The effect of liquid upward velocity and hydraulic retention time on granulation in UASB reactors treating wastewater with a high sulphate content

The effect of hydraulic retention time and liquid upward velocity on the granulation process and the competition between sulphate-reducing and methane-producing bacteria during anaerobic treatment of sulphate-containing wastewater was studied. The results showed that hydrogen, generated during the anaerobic mineralisation process, was completely oxidised by sulphate-reducing bacteria. Acetate was oxidised by both sulphate-reducing and methane-producing bacteria. The fraction of acetate used by sulphate reducers relative to methanogens increased with time, resulting in a predominance of sulphate-reducing bacteria, especially at relative long hydraulic retention time (40 h). The granulation process was favoured by the combination of high upward velocity and short hydraulic retention time. Very thin filaments (possibly sulphate reducers) may serve as primary nuclei for the attachment of Methanothrix, which starts the granulation process. No difference between the attachment capacity of sulphate-reducing and methanogenic bacteria was found.