Study on membrane bioreactor-ultra low-pressure selective nanofiltration for municipal wastewater treatment

A pilot-scale experimental device of membrane bioreactor-ultra low-pressure selective nanofiltration (MBR-DF) was constructed to improve the quality of recycled water and the reduction of sludge. The MBR-DF hybrid system was used to treat the municipal sewage and operated in a zero-sludge manner, and a conventional activated sludge process (CAS) was established at the same time. The influent contents of COD, $NH^{+}_{4}$-N, TN and TP were 87.0-165.7, 14.0-31.0, 14.2-32.4, and 2.5-3.3 mg/L, respectively. The results showed that the concentrated water zero-emission MBR-DF system had a good removal effect on COD, $NH^{+}_{4}$-N and $PO_{4}^{3-}$, with the removal rates of 95.7%, 99.0% and 68.5%, respectively. The removal rates of TDS and $HCO^{-}_{3}$ were 13.5% and 31.1% accordingly, and the total hardness of the effluent could be reduced effectively. The DF membrane had a significant retention effect on $PO_{4}^{3-}$ and $SO_{4}^{2-}$, had a poor retention effect on $NO^{-}_{3}$ and had no retention effect on $HCO^{-}_{3}$. After the influent DOM was passed through MBR-DF system, only a small amount of tryptophan-like substances remained. The UV₂₅₄ reduction rate of the influent DOM was 94.9% and the DOC removal rate was above 98.0% in the MBR-DF system. The degree of conversion of humus to non-humus was higher in the effluent DOM. The removal rates of 17-β estradiol (E2) by MBR-DF system and CAS system were 99.9% and 70.8%, respectively. Therefore, MBR-DF system could effectively reduce the health hazards and ecological threats of effluent in recycling.