Effects of different biofilm carriers on biogas production during anaerobic digestion of corn straw
Introduction
With the ongoing global warming, ever-increasing energy costs, and high cost in crop straw disposal, more attention should be paid to the efficient disposal of biomass, such as its bioconversion into methane-rich biogas (Yan et al., 2015). China is one of the large agricultural countries in the world and produces maximum crop straw residues every year, reaching more than 600 million metric tons (Li et al., 2014) in which corn straw (CS) accounts for about 25.0% of total crop straw (MOA, 2011). The increased generation of crop straw residues has a wide range of adverse effects on human health, energy, and environment safety (Zhou et al., 2016, Khoramnia et al., 2011). Anaerobic digestion (AD) technology is becoming an excellent choice for sustainable management of crop straw, and AD of crop straw with the aim of energy production has been widely used owing to its advantages of reducing greenhouse gas emissions and utilizing the wastes by substituting fossil fuels (Zhou et al., 2016, Yan et al., 2015, Rafique et al., 2010). However, the lignocellulose complex composed of cellulose, hemicellulose and lignin can considerably affect hydrolysis, which is the rate-limiting step in AD of CS (Vivekanand et al., 2012, Singh et al., 2015). A recent study revealed that the limitations of existing AD reactor include low mass transfer efficiency, short hydraulic retention time, slow start-up, and sensitivity to shock loads (Matsumoto et al., 2012). Thus, some new methods must be developed to stabilize and enhance the mass transfer efficiency between substrate and microorganisms, such as addition of biofilm carriers (Gong et al., 2011).
The microbial community compositions in an AD reactor comprise three typical populations, namely fermentative bacteria, acetogenic bacteria, and methanogenic bacteria. Among them, methanogenic bacteria are more sensitive to changes in the environmental conditions owing to their slower growth, when compared with fermentative and acetogenic bacteria. In view of the significance of densities and species of methanogenic bacteria in AD reactor, emphasis should be placed on developing and maintaining a stable, viable, and large population of methanogenic bacteria (Zhou et al., 2017). The selection of high-efficiency biofilm carriers plays an important role in enriching high-density methanogens to improve biogas and methane production as well as prevent the microorganisms from being washed out in the effluent (Júnior et al., 2015). Biofilm carriers in AD reactors can potentially increase the reactor productivity by retaining microorganisms, thus enriching the amount of methanogens (Gong et al., 2011). The development of biofilm carriers includes the process of improving material properties, and several biofilm carriers have been evaluated for enriching microorganisms (Habouzit et al., 2014). In view of the characteristics of the materials used, traditional biofilm carriers can be mainly classified into two categories, namely, fibrous and granular carriers. However, the pores of granular carriers can get clogged with the biofilm growth, which is a noticeable disadvantage of using this type of carriers (Escudié et al., 2011). Fibrous carriers can overcome this problem as long as the surface is sufficiently (Leyva-Díaz et al., 2017). Nevertheless, biocompatibility with the predominant microorganisms must be considered while selecting the most suitable biofilm carriers (Escudié et al., 2011).
Despite the general use of biofilm carriers in manure and wastewater treatment, there is little literature reporting the use of biofilm carriers for improving CS digestion in AD reactor. Therefore, the goal of the present study was to evaluate the performance of biofilm carriers in improving biogas and methane production during CS digestion in AD reactor and discuss the underlying microbial mechanism.
Section snippets
Materials
The inoculum was collected from the fermentation tank of a biogas station of Nanjing Tech University, China, and stored at 4 °C prior to use. The total solids (TS) content was 2.22% ± 0.02% of the slurry. CS was obtained from a corn field around the Nanjing Tech University. The TS and VS contents of the CS were 90.35% ± 0.12% and 91.35% ± 0.11%, respectively. The contents of cellulose, hemicellulose, and lignin were 33.21% ± 0.09%, 21.63% ± 0.13%, and 17.69% ± 0.02%, respectively.
Biofilm carriers
The characteristics of an
Effects of the biofilm carriers on the pH of the systems in an entire anaerobic fermentation cycle
Throughout the experiment, the pH of the systems was maintained within a neutral range, which caused little rancidity in the systems. The pH variations in the system with PPF and the control system without a carrier were relatively more stable, when compared with those in the other three systems. Although methanogens are known to exist in extreme pH environments, most of the methanogenic bacteria function in a pH range of 6.7–7.4, with an optimal pH of 7.0–7.2, and the rate of methane
Conclusion
In this study, an obvious improvement in biogas and methane production was observed when fibrous biofilm carriers were added to the fermentation system. The PPF system presented the highest biogas and methane production in an entire anaerobic fermentation cycle, which was 44.80% and 49.84% higher than those noted in the control, respectively. In addition, the PPF system exhibited the highest TS, VS, and COD removal efficiency. High-throughput sequencing analysis showed that the dominant species
Acknowledgements
This work was supported by the National Basic Research Program of China (2013CB733500), the National Key Research and Development Program of China (2016YFE0112800), and the Key Science and Technology Project of Jiangsu Province (BE2016389).
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