Extracellular biological organic matters in sewage sludge during mesophilic digestion at reduced hydraulic retention time

To operate an anaerobic digester at low hydraulic retention time (HRT) is welcome in practice. This study characterized the extracellular biological organic matter (EBOM) and supernatant organics for a sewage sludge digested in a lab-scale mesophilic digester (5 l) running at an HRT of 20, 15 or 10 d. The hydrophilic and hydrophobic acid fractions were the principal components in the sludge EBOM. The hydrolysis rates for hydrophobic acid fraction related EBOM at 10 d HRT and that of hydrophilic fraction related proteins in supernatant at 20 d HRT limited the anaerobic processes. Improved hydrolysis of soluble hydrophilic fraction assisted improving digester performance at 20 d HRT. To shorten digestion HRT, efficiency of hydrophobic acid fraction hydrolysis has to be practiced.