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

Highlights

• Microbial community shifts with inhibition was investigated in CH₄ fermentation (55 °C).

• TAN cased VFA accumulation and then synergistically inhibited process over 6 g/L.

• The unrecoverable Archaeal dynamic affected by ammonia makes the resilience weak.

• Methanothermobacter has higher tolerance than Methanosarcina on ammonia inhibition.

• Hydrolytic and acidogenic bacteria dynamic were significantly after inhibition.

Abstract

The thermophilic methane fermentation of chicken manure (10% TS) was investigated within a wide range of ammonia. Microbiological analysis showed significant shifts in Archaeal and Bacterial proportions with VFA accmulation and CH₄ formation before and after inhibition. VFA accumulated sharply with lower methane production, 0.29 L/g VS, than during the steady stage, 0.32 L/g VS. Biogas production almost ceased with the synergy inhibition of TAN (8000 mg/L) and VFA (25,000 mg/L). Hydrogenotrophic Methanothermobacter thermautotrophicus str. was the dominate archaea with 95% in the inhibition stage and 100% after 40 days recovery compared to 9.3% in the steady stage. Aceticlastic Methanosarcina was not encountered with coincided phenomenal of high VFA in the inhibition stage as well as recovery stage. Evaluation of the microbial diversity and functional bacteria indicated the dominate phylum of Firmicutes were 94.74% and 84.4% with and without inhibition. The microbial community shifted significantly with elevated ammonia concentration affecting the performance.

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 ${\text{NH}}_4^{+}$, 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).