Role and significance of co-additive of biochar and nano-magnetite on methane production from waste activated sludge: Non-synergistic rather than synergistic effects

[Display omitted] •Synergisms weren’t formed with co-additive of BC and NM.•The methane yield decreased by 24.7 and 38.3% compared to that in BC and NM.•The direct interspecies electron transfer contributed by NM was reduced by BC.•The NM adsorbed on BC were converted to nano-hematite.•The electron-active microorganisms were reduced at suspended and attached stages. To realize energy-self-sufficient operation of wastewater treatment plants is a key step for carbon–neutral goal, and conductive materials have been widely used to build direct interspecies electron transfer (DIET) to promote methane production from waste activated sludge (WAS). However, the role and significance of co-additive of biochar (BC) and nano-magnetite (NM) have not been reported, and whether synergistic effects can be formed or not is still unknown. This study investigated the effects of co-additive of BC and NM by stepwise and simultaneous strategies on anaerobic digestion. Non-synergistic rather than synergistic effects were observed. The methane production reduced to 122.4 mL/g volatile suspended solids in simultaneous co-additive strategy, which was 24.7 and 38.3% lower than that in sole BC and NM, respectively. Differently, methane productions in stepwise additive strategies were higher than that in sole BC but lower than that in sole NM. Mechanism studies indicated that although NM accelerated WAS solubilization and hydrolysis, BC adsorbed soluble organics, reducing available substrates for methanogens at suspended stage. Also, extracellular polymeric substances were decreased, weakening electrochemical functions of redox-active shuttles. Besides, the DIET contributed by NM was reduced by the adsorption of BC to NM, and the NM adsorbed on BC were converted to nano-hematite, which presented inhibitions to methanogenesis. Moreover, the electron-active microorganisms, such as Clostridium and Methanobacterium, were all reduced at suspended and attached stages with the presence of BCs, limiting potential contributions of DIET on methane production improvement. This work may provide some new insights for additive strategies of BC and NM for methane production enhancement from WAS.