Transients of micropollutant removal from high-strength wastewaters in PAC-assisted MBR and MBR coupled with high-retention membranes

•MBR configurations for extending micropollutant retention times are investigated.•Regular MBR, PAC-assisted MBR and MBR-RO with retentate recirculation are evaluated.•Presence of EDG in organic pesticides increases their likeliness to be removed.•Low concentrations of PAC from coconut endocarp enhances removal of diuron.•Closed-loop MBR-RO is the most effective approach for general pesticide removal. [Display omitted] Removal of micropollutants from wastewaters is crucial to ensure safe water reuse and protect natural watercourses. Although membrane bioreactors (MBRs) yield improved degradation of organic compounds, hydraulic retention times are often too short to satisfy acceptable removal rates of recalcitrant organics. Often, results regarding micropollutant removal in treatment plants are susceptible to uncontrolled feed concentrations, which are concomitant to seasonality and consumer habits. In this work, we investigate the concentration transients of four selected pesticides (carbendazim, diuron, 2,4-D and atrazine), which were constantly dosed to a MBR pilot plant treating high-strength industrial effluents (COD values >4000mg/l). In addition to the regular MBR operation, two feasible means of extending pesticide retention in bioreactors were evaluated: (i) addition of small concentrations of powdered activated carbon (PAC) into the activated sludge and (ii) coupling of the PAC-assisted MBR and a reverse osmosis unit (RO) with recirculation of the retentate. The aim of this work is to provide reliable information on the fate of micropollutants within wastewater treatment plants using different configurations under controlled feed conditions. Results have shown that carbendazim was the only pesticide efficiently (>80%) removed during regular MBR operation, which has been attributed to the presence of electron donating groups attached to its aromatic ring structure. Improved retention of diuron by addition of PAC enhanced its long-term removal, whereas the effect of PAC addition on the removal of 2,4-D and atrazine was only temporary, thus being mainly attributed to an adsorption effect. Additionally, the function of PAC as platform for biofloc formation limited sludge production and slowed down membrane fouling, further improving the general performance of the MBR. The MBR-RO hybrid process was the most effective one in increasing the residence time of pesticides in the bioreactor, regardless of their functional groups and properties, thus facilita