| Summary: | The two-stage anaerobic digestion (AD) technology attracts increasing attention due to its ability to collect both hydrogen and methane. A two-stage AD system feeding with food waste and waste activated sludge was investigated in order to achieve higher energy yield and organics removal. The two-stage process consists of a thermophilic H<sub>2</sub>-reactor and a mesophilic CH<sub>4</sub>-reactor, achieved the highest hydrogen and methane yields of 76.8 mL/g-VS and 147.6 mL/g-VS at hydraulic retention times (HRTs) of 0.8 d and 6 d, respectively. The co-digestion process in this study required much less external alkalinity to maintain the pH values than sole food waste digestion in the literature. Compared with the single-stage mesophilic methane AD process, the two-stage AD system had better performance on operation stability, biogas and energy yields, organics removal and chemical oxygen demand (COD) conversion at high organic loading rates (OLRs). According to the TA-cloning analysis, the dominant bacteria in H<sub>2</sub>-reactor was closely related to <i>Clostridium</i> sp. strain Z6 and species <i>Thermoanaerobacterium thermosaccharolyticum</i>. The dominant methanogens in two-stage and single-stage CH<sub>4</sub>-reactor were recognized as acetotrophic methanogens and hydrogenotrophic methanogens, respectively. The presence of the genus <i>Nitrososphaera</i> in the two CH<sub>4</sub>-reactors might contribute to the low NH<sub>4</sub><sup>+</sup>-N concentration in digestate and low CO<sub>2</sub> content in biogas.
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