MBR-NF Hybrid System And Its Engineering Application For High-quality Reclaimed Water Production

Municipal wastewater reclamation is believed to be a promising process to overcome the growing pressure on water resources.To overcome the water quality problem in wastewater reclamation,new technologies such as reverse osmosis?RO?and nanofiltration?NF?have been applied as a polishing treatment.The integrated membrane systems using low pressure,i.e.,microfiltration?MF?and ultrafiltration?UF?followed by membrane systems using high pressure,i.e.,NF and RO,or membrane bioreactors?MBRs?coupled with NF and/or RO process,have been widely used to enhance the removal of micro-pollutants in the recycled water and mitigate fouling formation on the NF and RO membranes for municipal wastewater reclamation.Based on an effluent with a significantly lower organics concentration,MBR is preferred to be the pretreatment process for the high-pressure membranes,compared to conventional activated sludge process with teriary membrane filtration.NF membranes as an alternative to RO can be an acceptable barrier for trace organic containminants?TrOCs?because its removal performance approaches that of RO,and because of reduced operation and maintenance costs in long-term project implementation with higher flux rates at low operating pressure.This work aims to develop and demonstrate the MBR-NF hybrid system with a high-quality permeate for reclamation of municipal wastewater.The removal characteristics and separation mechanisms of different valence ions were investigated on the DF30 NF membrane,a product with independent intellectual property,and then a NF process design software was developed with the application of an optimized NF separation model.The molecular weight cutoff of DF30 membrane is400.The low rention of bicarbonate provides an important basis for high recovery of the DF30 membrane system.A full-scale MBR system with post endogenous denitrification was carried out to enhance TN removal.It was the key factor to keep the DO at 0.5 mg/L in the aerobic tank for a high simultaneous nitrification-denitrification efficiency.As a result,the removal rate of TN was increased by more than 10%.The energy consumption for membrane scouring could be reduced by more than 30%in the MBR system with open slot aeration structures of membrane cassettes and high-low intermittent scouring strategy,compared with the conventional continuous mode.The critical recovery rate of the DF30 membrane system was found in terms of CaSO₄ scaling with the feed of MBR effluent.The NF system ran stablely at 90%recovery and the membrane fouling was well controlled with an antiscalant dosage of 2³ mg/L for capturing calcium ion,combining acid-base chemical cleaning.A full-scale MBR-NF system was monitored and investigated to evaluate the long-time effects of contaminants removal,including conventional pollutants and trace organic contaminants?TrOCs?rention,and its economic efficiency as well.The NF membrane fouling was well controlled for a high recovery long-time operation in the full-scale system.The results showed that the MBR-NF permeate could meet with the limitations in class A,the local standards for water reuse in Beijing?DB11/890-2012?,and the class?standard of suface water quality in China?GB3838-2002?except for TN.TrOCs could be highly removed in the integrated MBR-NF system with a retention rate of 86%.The effluent quality of NF concentrate treated by Fenton technology could meet with the limitations in class B,the local standards for water reuse in Beijing?DB11/890-2012?,and the suface water class?standard?GB3838-2002?except for TN less than 10 mg/L.The direct operational cost of the MBR-NF system,including the cost of the concentrate treatment,is 1.86 RMB/m³,and 2.89 RMB/m³ the overall cost as well.The membrane cassettes with open slot aeration structures have been applied in 16 large-scale municipal wastewater treatment plants with MBR technology in China.The total capacity of those plants reaches 1 million and 13 thousand m³/d.The specific energy consumption was 0.41 kWh/m³.It decreased by 34.9%,compared to 0.63 kWh/m³ surveyed in 2012by the research group of the Major Science&Technology Projects for Water Pollution Control and Management of China?2011ZX07317-02?.The MBR-NF technology has been widely used with an accumulative total capacity of 99 thousand m³/d in China currently.