A novel membrane cleaning apparatus was constructed and used to study the cleaning of 2540elements(2.5inch) spiral NF membrane with gasliquid twophase flow process.The twophase flow was formed by mixing the cleaning liquid with compressed air.The effects of cleaning liquid flow rate

air velocity

ratio of gas to liquid

and cleaning time on the rejection rate and flux recovery ratio(FRR)of NF membrane were investigated systematically.The results showed that high FRR of NF membrane was achieved by relatively low cleaning liquid flow rate about 0.1 L·min-1.NF membranes with different active areas had different critical cleaning flow rates.Above the critical flow rate

FRR of NF membrane increased slowly.FRR of NF membrane increased with air velocity.For polyamide composite NF membrane

the upper limit was 14 m·s-1 and higher air velocity would decrease the rejection rate of membrane.FRR of NF membrane was effectively improved when the gas to liquid ratio was from 1000∶1 to 3000∶1.The ratios lower than 1000∶1 or higher than 4000∶1 would decrease FRR.FRR was higher than 95% for cleaning NF membranes in a short 15 min cycle of the twophase flow process.