Anaerobic co-digestion (AcoD) has been widely applied to the disposal of waste activated sludge (WAS), since it optimizes the C/N ratio and decreases the buffer capacity, which dose not solve the problem involved in the slow methanogenic metabolism that limits methanogenesis yet. In this study, a strategy of initially fermenting the polysaccharide-rich organic wastes to produce the ethanol-contained fermentation liquid (EFL) that was then mixed with WAS for further AcoD was proposed, with the aim of establishing direct interspecies electron transfer (DIET). The results showed that, the methane production rates and organic removal efficiencies in the digesters treating WAS mixed with EFL were significantly higher than that in the single-phase AcoD system about 30% and 6-7%, respectively, but the improved performances in the digesters treating WAS mixed with the neutral-pH fermentation liquid (FL) were not significant, which resulted in a low-efficiency energy recovery. The conductivity of sludge fed with WAS mixed with EFL were about 3-5 folds higher than that with the neutral-pH FL, suggesting that the DIET-based metabolism was established. Together with the special enrichment of Fe(III)reducing genus involved in Rhodoferax and high-abundance Methanospirillum capable of producing the electrical pili, a novel DIET between Rhodoferax and Methanospirillum was inferred. (C) 2019 Elsevier Ltd. All rights reserved.