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[Display omitted] • Biogas recirculation may create multifunctional granule AD system of sewage sludge. • Biogas recirculation accelerates CH 4 recovery rate from sludge by 31–44%. • Simultaneous sludge stabilization and aggregation can be realized in this system. • This system can improve net energy profit by 78–85% for sewage sludge treatment. This study investigated the possibility of biogas recirculation-driven anaerobic granular sludge system for sewage sludge treatment, aiming to develop an energy sufficient and multifunctional anaerobic digestion (AD) system for sewage sludge with biogas upgrading, sludge stabilization and self-aggregation. Results show that biogas recirculation could enhance the CH 4 production rate by 31–44% and shorten the lag-phase duration to 0.08–0.2 day with simultaneous increment of CH 4 content (> 83% in this study). The reason is mainly associated with the stronger interspecies electron transfer under the biogas recirculation condition. In addition, 37–40% better dewaterability of the digested sludge was achieved, implying the occurrence of self-aggregation of microbial cells induced by biogas recirculation. Energy balance analysis reflects that this sewage sludge treatment system could enhance the net energy recovery by 78–85%. Moreover, almost no obvious influence was noticed on the seed granules' composition and properties. These findings suggest that the biogas recirculation-driven anaerobic granular sludge system could be a promising alternative for sewage sludge treatment, which can improve biogas quality and sludge dewaterability simultaneously towards sludge self-aggregation with no addition of other chemicals. [ABSTRACT FROM AUTHOR]

Most previous studies have focused on the effects of salt on anaerobic digestion (AD) for hydrogen or methane production. However, the effects of salt on two-stage AD and the related approaches to mitigate the adverse effects of high salinity on hydrogen and/or methane production were seldom addressed. In this study, addition of Air-nanobubble water (Air-NBW) was adopted to mitigate the inhibition of high salinity on co-production of hydrogen and methane from two-stage AD of food waste (FW). In the Air-NBW added reactors with 0–30 g NaCl/L, hydrogen yield was increased by 21–65% with the subsequent methane yield elevated by 14–43% when compared to the corresponding deionized water (DW) group. This study for the first time confirmed that when two-stage AD of FW was exposed to the same salinity level, addition of Air-NBW could enhance enzymatic activities at the individual stage. Results of electron transport system (ETS) activity further demonstrate that addition of Air-NBW may promote the electron transfer associated with the synthesis of hydrogen and methane. Therefore, an efficient approach for hydrogen and methane recovery from the two-stage AD of FW under high salinity was proposed through improving microbial electron transfer and corresponding enzymatic activities at each stage via Air-NBW addition. [Display omitted] • Air-nanobubble water (NBW) reduced salt inhibition on H 2 and CH 4 production. • Air-NBW addition shortened lag phase of the first stage AD at 0–30 g NaCl/L. • Enzyme activities were enhanced by Air-NBW even at high salinity of 20–30 g NaCl/L. • Air-NBW may promote the electron transfer linking the synthesis of H 2 and CH 4. [ABSTRACT FROM AUTHOR]