Recalcitrant Organics Emerging from Biological Treatment of Kraft Mill Effluents

Abstract

Abstract Recalcitrant organic matter (ROM) in combined kraft mill effluents is that organic matter remaining in the effluents after primary and secondary treatment. Recalcitrant organic matter comprises of both high molecular weight (HMW) and low molecular weight (LMW) components and is of interest, since environmental regulators are considering placing limits on final effluent COD and colour. Biologically treated pulp mill effluent was fractionated by ultrafiltration to study the contributions of the high and low molecular weight recalcitrant organics towards final effluent COD and AOX. Batch biodegradation tests were carried out on lab-generated biotreated effluent from lab scale sequencing batch reactors operating at 35, 45, 55 and 60°C, to investigate if the residual recalcitrant fraction could be further degraded. Biodegradation tests involved the optimization of the microbiological medium by the addition of either an alternate carbon source (glucose) or a carbon-nitrogen substrate (yeast extract). Treatment temperatures and nutrient levels were varied and the effect of each of these four factors on the biodegradability of the recalcitrant fractions was studied. The recalcitrant portion was found to be resistant to further biodegradation, even under optimized microbiological conditions. The HMW fraction of the ROM obtained from final biotreated effluent from a bleached kraft pulp mill (HMW ROMMill) was studied for its ability to bind other organic model pollutants in an aqueous environment. Pentachlorophenol (PCP) was tested for its binding onto the HMW ROMMill, using toxicity as a surrogate parameter for binding, in the Microtox™ test. Equilibrium dialysis studies were carried out to investigate the ability of HMW ROMMill to bind 14C-Benzopyrene (BaP) and 3H-dehydroabietic acid (DHA). Microtox™ studies failed to indicate the binding of PCP onto HMW ROMMill. BaP and DHA however did bind onto HMW ROMMill. BaP binding onto HMW ROMMill was higher than DHA binding, possibly due to its hydrophobicity. Also, increasing the dissolved organic carbon concentration of HMW ROMMill led to a decrease in the partition coefficient values for both BaP and DHA.