Effects of different primary substrate concentrations on 2,4-dichlorophenol removal by aerobic granular sludge

In this study, the aerobic granular sludge technology was applied for the degradation of 2,4-dichlorophenol (2,4-DCP, up to 30 mg/L), with sodium acetate (NaAc) as the primary (growth) substrate: since dosage of an external carbon source may represent a relevant cost at larger scale, the effects of decreasing NaAc influent concentrations (from 800 to 400 mg/L) on process performance were evaluated, in order to reduce operating costs. High NaAc concentration in the influent, a key aspect during process start-up, was shown to play a minor role for granular sludge long term operation: both NaAc and 2,4-DCP were completely removed during the whole experimental activity and the observed chloride release was found to be stoichiometric (indicating the complete mineralization of the toxic compound). The increase of the applied 2,4-DCP/NaAc ratio from 0.025 to 0.075 also influenced microbial population dynamics, and probably forced granular biomass to enhance self aggregation as a defensive strategy against the toxic effects of 2,4-DCP: the increase of granules density (from 68±2 up to 124±6 gTSS/Lgran) was accompanied by enhanced settling ability and better effluent quality. As shown by FISH, no inhibiting effects were observed on overall microbial activity and biodiversity of granular biomass at increasing 2,4- DCP/NaAc ratios.