Kinetics for the Methanogen’s Death in the Acidic Environments

This study focuses on the inactivation of methanogens under acidic environment that may arise due to overloading of anaerobic reactors. Two types of methanogen-enriched cultures were prepared in the lab-scale reactors using acetate and formate as substrate. Each culture was subsequently incubated in a batch reactor for 6 days under different pH conditions with one of the VFAs of formate, acetate, propionate, butyrate, valerate or phosphate buffer solution. Propidium-monoazide quantitative polymerase chain reaction (PMA-qPCR) analysis and the methane production test revealed that the methanogenic archaea were highly sensitive to the acidic environment. Under the moderate pH of 6.5–7.5, no significant change in cellular decay was observed. However, at pH below 6.5 the decay rate was accelerated leading to archaea’s inactivation. At pH 5.0, the archaeal specific decay rates were elevated as high as 40 times of that at pH 7.0. When the operational pH was the same in the experiments, the cellular decay rate was comparable between the batch test with VFA and that without VFA. These observations strongly suggest that the methanogen decay is caused by low pH rather than the elevated concentrations of VFA compounds (15–40 mM of undissociated VFA) during acidic failure of anaerobic digester.