Study of an enhanced dry anaerobic digestion of swine manure: Performance and microbial community property
Graphical abstract
Introduction
As the intensification of animal feeding, large amounts of livestock manure were generated worldwide. Among them, swine manure comprised a large fraction (Zheng et al., 2015). Its annual yield was estimated to be 208 million tons (Zheng et al., 2015). Furthermore, swine manure was a kind of pollutant which not only contained high organic compounds, large amount of pathogens but also emitted bad order. Improper treatment would result in the deterioration of atmosphere, water body and soil. Thus, severe threat was posed to both of the environmental safety and human health.
Anaerobic digestion (AD) is one of the most promising technique to treat swine manure, for it could not only treat swine manure but also generate biogas (Zheng et al., 2015). Compared to low solid anaerobic digestion (solid content of less than 10%) and semi-dry anaerobic digestion (solid content of 10% to 20%), dry anaerobic digestion was conducted at solid content higher than 20% (Chen et al., 2015). It had the significant advantages of smaller reactor, lower energy consuming and less digestate generation. However, its application was still limited because of its poor methane production, long solid retention time (SRT) and low organic loading rate (Chen et al., 2015). For example, Chen et al. (Chen et al., 2015) conducted dry anaerobic digestion of swine manure (solid content of 20–35%) at the SRT of more than 56 days. Saady and Masse (Saady and Massé, 2015) also evaluated the feasibility of dry anaerobic digestion (solid content was 27%) of dairy manure and wheat straw, the methane production rate was only around 187.3 mL CH4/g VS. Dry anaerobic digestion (solid content of around 27%) of pig manure and grass silage was performed at the Organic loading rate (OLR) of 1, 1.5, 2 and 3 kg VS m−3 d, the methane production rates were only 115.2, 114.4, 145.6, 196.5 mL CH4/g VS respectively (Xie et al., 2012).
Thermal treatment could accelerate the hydrolysis of AD (González-Fernández et al., 2008), and thus improve methane production rate of different organic compounds, e.g. sludge (Liao et al., 2016), food waste (Ariunbaatar et al., 2015), rice straw (Zhang et al., 2015) and kitchen waste (Li and Jin, 2015). For example, Gonzalez-Fernandez et al. (González-Fernández et al., 2008) confirmed an increase of 34.8% in methane production rate after swine manure (2.7% solid content) was treated at 170 °C for 30 min. Thermal treatment (solid content of 5%) was conducted at 70 °C and 90 °C for 3 h, and increased the methane production rate of pig manure by 70% and 89% respectively (Carrère et al., 2009). Likewise, an enhancement of 45% in methane production rate was observed when pig slurry was pretreated at 80 °C in Bonmatí et al.’s study (Bonmatí et al., 2001). However, thermal treatment was usually used to enhance the anaerobic digestion at low solid content, its enhancement on dry anaerobic digestion was not yet studied.
In this study, an enhanced dry anaerobic digestion of swine manure (thermal treatment + dry anaerobic digestion) was proposed. The performance of the process was investigated at different solid retention time (SRT) in term of volatile solids (VS) removal rate, methane production and reactor stability. Then, compared with the control group (dry anaerobic digestion without any treatment), the enhancement of thermal treatment on dry anaerobic digestion was evaluated. Furthermore, microbial structure and specific methanogenic pathway of the process were firstly revealed.
Section snippets
Swine manure and inoculum
Raw swine manure was used as feedstock in this study, and was collected biweekly from a pig farm in Qinghe district in Beijing, China. Inoculum was originated from a biogas plant treating pig manure. Both of manure and inoculum were stored in refrigerator at 4 °C before the experiment. Characteristics of raw swine manure and inoculum were shown in Table 1.
Reactor design and experiment set-up
Reactors were designed for “thermal treatment + dry anaerobic digestion”. Thermal treatment was conducted via a continuous stirred tank
Control experiment
In order to evaluate the enhancement of “thermal treatment + dry anaerobic digestion”, dry anaerobic digestion without any treatment was conducted as control experiment. At the SRT of 29 d, severe foaming occurred and the performance became deteriorated, indicating that the system was overloaded. Thus, the control experiment terminated at the SRT of 29 d. VS removal rate of the control experiment was estimated to be 39.6% and 19.4% at SRT of 41 d and 29 d respectively. The methane production
Conclusion
In this study, an enhanced dry anaerobic digestion of swine manure (thermal treatment + dry anaerobic digestion) was firstly proposed. The enhancement of the thermal treatment was also identified via semi-continuous experiment. Results indicated that VS removal rate and methane production rate reached 71.4% and 416.0 mL CH4/g VS respectively at SRT of 35 d. Thermal treatment could enhance the methane production rate of dry AD by 390%. Microbial community indicated that hydrogenotrophic
Acknowledgements
Financial support for this research was provided by the Natural Science Foundation of China (51611130119, 51678338), and National Major Science and Technology Project about Water Pollution Control (2014ZX07114001).
Notes
All authors declare no actual or potential conflict of interest.
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2022, EnergyCitation Excerpt :Therefore, GAC addition narrowed the difference in biogas production rate between the dry and wet process of anaerobic digestion. As shown in Table 4, the maximum OLR of dry anaerobic digestion of swine (or pig) manure reported in the literature was approximately 8.0 g VS/L/d. Hu et al. [32] have studied the effect of thermal treatment on dry anaerobic digestion of swine manure. When the OLR was approximately 7.9 g VS/L/d, the volumetric biogas production rate was approximately 0.85–0.92 L/L/d, the system experienced deterioration and overloading, and the highest volumetric biogas production rate got at the OLR of 5.2 g VS/L/d. Gong et al. [33] have conducted dry anaerobic digestion of pig manure, corn straw and municipal sludge at pilot scale.