Laboratory-scale bioaugmentation relieves acetate accumulation and stimulates methane production in stalled anaerobic digesters

Abstract

An imbalance between acidogenic and methanogenic organisms during anaerobic digestion can result in increased accumulation of volatile fatty acids, decreased reactor pH, and inhibition of methane-producing Archaea. Most commonly the result of organic input overload or poor inoculum selection, these microbiological and biochemical changes severely hamper reactor performance, and there are a few tools available to facilitate reactor recovery. A small, stable consortium capable of catabolizing acetate and producing methane was propagated in vitro and evaluated as a potential bioaugmentation tool for stimulating methanogenesis in acidified reactors. Replicate laboratory-scale batch digesters were seeded with a combination of bioethanol stillage waste and a dairy manure inoculum previously observed to result in high volatile fatty acid accumulation and reactor failure. Experimental reactors were then amended with the acetoclastic consortium, and control reactors were amended with sterile culture media. Within 7 days, bioaugmented reactors had significantly reduced acetate accumulation and the proportion of methane in the biogas increased from 0.2 ± 0 to 74.4 ± 9.9 % while control reactors showed no significant reduction in acetate accumulation or increase in methane production. Organisms from the consortium were enumerated using specific quantitative PCR assays to evaluate their growth in the experimental reactors. While the abundance of hydrogenotrophic microorganisms remained stable during the recovery period, an acetoclastic methanogen phylogenetically similar to Methanosarcina sp. increased more than 100-fold and is hypothesized to be the primary contributor to reactor recovery. Genomic sequencing of this organism revealed genes related to the production of methane from acetate, hydrogen, and methanol.

Keywords: Acetoclastic; Anaerobic digestion; Bioaugmentation; Methanogen; Thermophilic.

Fig. 1

Accumulated methane (circle) and acetate (triangle) in digestate from bioaugmented and media control reactors R1–R5. Digesters were treated with either an acetoclastic consortium or sterile medium on day 28 (indicated by the dotted line)

Fig. 2

Digestate pH in bioaugmented (a) and media control (b) reactors R1–R5. Digesters were treated with either an acetoclastic consortium or sterile medium on day 28 (indicated by the dotted line)

Fig. 3

Normalized methane as a percentage of total biogas produced in bioaugmented and media control reactors R1–R5. Digesters were treated with either an acetoclastic consortium or sterile medium on day 28 (indicated by the dotted line)

Fig. 4

Gene abundances of bacterial cpn60 for OTU1109 (circle) and OTU2923 (triangle), and archaeal thermosome for OTU805 (cross) and OTU795 (square) in the digestate of bioaugmented and media control reactors R1–R5. OTU2923 and OTU795 were only detected after bioaugmentation treatment. Digesters were treated with either an acetoclastic consortium or sterile medium on day 28 (indicated by the dotted line)