The effect of pH profiles in methanogenic aggregates on the kinetics of acetate conversion

Due to the conversion of acetic acid into the weaker carbonic acid and methane, the pH inside methanogenic aggregates is higher than in the bulk liquid. The pH profiles in aggregates were measured with pH microelectrodes. These profiles strongly determine the macro-kinetics of the aggregate, by their influence on the values of the growth parameters Ks and μmax. Acetate transport resistances were not limiting for the conversion rate in methanogenic aggregates. Nevertheless, the effectiveness factor η did not approach unity, but amounted to 0.57–0.62 in the acetate concentration range relevant for most methanogenic reactors. The value of η is determined almost entirely by the pH profiles inside the aggregates. It was concluded that for the physical/mathematical description of the conversion in methanogenic aggregates, information on the pH gradients and the pH dependency of the growth parameters is indispensable. Batch experiments showed that acetate uptake by aggregates was not coupled directly to methanogenesis. Consumed acetate was not converted instantaneously to methane, suggesting the conversion to proceed via a pool of acetate or reserve material.