REMOVAL OF ORGANIC MICROPOLLUTANTS IN LABORATORY ACTIVATED-SLUDGE REACTORS UNDER VARIOUS OPERATING-CONDITIONS - SORPTION

The sorption to activated sludge biomass of four model chemical pollut ants, 2,4-dichlorophenol (2,4-DCP), 2,4,6-trichlorophenol (2,4,6-TCP), pentachlorophenol (PCP) and lindane, was studied in laboratory experi ments. Sorption isotherms were determined for three biomass samples: a sludge sample from a municipal waste/water treatment plant and two re sulting laboratory cultures grown on synthetic peptone sewage at solid s retention times (SRT) of 2.7 and > 100 days, respectively. The sorpt ion experiments were performed in test tubes with various concentratio ns of model compounds added. Based on linear sorption coefficients, th e removal by sorption was calculated for activated sludge reactors ope rated at various conditions. This made it possible to quantify that pa rt of the observed removal of the model compounds, which was due to bi odegradation, and that part which was due to sorption. Removal by sorp tion was only important for PCP and was up to 50% of the total removal at short SRTs (< 3 days), decreasing to 5-10% at higher SRTs (> 14 da ys). There are two reasons to expect that sorption in bioreactors with low SRTs will play a more important role than for bioreactors with hi gh SRTs: (i) at low SRT the slow growing specific degraders will be wa shed out from the reactor system. Consequently, biodegradation is stop ped and the concentration level in the bioreactor increases, which lea ds to an increased sorption and (ii) a higher mass-flow of wasted biom ass will contribute to an increased removal of pollutants in the sorbe d phase. The linear sorption coefficients for 2,4-DCP and 2,4,6-TCP we re a little higher than expected from log K(ow) correlations, for PCP much higher than expected and for the non-polar compound lindane withi n the expected range. For the chlorinated phenols, this was probably d ue to some sorption of the ionized phenolate fractions.