Start-up of a Granular sludge Sequencing Batch Reactor for the treatment of 2,4-dichlorophenol contaminated wastewater

Chlorophenols are characterized by high toxicity even at relatively low concentrations, strong tendency to bioaccumulate and persistency in water and soil. In this study, a Granular sludge Sequencing Batch Reactor (GSBR) was started-up for the biological aerobic treatment of wastewater containing 2,4-dichlorophenol (2,4-DCP), in presence of readily biodegradable sodium acetate (NaAc) as the biogenic substrate. Different influent concentrations of NaAc (420-800 mg/L) and 2,4-DCP (0-20 mg/L), as well as different operating conditions (i.e. cycle length) were tested in order to determine the optimal strategy for successful GSBR start-up: stable granulation and complete 2,4-DCP removal with stoichiometric chloride release (indicating the complete dechlorination of the toxic compound) were achieved only when high NaAc influent concentration and volumetric organic loading rates (800 mg/L and 1.9 kgCOD/m3?d, respectively), prolonged reaction phase (cycle time of 4 hours) and gradual increase of 2,4-DCP concentration in the influent up to 20 mg/L were applied, thus providing useful information for process optimization in view of future scale-up. Granules were initially colonised by fungi which disappeared throughout the experimental activity, and complete 2,4-DCP removal was mostly due to bacterial activity.