The effects of organic acids on the methanogenic degradation of the landfill leachate molecules butyrate and valerate

The effects of elevated volatile fatty acid (VFA) concentrations, as distinct from pH, on the methanogenic degradation of butyrate and valerate were determined. Microbial associations capable of degrading butyrate or valerate were enriched from municipal refuse and stabilised in multi-stage continuous culture systems. The responses of these associations to each of the applied acids (100 mmol l−1 acetate, propionate, or butyrate) indicated that the butyrate- and valerate-degrading associations were characterized by different populations of acetogenic and sulphate-reducing bacteria, despite being enriched from the same refuse source. The sulphate reducing bacteria were generally more susceptible to elevated VFA concentrations than the methanogens. Propionate promoted methanogenesis in the presence of bacteria capable of metabolising this fatty acid, but had an inhibitory effect on butyrate oxidation. In most cases, 100 mmol l−1 acetate indirectly inhibited methane production. It was thus recommended that leachate concentrations of acetate and propionate should be closely monitored, since the onset of methanogenesis in landfill sites may be delayed by elevated levels of these volatile fatty acids, rather than the concomitant reduction in leachate pH.