Control method of organic pollution of reverse osmosis membrane
With the shortage of water resources, seawater desalination and sewage reuse have attracted more and more attention. Reverse osmosis is considered as a feasible and effective technology in seawater desalination and wastewater reuse. However, membrane fouling is inevitable in the operation of reverse osmosis system, which has become the bottleneck restricting its application. Membrane pollution will not only reduce the water yield and water quality, but also increase the operating pressure, increase the cost of water treatment, but also affect the membrane itself.
Organic pollution is the biggest and most difficult problem faced by seawater desalination and sewage reuse. 50. Weinrich et al. Concluded that 40% of the water production decline in seawater desalination reverse osmosis is caused by organic matter and microbial pollution. Humic acid, polysaccharide and protein in organic matter will lead to serious membrane pollution. Therefore, it is imperative to study the organic pollution control of reverse osmosis membrane.
At present, the organic pollution control methods of reverse osmosis membrane mainly include water pretreatment, optimization of operating conditions, addition of scale inhibitor, membrane surface modification and cleaning, which can slow down the organic pollution of reverse osmosis membrane to a certain extent.
1 water inlet pretreatment
The performance of reverse osmosis membrane is closely related to the influent quality, so it is very necessary to pretreat the influent. Traditional pretreatment methods include pH adjustment, coagulation, deep filtration, adsorption, dissolved air flotation and low-pressure membrane filtration (ultrafiltration and microfiltration), which can effectively remove suspended solids, turbidity and organic matter.
H. Huang et al. Pretreated the reverse osmosis influent with ultrafiltration, magnetic ion exchange resin ultrafiltration and magnetic ion exchange resin coagulation ultrafiltration. Ultrafiltration can effectively remove the high molecular weight natural organic matter in the influent and control the organic pollution of reverse osmosis membrane within the test time (48h). Adding magnetic ion exchange resin before ultrafiltration can increase the removal of medium molecular weight natural organics with relative molecular weight of about 700 ~ 900 and low relative molecular weight natural organics with relative molecular weight less than 200. Compared with magnetic ion exchange resin ultrafiltration, magnetic ion exchange resin coagulation ultrafiltration only slightly improves the removal of medium relative molecular weight and low relative molecular weight components. Both combined processes can slow down the organic pollution of reverse osmosis membrane.
S. Jeong et al. Studied three submerged membrane mixing systems (smhss) as pretreatment systems for seawater desalination. The experimental results show that the submerged membrane coagulation adsorption mixing system (smcahs) has the best effect. Containing 0.5mg/l Fe3 + and 0.5g/l powdered activated carbon can remove more than 72% of dissolved organic carbon (DOC), especially a large number of biopolymers and humus, which significantly slows down the trend of pollution.
J. A. Lopez Ramirez et al. Pretreated the secondary effluent of activated sludge treatment unit, which is divided into three levels: enhanced treatment, moderate treatment and minimum treatment. The performance of the membrane changes with the pretreatment. It is suggested to carry out enhanced pretreatment (coagulation with ferric chloride and polyelectrolyte and precipitation at high pH) to protect the membrane.
F. C. Kent et al. Studied the effects of membrane bioreactor (MBR) and traditional activated sludge plus tertiary membrane filtration (cas-tmf) on reverse osmosis pollution. The results show that the effect of membrane bioreactor on reducing reverse osmosis pollution is better than that of traditional activated sludge plus three-stage membrane filtration.
2 optimize operating conditions
The operating conditions of reverse osmosis system, including temperature, cross flow rate and initial flux, will affect the degree of organic pollution of reverse osmosis membrane. Therefore, on the basis of meeting the production needs, controlling the initial flux slightly lower than the critical flux, higher cross flow rate and appropriate temperature are conducive to slow down the organic pollution of reverse osmosis membrane.
H. Mo et al. Found that when the pH was 4.9 and 7 respectively, the zeta potential slightly became positive with the increase of temperature, resulting in the decrease of electrostatic repulsion between bovine serum protein molecules and between bovine serum protein and membrane surface, accelerating the accumulation of bovine serum protein on membrane surface, and aggravating the protein pollution of reverse osmosis membrane. Correspondingly, the higher the temperature is, the faster the water flux decreases, which also indicates that the more protein pollution of reverse osmosis membrane is at higher temperature. Therefore, controlling the appropriate temperature will slow down the protein pollution of reverse osmosis membrane.
