Study On Desalination Process Of Membrane Separation In Industrial Reverse Osmosis Concentrates

Reverse osmosis technology,an efficient water treatment technology,has a good application effect in many industries.During the process of treating secondary biochemical effluent from the water purification workshop of petrochemical enterprises using reverse osmosis technology,approximately 25-30%of reverse osmosis concentrates?ROC?will be produced which contains a large number of inorganic salt ions(mainly including F^-,Cl^-,treatment methods are not suitable for processing of ROC,especially inorganic ions in ROC,which rarely participate in the reaction process.Aiming at the desalination of industrial ROC,this research introduced a nanofiltration process.First,advanced oxidation process and ultrafiltration were used for the ROC pretreatment.The pretreatment can not only effectively eliminate the subsequent nanofiltration membrane contamination,but also remove refractory organic pollutants in ROC synergistically.The effects of different influencing factors on chemical oxygen demand?COD?and UV254 were investigated by single-factor experiments to determine the optimal feed for nanofiltration experiments.Then,the optimized experimental conditions for nanofiltration desalination were explored by single factor and response surface method?RSM?optimization experiments,and SEM-EDS and FTIR were employed to analyze membrane fouling.By analyzing the types and properties of the pollutants,the appropriate cleaning methods and agents are selected eventually.O₃-assisted UV-Fenton method coupled with nanofiltration technology was adopted to process industrial ROC.The pre-treatment single-factor experiment results show as follows,the p H is at 3.0,the dosage of H2O2 is 10 m M,the dosage of Fe²⁺is 0.2 g/L,the irradiation time of UV is 90 min,the dosage of O3 is 15 mg/L.The removal rates of COD and UV254were 71.4%and 79.6%,respectively.Four types of commercial nanofiltration membranes?NF90,DK,NT101,NT103?were used for desalination experiments.It was found that the flux of the nanofiltration membrane after pretreatment had increased significantly.The effect of the nanofiltration membrane on the rejection of main ions under the different operating pressures and feed temperatures was further studied.The results showed that the ion rejection rised with the increase of operating pressure and decreased with the temperature going up.Compared with DK membrane and NT101 membrane,NF90membrane and NT103 membrane have better rejection of monovalent ions,while DK membrane and NT101 membrane can achieve the separation of monovalent and divalent ions effectively.The advanced oxidation process-ultrafiltration pretreatment process indicate that ultrafiltration pretreatment can further trap colloids,and achieve stable operation of nanofiltration devices.RSM-BBD results showed that the maximum rejection of 97.75%was obtained under the optimized conditions of the 1.18 MPa operating pressure,6.79°C feed temperature,64.33 min filtration time.Under the verification experiment,the actual rejection of total soluble salts was 95.84%.There is a small discrepancy of 1.91±0.22%between predicted and validated rejection which proposes RSM is an appropriate tool for optimizing the removal rate of total soluble salts in NF desalination of pretreated ROC.The new membrane,the membrane after use,and the cleaned membrane were characterized by SEM-EDS and FTIR.The characterization results demonstrate that the fouling of the membrane was mainly caused by the contamination of elements such as Ca,Mg,Na,Fe,Pb and Si and organic fouling.At the same time,the membrane flux ratios of the two nanofiltration feed waters including ROC and the pretreated ROC for 12 hours were compared and analyzed,and the results showed that the former contaminated membrane flux was reduced to 24%,while the latter was attenuated to 60%.It indicated that the removal of organic matter by advanced oxidation process could effectively reduce the contamination of nanofiltration membrane and prolong the service life of nanofiltration membrane.After chemical cleaning of the contaminated membrane,it was found that the combined cleaning agent could achieve better efficiency than the single-cleaning cleaning.The sequence of organic pollution and of inorganic pollution can well restore the membrane flux and regenerate the polluted membrane.