Addressing the influence of homoacetogenesis and syntrophic oxidation of acetate on methanogenic activity via the use of inhibitors

Methanogenic activity tests are a cost-effective tool for controlling the operation of full-scale anaerobic reactors. They serve as indicators of inoculum quality and provide an estimate of the maximum methane production rate from either acetate (in the aceticlastic activity test) or hydrogen and carbon dioxide (in the hydrogenotrophic activity test). This work addresses an often neglected point: the role of the reversible homoacetogenesis route in methane production during the test, which could distort the kinetic parameters obtained for each group of methanogenic archaea (aceticlastic and hydrogenotrophic). To investigate this phenomenon, and for the first time, a dual inhibitor methodology was developed based on performing activity tests with different concentrations of an archaeal inhibitor, 2-bromoethanesulfonate (BES), to selectively inhibit methanogenesis and a bacterial inhibitor, ampicillin, to inhibit reversible homoacetogenic reactions. For the inoculum source studied, it was found that hydrogen consumed by the homoacetogenic pathway was less than 5 % of the total hydrogen consumption in the hydrogenotrophic activity tests, whereas in the aceticlastic activity tests, the methane formed by the alternative homoacetogenic pathway became important only if aceticlastic methanogens were inhibited. In addition, the monitoring of an industrial reactor is analyzed considering these results, confirming the importance of the hydrogenotrophic pathway and the resilience of the sludge.