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 MgCl 2 addition. Results showed that 86% of methane content could be obtained by the intermittent biogas recirculation during AD. When MgCl 2 was further added, soluble orthophosphate and ammonia nitrogen were reduced by 87% and 19% during 17 days' AD. At the same time, around 120 mg/g-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 MgCl 2 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. Image 1 • A novel anaerobic digestion (AD) system was established in this study. • One AD reactor achieved biogas upgrading, nutrient recovery and sludge conditioning. • Biogas recirculation increased CH 4 content (86%) in the biogas from AD of sludge. • 93% of total phosphorus and 69% of total nitrogen were reserved in solid digestate. • Sludge dewaterability was enhanced by 37% using the novel AD system. [ABSTRACT FROM AUTHOR]