About pollution of Reverse osmosis system

Causes of pollution of reverse osmosis system

1) The influent water quality changes frequently, which increases the load of pretreatment. Due to the increase of impurities such as inorganic matter, organic matter, microorganisms, particles and colloids in the influent water, the probability of membrane fouling increases

2) The chemical cleaning is not timely or the cleaning method is incorrect

3) Improper dosing of treatment agents

4) The membrane element is worn out

Manifestations of reverse osmosis system contamination

During the reverse osmosis operation, due to the selective permeability of the membrane, some solutes accumulate near the membrane surface, resulting in membrane fouling. The common symptoms of fouling are as follows: one is biological fouling (symptoms appear gradually) organic deposits are mainly living or dead microorganisms, hydrocarbon derivatives, natural organic polymers and all carbon-containing substances. The initial manifestations are increased salt rejection, increased pressure drop and decreased permeate. Another is colloidal fouling (symptoms gradually appearing). During the membrane separation process, the concentration of metal ions and the change of pH value of the solution may be due to the deposition of metal hydroxides (mainly represented by Fe(OH)3), causing fouling. Blocking. Initially, the salt rejection rate decreased slightly, and gradually increased, and finally the pressure drop increased and the permeate decreased. There is also particle fouling during the operation of the reverse osmosis system, if there is a problem with the security filter, it will cause particles to enter the system, resulting in particle fouling of the membrane.

Initially, the concentrated water flow rate increases, the desalination rate does not change much at the initial stage, the water production rate gradually decreases, and the system pressure drop increases rapidly. The last common thing is chemical scaling (symptoms appear quickly). When the feed water contains high Ca2+, Mg2+, HCO3-, CO32-, SO42- plasma, CaCO3, CaSO4, MgCO3 and other scales will be deposited on the membrane surface. It is manifested as a decrease in desalination rate, especially in the last stage, and a decrease in water production. Membrane fouling is the main reason for the decrease of membrane permeate flux. The blockage of membrane pores and macromolecular solutes increases the membrane filtration resistance; the solutes are adsorbed on the inner wall of the pores; the formation of a gel layer on the membrane surface increases the mass transfer resistance. The deposition of components in the membrane pores will cause the membrane pores to be reduced or even blocked, which actually reduces the effective area of the membrane. The additional resistance generated by the fouling layer formed by the deposition of components on the membrane surface may be much greater than the resistance of the membrane itself, so that the permeate flow has nothing to do with the permeability of the membrane itself. This effect is irreversible, and the degree of pollution is related to the concentration and properties of the membrane material, the solvent in the retention solution, and the macromolecular solute, the pH value of the solution, the ionic strength, the charge composition, the temperature, and the operating pressure. Flux drops by more than 80%.

In the operation of the system, membrane fouling is a very difficult problem. Its appearance has caused a significant decline in the removal rate, water permeability, and membrane flux of the reverse osmosis device. At the same time, it has increased the operating pressure of each section, prompting the operation and The rising cost seriously affects the service life of the membrane and the development and utilization of reverse osmosis technology.

Solution to the pollution of the reverse osmosis system

1) Perfect preprocessing

For each membrane device, people hope that it can play its role to the maximum, hope to have the highest desalination rate, the largest water permeability and the longest possible life. To achieve the above three points, the quality of the water supply is very important, so The raw water entering the membrane device must have good pretreatment. Reasonable pretreatment is very important for the long-term safe operation of reverse osmosis equipment. With pretreatment that meets the water quality requirements of reverse osmosis, it can ensure that the product water flow rate remains stable; the desalination rate maintains a certain value for a long time; the product water recovery rate can remain unchanged; the operating cost reaches the lowest; the membrane service life Wait longer.

2) Periodic chemical cleaning to restore membrane performance

Although the feed liquid has undergone various pretreatment measures, deposition and fouling may occur on the surface of the membrane after long-term use, which will block the membrane pores and reduce the water production. Therefore, regular cleaning of the polluted membrane is necessary. However, the reverse osmosis membrane system cannot be cleaned until the pollution is serious, which will increase the difficulty of cleaning, increase the number of cleaning steps and prolong the cleaning time. It is necessary to correctly grasp the timing of cleaning and remove dirt in time.

3) Pay attention to the operation when shutting down. Since the concentration of inorganic salts on the concentrated water side of the membrane is higher than that of the raw water, it is easy to scale and pollute the membrane. When preparing to shut down, low-pressure flushing should be performed to discharge the wastewater in the membrane shell.

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