Removal of organic micropollutants in laboratory activated sludge reactors under various operating conditions: sorption

Abstract

The sorption to activated sludge biomass of four model chemical pollutants, 2,4-dichlorophenol (2,4-DCP), 2,4,6-trichlorophenol (2,4,6-TCP), pentachlorophenol (PCP) and lindane, was studied in laboratory experiments. Sorption isotherms were determined for three biomass samples: a sludge sample from a municipal waste/water treatment plant and two resulting laboratory cultures grown on synthetic peptone sewage at solids retention times (SRT) of 2.7 and > 100 days, respectively. The sorption experiments were performed in test tubes with various concentrations of model compounds added. Based on linear sorption coefficients, the removal by sorption was calculated for activated sludge reactors operated at various conditions. This made it possible to quantify that part of the observed removal of the model compounds, which was due to biodegradation, and that part which was due to sorption. Removal by sorption 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 days). There are two reasons to expect that sorption in bioreactors with low SRTs will play a more important role than for bioreactors with high SRTs: (i) at low SRT the slow growing specific degraders will be washed out from the reactor system. Consequently, biodegradation is stopped and the concentration level in the bioreactor increases, which leads to an increased sorption and (ii) a higher mass-flow of wasted biomass will contribute to an increased removal of pollutants in the sorbed phase. The linear sorption coefficients for 2,4-DCP and 2,4,6-TCP were a little higher than expected from log K ow correlations, for PCP much higher than expected and for the non-polar compound lindane within the expected range. For the chlorinated phenols, this was probably due to some sorption of the ionized phenolate fractions.