Abstract
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 H2-reactor and a mesophilic CH4-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 H2-reactor was closely related to Clostridium sp. strain Z6 and species Thermoanaerobacterium thermosaccharolyticum. The dominant methanogens in two-stage and single-stage CH4-reactor were recognized as acetotrophic methanogens and hydrogenotrophic methanogens, respectively. The presence of the genus Nitrososphaera in the two CH4-reactors might contribute to the low NH4+-N concentration in digestate and low CO2 content in biogas.
Highlights
Anaerobic digestion (AD) is an effective technology for waste activated sludge (WAS) treatment and biogas production [1]
Compared to the reported semi-continuous hydrogen production process, the hydrogen yield in this study is much higher than 129.1 mL/g-VSremoved [9] and 32–48 mL/g-volatile solid (VS) [10] obtained from mesophilic co-digestion of sludge and Food waste (FW), suggesting the advantages of thermophilic condition on anaerobic fermentative hydrogen production
The soluble carbohydrates removal efficiency was found to be above 94% in all operation conditions. These results suggests that carbohydrates was the major substrate for hydrogen and volatile fatty acid (VFA) production, which is accordance with the reported result [32]
Summary
Anaerobic digestion (AD) is an effective technology for waste activated sludge (WAS) treatment and biogas production [1]. The WAS has low carbon/nitrogen ratio (C/N), which could limit biogas production. The digester for methane production was supposed to be stable when the total volatile fatty acid (TVFA)/alkalinity ratio was lower than 0.4 [3,4,5], it was found that the TVFA/alkalinity was 0.32–0.35 and 0.59 for the co-digestion process and sole FW digestion, respectively [6]. Many studies focused on methane production by single-stage AD process from co-digestion of WAS and FW [3,4,7,8], and the on-sit pilot-scale co-digestion system is reported [6]. The operational comparison of single-stage and two-stage AD process with FW or thin stillage as substrate have been reported, Energies 2019, 12, 2748; doi:10.3390/en12142748 www.mdpi.com/journal/energies
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