Removal of trace organic chemicals by membrane bioreactors and hybrid processes

Membrane bioreactors (MBRs) can effectively remove a wide range of trace organic contaminants (TrOCs), yet their fate and recalcitrant contaminant removal during MBR treatment is uncertain. This study focused on revealing the fate and removal of TrOCs during MBR treatment, membrane distillation (MD) and novel membrane distillation bioreactor hybrid treatment (MDBR). It also aimed to elucidate the effect of physicochemical properties (namely, molecular structure, log D, and volatility) on the fate and removal of TrOCs during MBR, MD and MDBR treatment. The effect of molecular properties on the fate of trace organic contaminants in the aqueous and solid phases during wastewater treatment by MBR was comprehensively examined. A set of 29 TrOCs was selected to represent pharmaceuticals, steroid hormones, phytoestrogens, UV-filters and pesticides that occur ubiquitously in domestic wastewater. Both adsorption and biodegradation/transformation were found to be responsible for the removal of TrOCs by MBR treatment. Molecular structure had an important effect on the biodegradability of a compound while adsorption was the dominant removal mechanism for hydrophobic (log D \u3e3.2) compounds. Compounds with high log D (log D \u3e3.2) but which were readily biodegradable did not accumulate in sludge. By contrast, recalcitrant compounds with a moderate hydrophobicity, such as carbamazepine, accumulated significantly in the solid phase. The results provided a framework to predict the removal and fate of TrOCs by MBR treatment. This study also investigated the fate of eight N-nitrosamines during MBR treatment. The results suggest that biodegradation is mainly responsible for the removal of N-nitrosamines during MBR treatment. Other removal mechanisms (e.g. adsorption to sludge, photolysis and volatilization) were insignificant. N-nitrosamine removal efficiencies were found to be from 24 to 94%, depending on their molecular properties. High removal efficiencies of N-nitrosamines such as Nnitrosodimethylamine and N-nitrosodiethylamine could be explained by the presence of strong electron donating functional groups (EDG) in their structure. In contrast, Nnitrosomorpholine possessing the weak EDG morpholine was persistent to biodegradation. The removal efficiency of N-nitrosomorpholine was the lowest amongst all N-nitrosamines investigated. The feasibility of MD for removing a set of common TrOCs was then examined. The results suggest that the rejection and fate of TrOC during MD are governed by compound volatility and, to a lesser extent, hydrophobicity. All TrOCs with pKH\u3e 9 (which can be classified as non-volatile) were well removed by MD. Among the 29 TrOCs investigated, three compounds (i.e. 4-tert-octylphenol, 4-tertbutylphenol and benzophenone) possess moderate volatility (pKH\u3c 9) and therefore had the lowest rejection efficiencies of 54, 73 and 66%, respectively. In addition, the fate and transport of the TrOCs during the MD process was also investigated. Hydrophilic TrOCs having negligible volatility were concentrated in the feed, while compounds that are hydrophobic or moderately volatile were substantially lost through adsorption or evaporation. When MD treatment was integrated with a thermophilic MBR, near complete removal (\u3e95%) of all 29 TrOCs investigated was achieved despite their diverse physicochemical properties (i.e. hydrophobicity, persistency and volatility). The results suggest that MD could be a promising posttreatment used in conjunction with thermophilic MBR for TrOC removal. The removal of TrOCs by a novel membrane distillation – thermophilic bioreactor (MDBR) system was then examined. Salinity build-up and the thermophilic conditions to some extent adversely affected the performance of the bioreactor, particularly the removal of total nitrogen and recalcitrant TrOCs. While most TrOCs were effectively removed by the thermophilic bioreactor, compounds containing electron withdrawing functional groups were resistant to biological treatment and their removal efficiency by the thermophilic bioreactor was low (0 to 53%). However, the overall performance of the novel MDBR system with respect to the removal of total organic carbon, total nitrogen, and TrOCs was high and was not significantly affected by the salinity build-up and thermophilic conditions of the bioreactor. All TrOCs investigated were highly removed (\u3e95%) by the MDBR system. Biodegradation, sludge adsorption, and rejection by MD contribute to the removal of TrOCs by MDBR treatment