Enhancement of methane production and antibiotic resistance genes reduction by ferrous chloride during anaerobic digestion of swine manure
Introduction
In recent years, it was estimated that China produced 3.8 billion tons per year of livestock and poultry manure, and the comprehensive utilization rate is less than 60% (Bluemling and Wang, 2018). Improper disposal of these wastes can cause significant environmental pollution, including surface and groundwater contamination, odour problems and the spread of pathogens (Goodrich and Schmidt, 2002). Consequently, the Action Plan for Livestock and Poultry Manure Utilization (2017–2020) was implemented by the Chinese government in 2017 to promote the utilization of livestock and poultry manure resources; in this plan, anaerobic digestion (AD) was the core treatment method for swine wastewater and manure (Ma et al., 2018). Veterinary antibiotics (including tetracycline and tylosin) as feed additives were widely used in intensive livestock and poultry farming, as antibiotics not only prevent disease, but also improve feed efficiency, and promote the growth of the livestock and poultry (Larson, 2015, Wang et al., 2008). However, approximately 30%-90% of antibiotics cannot be absorbed and were thus excreted in the feces or urine. Therefore, antibiotic residues and ARGs were found in many manure and organic sludge sources, leading to ARGs enrichment, such that farm animal feces now contains an important ARG library (Ji et al., 2012).
AD technology is a very effective technique for treating pig manure. It not only avoids pig manure pollution, obtains biogas clean energy, realizes waste recycling, and anaerobic digestion can remove pathogens and ARGs (Sui et al., 2016). However, low energy recovery efficiencies still limit its industrial application. (Bharathiraja et al., 2018). Therefore, research has recently focused on the improvement of AD and the concomitant reduction of ARGs in agricultural waste streams. In order to address both of these problems, many different treatment technologies have been investigated and utilized, such as chemical (Doǧan and Sanin, 2009), thermal (Song et al., 2004), enzymatic (Roman et al., 2006), ect. Unfortunately, most of these methods require high-energy inputs, making pretreatment processes costly (Wang et al., 2016).
In recent years, iron-based materials, was low-cost and non-toxic additives such as iron (Fe) iron oxide, and iron salts, which were applied in AD considered to be an effective means of increasing methane production and sulfate control from AD systems (Nordell et al., 2016, Wei et al., 2018, Yun et al., 2019). Iron-enhanced AD system has evaluated by life cycle assessment (LCA), which revealed that scrap iron was favorable in economy and could reduce both operational costs and carbon emissions for carbon-neutral oriented WWTPs (Wei et al., 2018). Previous studies have shown that adding zero-valent iron and magnetite to the AD of swine manure can not only strengthen methane production but also promote the reduction of ARGs in the AD residue (Zhang et al., 2018, Zhang et al., 2019b). However, due to the morphology of the zero-valent iron and magnetite, the optimum addition amount can be up to up to 20 g/L based on the molar mass of iron. In contrast, Qin et al., 2019 found that adding an appropriate amount of ferrous chloride or FeCl2, (determined to be 0.2 g/L) during sludge AD could increase methane production by 6.4% (Qin et al., 2019). In addition, the appropriate dosage of FeCl2 had the potential to create a favorable environment AD environment, including optimization of VFAs, COD and NH4+-N in the sludge system. Another benefit of FeCl2 additions on AD performance was the direct promotion of protease, dehydrogenase, and cellulase activities (Qin et al., 2019, Zhang et al., 2016a, Zhang et al., 2016b). Other studies have shown that Fe2+ considerably enhanced the utilization of acetate, propionate, and H2 during the AD of organic matter at 37 °C (Sai Ram et al., 2000). The current literature supports the possibility of enhancing methane production using appropriate FeCl2 additions, but the optimum concentrations and the mechanism of FeCl2 effects have not been resolved for the AD of swine manure. There was also very limited information available on the potential effects of FeCl2 additions on ARG dynamics during AD of swine manure, so research on the potential of FeCl2 as a benficial additive in the AD of swine manure is needed.
In this study, we used batch experiments to optimize and characterize the effects of FeCl2 additions, including the potential mechanisms of the effects of FeCl2, on the performance and methane production dynamics of swine manure AD. And the fate and changes of ARGs in response to FeCl2 additions were also investigated using a high-throughput quantitative PCR technology including 296 primer sets targeting almost all major classes of ARGs.
Section snippets
Experiment setup
Swine manure and inocula were collected from a swine farm in Beijing, China and immediately stored at 4 °C until further use. The physicochemical properties of swine manure and inoculated sludge were shown in the supporting information. FeCl2·4H2O (99% metals basis; CAS, 3748-10-9) was purchased from Aladdin Reagent Co. Ltd., China. Batch experiments were conducted at 37 °C and performed by AMPTS II instrument (Bioprocess Control AB, Sweden) as described previously (Lu et al., 2019). Briefly,
Enhancement of methane production by FeCl2 addition and evolutions of VFAs
The paired samples t-test indicated that ferric oxide significantly influenced the methane production during AD of swine manure at FC-5 (p < 0.01). FeCl2 increased the accumulative methane production by maximum 21.5% from 221.5 mL CH4 g−1VSadded (CK) to 269.1 mL CH4 g−1VSadded when adding 5 mmol/L FeCl2 (FC5; Fig. 1a). However, a low cumulative methane production was observed with supplementation levels higher than 5 mmol/L FeCl2. The increase in methane production occurred mainly during the
Conclusions
FeCl2 could both improve the accumulative methane production and reduce the abundance of total ARGs during the AD of swine manure. The accumulative methane production was increased by 21.5% at FC5, and the maximum reduction of total ARGs was increased by 33.3% at FC25. The reduction of both pathogenic microorganisms and MRGs was also enhanced. FeCl2 intensified the utilization of acetate and propionate by considerably enhancing H2 utilization in microorganisms and DIET. The bacterial community
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was financially supported by the National Major Science and Technology Projects for Water Pollution Control and Management of China (2017ZX07102-002), the National Natural Science Foundation of China (51808540), the National Key Research & Development Plan of China (No. 2016YFD0501405), the Jiangxi Key Research & Development Plan Program of China (20171ACG70018), and the Guangxi Key Research & Development Program of China (AB16380025).
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Tiedong Lu and Junya Zhang contributed equally to this work.