Biological removal of emerging micropollutants in nitrifying activated sludge at low temperatures

Growing concentrations of emerging micropollutants (EMs) such as pharmaceuticals, endocrine disruptors and personal care products are found in the aquatic environment worldwide. These substances could pose the risk on humans and animals due to their chronic ecotoxicity and persistence. The main route of EMs emission into the natural waters is through effluents of wastewater treatment plants (WWTPs). Operational сonditions can significantly affect biodegradation of EMs during wastewater treatment. In particular, low temperature limits biological processes due to slower metabolic reactions and decrease of bacterial diversity in activated sludge.In this thesis, biological removal of EMs from wastewater was studied at low temperatures (8-12°C). Different operational conditions were compared in order to increase removal efficiency and therefore enhance the quality of WWTPs effluents. Laboratory-scale Sequencing Batch Reactors (SBRs) and Membrane Bioreactors (MBRs) were used to mimic existing wastewater treatment processes. Data from full-scale WWTPs of Helsinki region were taken into account. The study presents the removal rates for widely-used EMs ibuprofen, diclofenac, carbamazepine, estrone, 17β-estradiol and 17α-ethynylestradiol. Altogether, obtained removal efficiencies were much lower compared to published data for higher temperatures. Biodegradation studies demonstrated that EMs might accumulate in activated sludge cells at large extent. Depending on the substance, this accumulation could be followed by biodegradation or by return of the compound to the wastewaters with the cell decay. Therefore, following the concentrations of EMs in solid phase of activated sludge is necessary for the assessment of biodegradation potentials. Sludge retention time (SRT) proved to be an effective operational tool for regulating biological processes during wastewater treatment. Prolongation of the SRT demonstrated positive effect on activated sludge performance and EMs biodegradation at low temperatures. At the same time, decrease of the temperature raised the negative pressure effect on activated sludge, limiting the removal potential of EMs as well as inhibiting nitrification. Thus, SRT optimum is closely related to the temperature fluctuations. Contribution of different microorganisms in performance of nitrifying activated sludge and EMs removal at cold temperatures is discussed in this study. In addition to six above mentioned EMs effect of antibiotics sulfadiazine and trimethoprim was studied. Described microbial communities significantly differed from typical nitrifying activated sludge at class level. EM removal potential and the need for further research of bacterial class Deltaproteobacteria and domain Archaea is proposed. Overall, prolongation of SRT improved stress resistance of microbial community resulting in stable performance of activated sludge.