Microbial community shifts and biogas conversion computation during steady, inhibited and recovered stages of thermophilic methane fermentation on chicken manure with a wide variation of ammonia
Introduction
The methane fermentation of organic solid wastes, such as the organic fractions of municipal solid waste and agricultural residues have been investigated extensively worldwide. This process serves organic waste stabilization and simultaneously generates energy, thus responding to two of society’s most urgent needs: the one for alternative clean energy and the one for more sustainable waste disposal. About 13 million tons of chicken manure (CM) are produced in Japan annually which is a typical agricultural waste well-suited to methane fermentation because it has a high fraction of biodegradable organic matter (Niu et al., 2013).
Compared to mesophilic fermentation (30–40 °C), thermophilic fermentation (50–60 °C) generally results in high methanogenic efficiency, is more economical to operate and eliminates pathogens with sanitizing effects. However, the thermophilic process is more sensitive to changes in the factor with affect operation, such as TS, pH, Volatile Fatty Acid (VFA), ammonia and toxic substrates(Abbassi-Guendouz et al., 2013). Free ammonia (FA) is pH and TAN (total ammonia nitrogen) depended in thermophilic fermentation, and is considered a key inhibitor in chicken manure methane fermentation (Niu et al., 2013). It has been widely accepted that FA is the cause of the inhibition with inactive enzymes and the ease of diffusivity into the cell membranes the NH3 ionized to , with results in a pH imbalance between inside and outside of the cell. This pH change affects both the transportation of the materials and leads to lower enzyme activity (Kadam and Boone, 1996).
Anaerobic digestion usually involves several consequent degradation phases, such as hydrolysis, acidogenesis, and then methanogenesis (Gujer and Zehnder, 1983). High N-content makes CM fermentation easily inhibited by ammonia. Hashimoto (1986) reported that both thermophilic and mesophilic processes are inhibited at a TAN of 2500 mg/L. Compared to the bacteria responsible for hydrolysis and acidogenesis, the inherently low growth rate of methanogen archaea makes the anaerobic systems sensitive to environmental changes (Xing et al., 1997). Different trophic level affects the entire community and thus reducing the efficiency of the process. As functionally establish the tolerance of microbial community for the environment press, it is important to investigate the microbial community in the reactor when characterizing the entire sequential metabolic process as a guide to operation conditions. A previous comparison of the diversity in different anaerobic bioreactors reveals that variations in reactor design, operating conditions and substrate composition have a strong impact on the development of microbial communities (Leclerc et al., 2004).
Although there are studies on microbial communities under thermophilic wet digestion conditions, only a few reports have focus on the characteristic microorganisms and microbial communities in thermophilic methane fermentation on manure (Yabu et al., 2011). Previous researches reported the microbial community mainly focuses on chicken manure composts (He et al., 2013). As such, more information is required about the microbial community in thermophilic CM fermentation in order to understand how to operate the process effectively. Therefore, in this work, a long-term methane fermentation process feeding with 10% TS of CM using a continuously stirred tank reactor (CSTR) was performed to investigate the performance and microbial community in the stable, inhibited and recovery stages of the reactor. The main inhibited factors of the process were also evaluated by principal components analysis (PCA).
Section snippets
CM properties
Original CM with a TS of 44.3% was kept in the refrigerator at 4 °C. Raw CM was diluted to 10 ± 2% TS content with tap water. The diluted CM was shredded into slurry using a heavy duty laboratory blender and was provided for the CSTR reactor. The shredded CM was stored in a substrate tank with 4 °C cooling water circulation to avoid microbial activity. The raw CM was pretreated to reduce nitrogen through ammonia fermentation and ammonia stripping. The ammonia stripped CM, hereafter referred to as
Performance of reactor
The whole experiment was divided into 5 phases. In phase I, pretreated CM was used to investigate low nitrogen CM fermentation performance. In phase II, raw CM was used and long term tested to analyze nitrogen accumulation and its inhibitory effects. In phase III, NH4HCO3 was added together to the raw CM to increase the TAN to an extreme high concentration. In phase IV, feed cased and diluted by 2 times with tap water at day 240 to control the TAN keep around 4000 mg/L. In phase V, the biomass
Conclusions
- (1)
0.74 m3/kg VS degraded of biogas produced and 70.93 g/kg VS degraded of ammonia nitrogen generated in thermophilic CM fermentation. Recovery stratagem of dilution was failed since after synthesized inhibition of ammonia and VFA.
- (2)
Hydrogenotrophic M. thermautotrophicus str has a higher ammonia tolerance shifting to the dominate archaea in the reactor. Aceticlastic methanogens are more sensitive to ammonia inhibition, thus revealing serious VFA accumulation.
- (3)
The dominant phylum of Firmicutes was 84.3%
Acknowledgements
This work was supported by funding from the Japan Society for the Promotion of Science (JSPS-24.02053). The authors would like to acknowledge the Engineering and service Cooperation of Hitachi Co. ltd for this study.
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