate, propionate, butyrate, glucose, microcrystalline cellulose and hydrogen and carbon dioxide. When the total solids content increased from 4.2% to 14.4%, the SMA of the digestate decreased 40%-50% for most substrates except hydrogen and carbon dioxide, mainly because of blocked mass transfer in the high-solids digestate. On the other hand, some features remained stable: acetotrophic methanogens exhibited more activity than hydrogenotrophic methanogens, butyrate fermentation rather than propionate fermentation was still the main metabolic pathway during the degradation of glucose and cellulose, and hydrolysis was still the rate-limiting step during the anaerobic digestion of cellulose.