Abstract
In this study, gas production from mixed anaerobic fermentation of rapeseed cake and sheep manure at low temperature (15.2-17.8°C) was investigated in Qinghai rural household biogas digesters to understand the temporal dynamics of key microbial populations involved in fermentations. Different raw material ratios resulted in significantly different effects on biogas yields and microbial community compositions over 40days. When the dry weight ratio of sheep manure to rapeseed cake was 1:2, the highest level of cumulative gas production was observed (122.92 m3·t-1). Firmicutes, Proteobacteria, Bacteroidetes, and Actinobacteria were the dominant bacterial phyla among the 29 digester samples (total relative abundances > 79.23%), followed by Synergistetes (4.09-10.7%). Lactobacillus was the most abundant genus in the biogas digesters with high rapeseed cake contents (average relative abundances: 14.68%), while Peptoniphilus exhibited higher abundances (12.69%) in the mixed treatments. In addition, unclassified Synergistaceae abundances (6.64%) were positively associated with biogas production variation among treatments. Bacteroides (5.74%) and Pseudomonas (5.24%) both accounted for larger proportions of communities in the digesters that used more sheep manure. Methanomicrobiales (66.55%) was the most dominant archaeal group among digesters, with Methanogenium (41.82%) and Methanoculleus (16.55%) representing the main gas-producing archaeal genera; they were more abundant in biogas digesters with higher sheep manure contents and higher rapeseed cake contents, respectively. VFAs and pH were the main factors associated with differences in microbial communities among the 29 samples. Specifically, VFA concentrations were positively correlated with Lactobacillus, Methanoculleus and Methanothrix abundances, while pH was positively correlated with Bacteroides, Pseudomonas, and Methanobacterium abundances.
Highlights
Rapeseed cake is a by-product that is obtained by pressing rapeseed to make oils
We investigated rural household biogas digesters that were used as fermentation devices to conduct lowtemperature (15.2-17.8°C) pilot experiments to evaluate biogas production by mixed anaerobic fermentation of rapeseed cake and sheep manure using seven different ratios of high solid content waste
The evaluation of low-temperature mixed anaerobic fermentation within seven biogas digesters using different raw material ratios indicated that a dry matter ratio of sheep manure and rapeseed cake of 1:2 corresponded to the highest cumulative biogas production (122.92 m3·t−1)
Summary
Rapeseed cake is a by-product that is obtained by pressing rapeseed to make oils. The cakes have abundant protein, amino acid, and other nutrient components and are agricultural solid wastes that can be reused as important resources. The selection of degradable raw materials (Li et al 2020), the fermentation of mixed substrate components (Shi et al 2021) and fermentations with high solid contents (Elmitwalli et al 2004) can adequately reduce environmental condition requirements for fermentation. They can partially compensate for the low gas production rates caused by low temperatures.
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