Abstract
Anaerobic digestion (AD) is an attractive technology for sludge treatment as it stabilizes sludge and produce renewable energy. However, problems such as low organic matter content and high heavy metals level are often encountered which severely limits the effectiveness of AD. In this study, the biochar-supported nanoscale zerovalent iron (nZVI-BC) was synthesized and used as additives during AD of sewage sludge to investigate the enhancement effects for methane production and its impacts on microbial structure at mesophilic temperature. nZVI-BC addition enhanced process stability by improving the generation and degradation of intermediate organic acids, but inhibitory effects were observed at high dosage. The methane content and cumulative methane yields were increased by 29.56% and 115.39%, respectively. Compared with AD without nZVI-BC, the application of nZVI-BC showed positive effect on improvement of metals (Cu, Cd, Ni, Cr, and Zn) stabilization in the digestate. Microbial community analysis illustrated that nZVI-BC addition could significantly increase the Shannon diversity index and Chao1 richness index of archaea, and meanwhile archaea were more diverse in nZVI-BC amended digesters than in control. It was notable that Methanosaeta dominated in all the digesters at genera level, while the relative abundance of hydrogenotrophic methanogens (Methanobacterium and methanospirillum) increased 35.39% in nZVI-BC amended digesters compared to the control, resulting in higher methane production. The results will guide development of microbial management methods to enhance the stability of AD process.
Highlights
Because of the increase in the wastewater treatment capacity, large amounts of sewage sludge as a by-product is produced and stockpiled in environment, which pose a potential environmental risk if not disposed appropriately
The decreased specific surface area of Nanoscale zerovalent iron (nZVI)-BC was attributed to the reason that the pore structure of biochar was completely blocked by nZVI particles (Zhu et al 2019)
366.37 ± 11.2 339.59 ± 11.29 244.49 ± 11.72 234.88 ± 9.02 207.50 ± 8.71 193.60 ± 7.79 microbial richness among the six different treatments, while the S6 samples shown the lowest. These results revealed that moderate dosage of nZVI supported on biochar composite (nZVI-BC) addition could increase the diversity of microbial community
Summary
Because of the increase in the wastewater treatment capacity, large amounts of sewage sludge as a by-product is produced and stockpiled in environment, which pose a potential environmental risk if not disposed appropriately. Environ Sci Pollut Res (2019) 26:10292–10305 and effective approaches are urgently needed prior to landfill or agricultural application due to the high bioavailability of heavy metals in sludge (Donatello and Cheeseman 2013; Wang et al 2018). The methane production increased rapidly in the start-up stage of the AD process (< 5–7 days), while the amount of methane production rose up little after digestion for 20 days This is because the initial methane production peak could correspond to the decomposition of the available soluble organic substrates in the sludge; afterwards, the methanogenic activities were inhibited due to the substantial accumulation of VFA and available organic substrates were being metabolized (Xi et al 2014). The maximum daily methane production was observed in the digesters with 2.50 g nZVIBC treatment, which yields 11.38 L kg−1 VS/day on day 5, while the other additives were unable to yield as high as that.
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