Value-adding conversion and volume reduction of sewage sludge by anaerobic co-digestion with crude glycerol

•Anaerobic co-digestion of sewage sludge and a crude glycerol was evaluated.•Co-digestion yielded methane, hydrogen, and 1,3-PDO as value-added products.•Increasing glycerol levels significantly increased hydrogen and 1,3-PDO production.•By adjusting glycerol content, hydrogen and methane fermentation was controlled.•Addition of crude glycerol promoted the solubilization of sewage sludge.

In this study, conversion of sewage sludge to biogas by anaerobic co-digestion with crude glycerol was examined. When 0.126 g/L crude glycerol was added to the reactor, only methane was produced. Upon addition of 5.04 g/L crude glycerol, hydrogen production occurred, and a significant amount of 1,3-propanediol (1,3-PDO) was generated in the liquid phase. On day 6, the dry weight was largely composed of organic acids (48%) and 1,3-PDO (17%), which are water-soluble. Degradation of 1,3-PDO was very slow, which is advantageous for recovery. Crude glycerol, which contains alkaline substances, promoted organic matter degradation by microorganisms, which possibly affected biogas and 1,3-PDO production. Addition of 0.630-2.52 g/L glycerol initially led to hydrogen production, followed by methane production a few days later, which stabilized within 1 week. In conclusion, adjustment of the crude glycerol concentration allows controllable conversion to value-added products for co-digestion.