Microflora communities which can convert digested sludge to biogas

ABSTRACT

In the present study, we developed several microflora communities that utilize digested sludge (DS), the recalcitrant waste product of anaerobic digestion, as a substrate for biogas production with the aim of their future application to DS recycling. Strict enrichment with DS as the sole nutrient source was introduced to culture microbes from soil and herbivore dung samples; microflora communities promoting stable levels of biogas production were obtained. The average methane and hydrogen yield from soil-derived microflora were 4.86 and 0.94 ml per 1.0 g DS, respectively. Notably, two microflora communities enriched from a riverbank sediment produced 20.79 ml and 14.10 ml methane from 1.0 g DS. By contrast, the methane and hydrogen yield for herbivore dung-derived microfloras were on average 1.31 ml and 1.87 ml per 1.0 g DS, respectively. Potent hydrogen-biogas producers were obtained from rabbit (4.12 ml per 1.0 g DS), goat (3.16 ml per 1.0 g DS), and sheep dung (2.52 ml per 1.0 g DS). The cultured microflora communities included representatives from the eubacterial genera, Clostridiaceae and Eubacteriaceae together with several anaerobic genera. Pseudomonas spp. are found in the riverbank sediment-derived microfloras, suggesting that the floras employ syntrophic acetate oxidation and hydrogentrophic methanogenesis (SAO-HM) pathway for methane production. The methanogenic microflora communities were dominated by bacteria from the Methanobacteriaceae family and unclassified archaea. Moreover, ascomycetous fungi and protists were found, implying that they act as oxygen scavengers and bacterial grazers, respectively. Enzymatic analysis suggested that the microfloras hydrolyze DS via cellulase, chitinase, and protease activities.

GRAPHICAL ABSTRACT

KEYWORDS:

• Digested sludge
• biogas
• microflora
• hydrolyzing enzyme
• environmental biotechnology

Acknowledgments

This study was funded by a Grant-in-Aid for Scientific Research (C: 17K00668) from the Japan Society for the Promotion of Science.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Author contribution

AK and KF: conceived and designed the experiment. AK, SO, YH, and KF: performed the research. AK, SO, YH, and KF: analysed data. AK and KF: wrote the paper. AK, NA, and KF: contributed to the critical appraisal of the paper.

Additional information

Funding

This work was supported by the Japan Society for the Promotion of Science [Grant-in-Aid for Scientific Research (C: 17K00668)].