Fate of phytosterols in pulp and paper wastewater treated in a simulated aerated stabilization basin

Abstract

Phytosterols are steroid chemicals produced by plants for the purposes of membrane function and hormone production. Phytosterols can cause endocrine disruption in aquatic species at very low concentrations and are suspected of contributing to endocrine disruption linked to pulp and paper effluent. Wastewater from the pulp and paper industry is often treated biologically in aerated stabilization basins (ASBs) that expose phytosterols to a range of redox zones. Phytosterol removal in ASBs varies and stigmasterol has even been shown to increase across the treatment system. Little is known about the microbial processes that occur within ASBs and their effect on phytosterol removal. The objective of this research was to assess the biotransformation potential of phytosterols in a simulated ASB treatment system and to improve understanding of the processes that occur within the various redox zones and their impact on the removal of phytosterols. To assess the biotransformation of phytosterols under aerobic conditions, three assays were conducted using a stock aerobic culture fed with pulp and paper wastewater. The assays tested three conditions: phytosterols present as a sole added carbon source, phytosterols with dextrin as an added carbon source, and phytosterols with ethanol as a solubilizing agent and added carbon source. Phytosterol biotransformation was found to be limited by low phytosterol solubility. When solubilized, phytosterol removal occurred in two phases: an initial near-linear removal, followed by accelerated removal during the culture's stationary stage, possibly due to the release of extracellular cholesterol oxidase. The anoxic and anaerobic biotransformation of phytosterols was examined through a series of three semi-batch cultures maintained under nitrate-reducing, sulfate-reducing and fermentative/methanogenic conditions, all developed from stock cultures fed with pulp and paper wastewater. Phytosterol removal was significant in the nitrate-reducing culture, although microbial activity and phytosterol removal declined in later stages. Phytosterol removal was also observed in the sulfate-reducing culture, although there was a significant lag period before removal occurred. No phytosterol removal was observed in the fermentative/methanogenic culture. Phytosterol biotransformation was also examined in the context of a lab-scale ASB fed continuously with pulp and paper wastewater. The steady-state ASB effluent and sediment characteristics were examined over three hydraulic retention times (HRTs). Effluent quality was not significantly affected by a change in HRT but sediment characteristics were significantly affected and, at shorter HRTs, phytosterols accumulated in the sediment. Wastewater bioassays demonstrated the release of phytosterols during the breakdown of solids. This research improves the understanding of biological processes within ASBs and their effect on phytosterol removal.