Digestion Properties of Intracellular Polymers and Extracellular Polymeric Substances and Influences of Extracellular Polymeric Substances on Anaerobic Digestion of Sludge

Anaerobic digestion is a method adopted to treat sludge, but its effectiveness is limited by the existence of extracellular polymeric substances (EPSs). For the first time, this study separated sludge into intracellular polymeric substances (IPSs) and EPSs to investigate each of their digestion properties and the influences of EPSs. It was found that (1) EPS anaerobic digestion was completed in 3 days and was much quicker than by IPS; (2) the removal of organic matter in EPSs was higher than in IPSs; and (3) the biogas yield of EPSs [266.6  mL/g volatile solids (VS)] was higher than that of IPSs (229.2  mL/g VS). To examine the influences of EPS on sludge anaerobic digestion, an EPS-removal sample was compared with the control sample, and it was found that EPSs could restrict biogas production during sludge anaerobic digestion. After removing EPSs, cumulative biogas production and biogas yield were improved by 9.4% and 28.8%, respectively. The whole biogas production process was also shortened by 7 days after removing EPSs. Future studies could decide whether EPSs should be removed based on their role in anaerobic digestion outcomes (e.g., prioritize the production of biogas or the removal of organic matter).