Anaerobic Digestion of Sewage Sludge with High Solid Content and Its Use as Fertilizer

Abstract

In local cities with decentralized small wastewater treatment plants (WWTPs), centralized anaerobic digestion at a WWTP that accepts more sludge from others can serve as the energy hub of the region. In this system, transportation of sewage sludge with a high solid content by vehicles and the use of digested sludge as fertilizer are promising. The present study evaluated the effect of high solid content on anaerobic digestion and on the quality of the digested sludge as a fertilizer in a 1,100-day continuous laboratory experiment. Mixed sewage sludge with 10% total solids was subjected to mesophilic anaerobic digestion with a volatile solids (VS) removal ratio of approximately 0.6. The acceptable organic loading rate was lower than 3.2 kgVS/(m 3 ·d). The viscosity increased to 8.5 dPa·s, but computational fluid dynamics analysis confirmed that mixing need not be upgraded under conditions of high solid content. Component and eluted material analyses of the digested sludge as a fertilizer confirmed that the high solid content in the sewage sludge does not limit the use of digested sludge as a fertilizer according to the Japanese Fertilizer Control Act. The results showed that this centralized anaerobic digestion system is useful for sludge management in small facilities.