Y. Yu et al. Found that with the increase of initial flux, the downward trend of water flux is more obvious. In addition, the humic acid contaminated layer is thick and dense at high initial flux, while the contaminated layer is loose and incomplete at low initial flux. The concentration polarization caused by high initial flux leads to the increase of humic acid and salt concentration on the membrane surface, which promotes the humic acid pollution of reverse osmosis membrane. Therefore, it is necessary to control the initial flux within a certain range to slow down the humic acid pollution of reverse osmosis membrane.
M. Sir and others found that when the operating pressure is lower than the critical pressure, there is only a concentration polarization layer above the membrane surface, and when the operating pressure exceeds the critical pressure, a pollution layer will be formed between the membrane surface and the concentration polarization layer. Therefore, it is recommended to control the initial flux slightly below the critical flux to minimize membrane fouling and maximize productivity.
3 add scale inhibitor
Scale inhibitors are often used to control inorganic salt scale pollution of reverse osmosis membrane, such as CaCO3 scale, CaSO4 scale, BaSO4 scale, silicon scale, etc. there are many relevant research reports. However, there are few studies on using scale inhibitors to reduce organic pollution, but it is an effective method to control organic pollution of reverse osmosis membrane.
Qingfeng Yang et al. Conducted a study on poly aspartic acid (PASP) to reduce humic acid pollution of reverse osmosis membrane. When the concentration of Ca2 + is within a certain range, Ca2 + will bridge between humic acid (HA) and PASP, forming a water-soluble complex ha CA PASP that is not easy to deposit on the membrane surface, so as to slow down the pollution of humic acid to reverse osmosis membrane, and the slowing effect is stronger with the increase of Ca2 + concentration. When there is no Ca2 +, PASP can also slow down the pollution of humic acid by combining - NH contained in the molecule with humic acid, but the inhibition rate is lower than that with Ca2 +. With the increase of the mass concentration of PASP (from 2 ~ 10mg / L), the inhibition rate increased. When the mass concentration of PASP was 10mg / L, the inhibition rate reached 91%. At this time, ha CA complex was stabilized in water by PASP. However, when the dosage was excessive (50mg / L), the inhibition rate of PASP decreased to 35%, and the water solubility of HA CA PASP decreased. Higher pH is conducive to the control of humic acid pollution. The inhibition rate of PASP at high pH is higher than that at low pH, because the electrostatic repulsion between humic acid macromolecules and membrane surface increases at high pH. After adding PASP, humic acid pollution is less affected by the changes of initial flux and cross flow rate, and decreases with the decrease of initial flux, the increase of cross flow rate and the decrease of inlet water temperature.
Qingfeng Yang et al. Also carried out the research on scale inhibitor to control protein pollution in reverse osmosis system. When the mass concentration of PASP was 2 ~ 10mg / L, the inhibition rate increased with the increase of reagent concentration, reaching 96% at 10mg / L, while when the dosage was excessive (50mg / L), the inhibition rate decreased, only 38%. The inhibition rate of another scale inhibitor lb-0100 was 65% when the mass concentration was 5mg / L, but it promoted the pollution of reverse osmosis membrane when the mass concentration increased to 50mg / L. Therefore, the determination of the optimal dosage concentration is very important, because the water solubility of the complex formed when the agent is overdosed will decrease. When the concentration of Ca2 + is within a certain range, Ca2 + will bridge between bovine serum albumin (BSA) and PASP to form a water-soluble complex BSA CA PASP that is not easy to deposit on the membrane surface, so as to slow down the pollution of bovine serum protein to reverse osmosis membrane, and the slowing effect increases with the increase of Ca2 + concentration. When there is no Ca2 +, PASP can also slow down the pollution of bovine serum protein by binding with bovine serum protein through - NH contained in the molecule, but the inhibition rate is lower than that with Ca2 +. This is roughly the same as the mechanism of PASP reducing humic acid pollution. After adding PASP, the pollution of bovine serum albumin on reverse osmosis membrane is less affected by the changes of pH, initial flux, cross flow rate and temperature, and the inhibition rate of PASP is higher at higher pH (higher than the isoelectric point of bovine serum albumin), lower initial flux, higher cross flow rate and higher temperature.
4 membrane surface modification
The fouling of reverse osmosis membrane is closely related to its surface properties. Generally speaking, the stronger the hydrophilicity of the membrane surface, the greater the pollution resistance, the smoother the surface, the lower the pollution probability, the higher the electrostatic repulsion, and the lower the pollution rate. Therefore, the membrane is often modified in combination with the treatment object to improve the anti pollution ability of reverse osmosis membrane.
