FATE OF EMERGING CONTAMINANTS IN WASTEWATER TREATMENT PLANTS

Abstract

Emerging contaminants (ECs) such as pharmaceuticals, personal care products, hormones, and industrial products found in raw and treated wastewater limit the reuse of reclaimed water. The study assessed the occurrence level and removal efficiency of 20 selected ECs by eight different full-scale wastewater treatment technologies over three years. The fate of the ECs during the treatment was investigated. The suitability of biological activated carbon to remove residual ECs in the secondary wastewater was checked. Based on the relative removal ranking criteria, the efficiency of different wastewater treatment technologies in removing ECs was found in the order: biological nutrient removal-based plants>waste stabilization pond>up-flow anaerobic sludge blanket technique>densadeg-biofor>anoxic-aerobic process>activated sludge process>combitreat integrated sequencing batch reactor. Environmental risk assessment at different trophic levels showed potential risk (Risk quotient > 1) from ciprofloxacin, sulfamethoxazole, trimethoprim, and triclosan for algae, triclosan for daphnia, and estrogens and triclosan for fish. Mass balance analysis revealed biodegradation as the major pathway for transformation/biodegradation, and sorption as the minor removal mechanism for ECs in all the WWTPs. The fraction of ECs sorbed on the sludge, measured as sorption distribution coefficient (Kd values), showed different sorption behaviour based on the type of sludge. The hydrophobic interactions measured in terms of octanol-water partitioning coefficient (Kow) and the hydrophobic-electrostatic interactions measured in terms of distribution coefficient (Dow) were insufficient in explaining the sorption behaviour of ECs. The full-scale BNR systems were separately investigated to understand the role of redox conditions and sludge recirculation. Removal in different redox conditions followed the order: aerobic>anaerobic>anoxic. A positive correlation existed between the concentration of ECs in the dissolved and particulate phases in all the redox conditions. The influence of redox conditions, F/M ratio, and microbial diversity in removing the ECs were evaluated by a 60 L pilot scale plant based on modified ludzack ettinger process with a moving bed bioreactor unit containing polyvinyl acetate (PVA) gel media, placed post anaerobic reactor. The total mass load removal of the 20 ECs was around 85%. However, when excluding the highly degradable compounds, caffeine and acetaminophen, mass load removal was 56%, highlighting the limited removal of antibiotics, anti-inflammatory drugs, carbamazepine, and hormones. The F/M ratio was inversely related to the removal of ECs with a correlation coefficient, r2 =0.84. The microbial diversity in the reactors was observed in the order aerobic>anoxic>PVA>anaerobic, which is directly related to the % removal of ECs. Considering only partial removal of ECs from the biological wastewater treatment, studies were conducted to explore the potential of biological activated carbon (BAC) in removing residual ECs in the secondary treated wastewater. The BAC removed chemical oxygen demand, UV254, and NH4-N by up to 60% and total organic carbon by about 40%. All ECs showed good removal efficiency after treatment with BAC. Compounds such as naproxen, carbamazepine, trimethoprim, and diclofenac which showed low removal in the WWTPs, were removed by >50% after treatment by BAC. Small-scale columns were studied to understand the role of sorption and biodegradation in removing ECs by BAC. Only the negatively charged and some neutral ECs were impacted by inhibition of bioactivity, indicating that the removal of such compounds is dependent on biodegradation, and the positively charged ECs are more sorption dependent in the BAC column. The results indicate that BAC is a sustainable and cost-effective tertiary/polishing technique in removing ECs from secondary treated wastewater.