Performance and interaction mechanism of integrating alkali pretreatment and immobilized electrodes on enhanced sludge digestion for methane production

Introduction of voltage or conductive materials have been proven to be effective strategies for enhancing the stability and efficiency of sludge digestion for methane recovery. This experiment simulated a two-phase anaerobic system, consisting of an acid-producing phase with alkali pretreatment and a methane-producing phase with immobilized electrodes and voltage enhancement. Various effective and comparative experiments were conducted to investigate the crucial roles of immobilized electrodes like carbon brush (CB) and nickel foam (NF), electroactive biofilm and voltage during the sludge methane-producing phase. The experimental results indicate that directly application of immobilized CB and NF electrode to promote sludge anaerobic digestion achieves better performance than additional voltage supply. The electrodes can increase methane production by 38% to 94%, whereas under additional voltage supply, methane production only increases by 22%-53% compared to the control. Through the analysis of key transformations in organic substrates and changes in microbial community structure, the inherent advantages of electrode materials in promoting the syntrophic methanogenesis processes are confirmed. Compared to suspended sludge, the types and relative abundance of typical syntrophic methane-producing microorganisms, Methanosarcina, and acetogenic bacteria, Syntrophomonas on the electrode surface significantly increase by 8.5% and 1.7%. This process is notably superior to the methane generation process dominated by hydrogenotrophic methanogensis after applying voltage. The results indicate that CB and NF are suitable conductive materials to promote anaerobic methane recovery, and their effectiveness surpasses that of applying voltage. These research outcomes provide new insights for the development of novel anaerobic digestion enhancement systems.