Qibo Cheng et al. Modified the surface of commercial composite reverse osmosis membrane and grafted N-isopropylacrylamide and acrylic acid. The modified membrane surface became more hydrophilic and negatively charged, and the water flux and desalination rate were improved under certain conditions. The results of bovine serum protein pollution test showed that the modification slowed down the deposition of pollutants on the membrane surface, increased the electrostatic repulsion between bovine serum protein molecules and the membrane surface and decreased the hydrophobic effect, which increased the pollution resistance.
Guodong Kang et al. Modified the surface of commercial composite polyamide reverse osmosis membrane and grafted polyethylene glycol derivatives. Compared with the unmodified membrane, the modified membrane has stronger resistance to the pollution of protein and cationic surfactant in water.
Sanchuan Yu et al. Coated a layer of natural hydrophobic polymer sericin on the surface of composite polyamide reverse osmosis membrane to improve its anti pollution performance. After coating sericin, the hydrophilicity of the membrane surface was enhanced, the negative charge increased and smoother, and the pure water permeability and salt permeability decreased. The anti bovine serum protein pollution ability of the modified reverse osmosis membrane increased, which also slowed down the deposition of pollutants and the decline rate of water flux.
5 cleaning
Although many efforts have been made to control membrane pollution, such as improving membrane performance, optimizing operating conditions and influent pretreatment, membrane pollution is still inevitable. Therefore, in order to ensure the successful application of membrane technology, it is very necessary to chemically clean the membrane to remove the surface pollution layer.
S. Lee et al. Studied the cleaning of organic pollution of reverse osmosis membrane by salt. The factors affecting salt cleaning efficiency include chemical factors (salt concentration, salt type and composition of organic pollution) and physical factors (cleaning contact time, cross flow rate, cleaning solution temperature and penetration rate). The results show that salt cleaning is particularly effective for reverse osmosis membranes contaminated by gel layers formed by hydrophilic organic pollutants, such as alginic acid and pectin. The possible cleaning mechanism is that the alginate gel layer expands during the salt washing process, resulting in the weakening of the gel network integrity. At this time, the ion exchange reaction between Na+ and Ca2+ occurs, resulting in the breakdown of the cross-linked alginate gel network. Ca2+ and alginate molecules are released into the bulk solution through mass transfer.
Xue Jin and EDTA were used for chemical cleaning of the membrane. It was found that it could destroy the interaction between calcium ions and carboxyl groups in alginate gel layer, so that alginate was separated from the surface of the membrane and the water flux of the reverse osmosis membrane could be recovered.
Sanchuan Yu et al. Cleaned the reverse osmosis membrane contaminated with bovine serum albumin with heat sensitive polymer (TRP). It is found that when the soaking temperature is lower than the low critical dissolution temperature (LCST), the dissolved Trp can diffuse into the bovine serum protein contaminated layer on the membrane surface. When the soaking temperature is higher than LCST, the Trp is insoluble, resulting in the loose structure of the contaminated layer on the membrane surface. The bovine serum protein contamination can be removed by washing. The cleaning efficiency of TRP is affected by the type and concentration of Trp and soaking time. Increasing the concentration and prolonging the soaking time are beneficial to improve the cleaning efficiency.
W. S. ang et al. Studied the cleaning effect of NaOH, EDTA, SDS and NaCl on reverse osmosis membrane polluted by mixed organic pollutants. The cleaning efficiency is affected by the type of cleaning agent, the pH of cleaning solution, cleaning time and the composition of contaminated layer. When NaOH is used alone, its breaking effect on the complex formed by mixed pollutants and calcium ions is limited. When the shear force is sufficient, high pH is conducive to improve the cleaning efficiency. EDTA, SDS and NaCl can effectively clean the reverse osmosis membrane polluted by mixed pollutants, especially at high pH and long cleaning time, and the optimal cleaning concentrations are 1.0, 10 and 50mmol / L respectively.
6 Conclusion
The organic pollution of reverse osmosis membrane is formed by the adsorption of organic matter on the membrane surface, which will not only lead to the rapid and significant reduction of water flux of reverse osmosis membrane, but also the membrane polluted by organic matter is difficult to be cleaned and almost irreversible. Therefore, it is very necessary to strengthen the monitoring of the type and concentration of organic matter in the influent of reverse osmosis system and select appropriate control methods at the same time.
At present, there are many studies on the organic pollution control of reverse osmosis membrane, and effective methods are put forward from different angles. The author believes that it is necessary to start from the reverse osmosis technology itself, select new membrane materials, improve the interfacial polymerization process, and moderately modify the surface of the mature membrane, so as to improve the anti pollution ability of reverse osmosis membrane. In addition, the cleaning method of organic pollution of reverse osmosis membrane is also one of the research directions in the future.