A novel anaerobic digestion system coupling biogas recirculation with MgCl2 addition for multipurpose sewage sludge treatment

Sewage sludge, one of the major byproducts from wastewater treatment plants (WWTPs), contains high contents of nutritional elements that can be recycled and reutilized to make up the overuse of limited resources on Earth. Currently, the reutilization ratio of sewage sludge is relatively low, most probably due to the high operation costs for its treatment and disposal in WWTPs. This study for the first time realized biogas upgrading, resources conservation and sludge conditioning simultaneously in one anaerobic digestion (AD) reactor for sewage sludge treatment through coupling biogas recirculation with MgCl2 addition. Results showed that 86% of methane content could be obtained by the intermittent biogas recirculation during AD. When MgCl2 was further added, soluble orthophosphate and ammonia nitrogen were reduced by 87% and 19% during 17 days' AD. At the same time, around 120 mgig-total solids (TS) of struvite was estimated to generate in the digested sludge, and the sludge dewaterability was enhanced by 37% when biogas recirculation was performed along with MgCl2 addition. At last, not only the enhanced electricity generation from biogas with higher methane content but also the increased fertilizer potential of the digestate were analyzed in the context of a local WWTP, reflecting the great potentials and profits of this novel AD system in industrial-scale sewage sludge treatment. (C) 2019 Elsevier Ltd. All rights reserved.