Combined pretreatment using CaO and liquid fraction of digestate of rice straw: Anaerobic digestion performance and electron transfer

[Display omitted] To improve anaerobic digestion (AD) efficiency of rice straw, solid alkaline CaO and the liquid fraction of digestate (LFD) were used as pretreatment agents of rice straw. The results showed that AD performance of rice straw with CaO − LFD pretreatment was optimal in different pretreatment methods of the CaO + LFD, CaO − LFD, LFD + CaO, CaO, and LFD. The maximum methane yield (314 ml⋅(g VS)−1) and the highest VFAs concentration (14851 mg⋅L−1 on day 3) of the CaO − LFD pretreatment group were 81% and 118% higher than that of the control group, respectively. Under the action of solid alkaline CaO, the bacteria of Clostridium, Atopostipes, Sphaerochaeta, Tissierella, Thiopseudomonas, Rikenellaceae, and Sedimentibacter could build up co-cultures with the archaeal of Methanosaeta, Methanobacterium, and Methanosarcina performing direct interspecies electron transfer (DIET) and improving AD performance of rice straw. Therefore, the combined pretreatment using CaO and LFD could not only pretreat rice straw but also stimulate co-cultures of microorganism to establish DIET enhancing AD efficiency.