| The shortage of water resources promotes the rapid development of membrane filtration technology in the field of reclaimed water treatment.Nanofiltration membrane has a high removal rate for dissolved organic matter and high valence inorganic ions in water.The biofouling of membrane and the treatment of nanofiltration concentrate hinder the application of nanofiltration technology in the field of reclaimed water treatment.In this study,an electro oxidation coupled with nanofiltration membrane process system（NF-EO）is proposed to treat the nanofiltration concentrate of secondary effluent,explore the removal effect of the system on inorganic and organic pollutants in the nanofiltration concentrate,so as to achieve the purpose of"zero liquid discharge",and explore the control effect of BDD in-situ electrolysis products on the biofouling layer on the vicinity of nanofiltration membrane.Firstly,the rheological characteristics of biofouling layer on the surface of nanofiltration membrane are related to the micro flow field on the membrane surface.The biofouling layer produced on nanofiltration membrane was measured using parallel plate rheometer operating in oscillatory and stable shear mode,and the shear stress on the membrane surface was simulated by computational fluid dynamics.The results show that the biofouling layer shows elastic characteristics,and shows stronger elasticity,greater viscosity and yield stress with the increase of COD content.Compared with the shear stress of the micro flow field on the membrane surface obtained by numerical simulation,the shear stress of the biofouling layer in the nanofiltration process is one order of magnitude higher than that of the floc layer formed,that is,the shear stress of the water flow are not able to effectively cause the biofouling layer to fall off,so it is necessary to find an electrochemical way to control the biological layer pollution on the surface of the nanofiltration membrane.Secondly,the electro oxidation（EO）reactor constructed with boron doped diamond（BDD）anode is used to treat the concentrated liquid produced by the secondary effluent of nanofiltration treatment sewage plant.In order to meet the discharge standard,the mixture of secondary effluent electrolyte and untreated secondary effluent is collected and sent back to the nanofiltration system to improve the recovery rate of the NF system.The results show that at the current density of 30 m A/cm~2,the active chlorine radical produced in the electrooxidation process makes most of the ammonia nitrogen oxidized to nitrogen within 30minutes.In addition,the EO reactor completely removed antibiotics,humic acid and bacteria from the nanofiltration concentrate under 60 minutes of long-time electrolysis.It is worth noting that in the long-term operation of up to 120 hours,the removal rate of organic pollutants in EO reactor remains at a stable value,the biofouling on the surface of nanofiltration membrane is reduced,and the water production flux is relatively stable.This study emphasizes the potential of NF-EO process as a"zero liquid discharge"method for the treatment of secondary effluent.In order to realize the sustainable control of biological pollution on the surface of nanofiltration membrane,the electro oxidation electrolyte of boron doped diamond（BDD）anode is used to mix with the chloramine produced in situ with the untreated secondary effluent as the influent of the subsequent nanofiltration process,so as to reduce the biofouling of membrane and realize"zero liquid discharge".The results showed that the concentration of chloramine reached the maximum at the current density of 30 m A/cm~2 for30 minutes.As the concentration of chloride ion is lower than 500 mg/L,the concentration of chloramine is greater than that of free chlorine.Because chlorine and chloramine are produced simultaneously in situ,the NF-EO hybrid process can completely remove antibiotics and bacteria.At the same time,the biofouling on the surface of nanofiltration membrane can also be effectively alleviated,and the stable flux can be maintained in the long-term operation of up to 160 hours.This research provides an effective"zero liquid discharge"strategy for the sustainable reuse of secondary effluent. |
