Hydrogen And Methane Production From Co-digestion Of Sludge And Food Waste In A Temperature-separated Two-stage Anaerobic Fermentation Process

Two-stage anaerobic fermentation process for hydrogen and methane production could effectively stabilize organic solid waste and produce biogas. However, as the fermentation substrate, the sludge could hardly achieve high biogas yield due to its low carbon/nitrogen ratio and low content of easily-degradable organic matters. Anaerobic fermentation with food waste as substrate might lead to p H drop because of the high content of easily-degradable organic matter and low alkalinity in food waste. In this study, co-digestion of sludge and food waste was performed to achieve substrate nutrition balance in a temperature-separated two-stage fermentation process for hydrogen and methane production.The energy yield from co-digestion of sludge and food waste could be higher than sole substrate fermentation during the two-stage fermentation process for hydrogen and methane production. The highest energy yield of 14.0k J/g-VSadded was obtained at food waste percentage of 85%. The dominant volatile fatty acids(VFA) were acetate and butyrate for co-digestion process, but the main VFA was acetate for sludge fermentation in hydrogen production process. The soluble carbohydrate was rapidly consumed in hydrogen production process and slightly increased in methane production process. The increase of ammonia and soluble protein demonstrated that the protein was hydrolyzed continuously. Hydrogen production was inhibited at initial p H of 4.0, whereas there were indistinctive differences between the hydrogen yields or methane yields at initial p H of 5.5-11.0, implying the co-digestion process was adaptive for p H variation. At the end of hydrogen production, the main VFA was acetate and butyrate at the initial p H of 4.0 and 11.0, respectively. During hydrogen production from co-digestion of sludge and food waste, the aeration pretreated seed sludge and thermophilic fermentation(55℃) were much more effective on hydrogen production and methanogens activity inhibition than heat treated seed sludge and mesophilic fermentation.The dynamic experiment of temperature-separated two-stage fermentation process for hydrogen and methane production from co-digestion of food waste and sludge combined digested sludge recirculation process could be steadily operated for more than 120 d. The highest hydrogen yield of 207.5m L/g-VSremoved was obtained at organic loading rate(OLR) of 39.6g-VS/L/d and hydraulic retention time(HRT) of 0.8d. The highest methane yield of 554.3m L/g-VSremoved was obtained at OLR of 4.4g-VS/L/d and HRT of 6d. The contents of hydrogen and methane in biogas produced in separate stage were 54.5%-60.9% for hydrogen and 71.5%-81.3%, respectively. The VFA produced in hydrogen production process were mainly acetate and butyrate. Compared to single-stage fermentation process, the two-stage fermentation process had advantages on energy yield, biogas production, volatile solid removal and operation stability. The two-stage fermentation process was much more adaptive to high OLR than single-stage fermentation process. Moreover, two-stage fermentation process could not only collect hydrogen but dramatically improve methane yield. The dominant microorganisms were analyzed by high-throughput sequencing technology and molecular cloning technology. It was found that the dominant bacteria was Class Clostridia in hydrogen production reactor, the dominant archaea was Class Methanomicrobia in methane production reactor of two-stage process and Family Methanobacteriaceae in single-stage methane production reactor, respectively.