Effect of upward velocity and sulphide concentration on volatile fatty acid degradation in a sulphidogenic granular sludge reactor

During anaerobic treatment of sulphate-containing wastewaters, sulphate- reducing, bacteria (SRB) compete with methane-producing bacteria (MPB) for the available electron-donors. In this work, the anaerobic treatment of a synthetic wastewater, consisting of a mixture of acetate, propionate and butyrate and high concentrations of sulphate (COD:sulphate ratio 0.5) was studied in an upflow anaerobic granular sludge bed reactor. The influence of the superficial upward liquid velocity (v(up)), the influent composition and reactor pH on the competition between SRB and MPB was investigated. At a v(up) of 2 m h^-1 and pH 8, 93-97% of the COD was degraded by SRB. With increasing v(up)-values, COD removal efficiencies decreased, while at a v(up) of 6 m h^-1 the fraction of COD removed by MPB rose to 23%. Elevation of the influent acetate concentrations, by decreasing the v(up) (lower recirculation) or by the use of an influent volatile fatty acid mixture with u higher acetate content, resulted in an increase of methanogenesis up to 41% of the total COD removal. In contrast, elevated levels of propionate and butyrate in the influent favoured the sulphate reducing process. A decrease of pH from 8 to 7 resulted in free hydrogen sulphide concentrations higher than 200 mg litre^-1. This strongly inhibited methanogenesis while SRB were hardly affected, with a subsequent decrease of the COD removed by MPB from 41 to 7% as a result.