The potential of the innovative SeMPAC process for enhancing the removal of recalcitrant organic micropollutants

•Complete OMPs mass balance in a combined system biological treatment plus PAC.•Improvement of the denitrification after PAC addition.•Enhancement of OMPs biotransformation after PAC addition.•Relation between hydrophobicity (logD) and sorption onto the PAC.•Progressive saturation of the activated carbon in the solid phase with the time. SeMPAC is an innovative process based on a membrane sequential batch reactor to which powdered activated carbon (PAC) is directly added. It was developed with the aim of obtaining a high quality effluent in terms of conventional pollutants and organic micropollutants (OMPs). High COD removal and nitrification efficiencies (>95%) were obtained already during the operation without PAC, although denitrification was enhanced by PAC addition. OMPs were followed in the solid and liquid matrixes so that biotransformation, sorption onto the sludge and adsorption onto the PAC could be assessed. Recalcitrant compounds, such as carbamazepine and diazepam, were readily removed only after PAC addition (>99%). Progressive saturation of PAC was observed, with increasing concentrations of OMPs in the solid phase. Removal efficiencies for recalcitrant compounds were used as indicators for new additions of PAC. An improvement in the moderately biodegradable OMPs removal was observed after PAC addition (e.g. fluoxetine, trimethoprim) which was attributed to the biofilm that grew onto the sorbent, as well as to adsorption onto PAC.