The original water quality with the design of water quality changes, so that the pretreatment load increases, due to inorganic substances, organic substances, microorganisms, granules and colloids and other impurities in the incoming water, so the membrane contamination increased.

3. Membrane contamination due to untimely cleaning and incorrect cleaning methods

In addition to the normal decay of membrane performance in the course of use, untimely cleaning and incorrect cleaning methods are also an important factor leading to serious membrane contamination.

4. No correct dosage of chemicals

Composite polyamide membranes in use, because of the poor residual chlorine resistance of polyamide membranes, in use without the correct dosing of chlorine and other disinfectants, coupled with the user's lack of attention to the prevention of micro-organisms, easily lead to microbial contamination.

5. Wear on the membrane surface

Membrane elements are blocked by foreign objects or the membrane surface is worn (e.g. sand, etc.), in this case, the system components should be detected by the detection method, and damaged components should be found, modified and pretreated, and the membrane elements replaced.

During reverse osmosis operation, due to the selective permeability of the membrane, certain solutes accumulate near the membrane surface, resulting in membrane fouling.

There are several common signs of fouling.

One is biofouling (symptoms appear gradually) Organic deposits are mainly live or dead microorganisms, hydrocarbon derivatives, natural organic aggregates and all carbonaceous substances. Initially, this is characterised by increased desalination rates, higher pressure drops and lower water production

Then there is the colloidal fouling (symptoms gradually appear) membrane separation process, the concentration of metal ions and changes in solution pH, are likely to be metal hydroxide (mainly represented by Fe(OH)3) deposition, resulting in fouling. This initially manifests itself as a slight reduction in desalination rate, which gradually increases and culminates in a higher pressure drop and lower water production.

There is also particulate fouling of the reverse osmosis system during operation, if there is a problem with the security filter, this can lead to particulate matter entering the system and causing particulate fouling of the membrane. Initially this is manifested by an increase in the flow rate of concentrated water, little change in the desalination rate at the beginning, a gradual reduction in water production and a rapid increase in system pressure drop.

The last common one is also chemical scaling (symptoms appear quickly) when the feed water contains high levels of Ca2+, Mg2+, HCO3-, CO32-, SO42- plasma, which produces CaCO3, CaSO4, MgCO3 and other scales deposited on the membrane surface. This is manifested by a decrease in the desalination rate, which is especially evident in the last section, as well as a decrease in water production.

Membrane contamination is the main cause of membrane permeate flow reduction

This includes an increase in membrane filtration resistance due to blockage of the membrane pores and macromolecular solutes; solute adsorption on the inner pore wall; and the formation of a gel layer on the membrane surface to increase mass transfer resistance. The deposition of components in the membrane pores will cause the membrane pores to be reduced or even blocked, effectively reducing the effective area of the membrane. The additional resistance created by the formation of a contaminated layer of components deposited on the membrane surface can be much greater than the resistance of the membrane itself, making the permeate flow rate independent of the permeability of the membrane itself. The effect is irreversible and the degree of contamination is related to the concentration and nature of the membrane material, solvent and macromolecular solutes in the retention solution, pH of the solution, ionic strength, charge composition, temperature and operating pressure, etc. In severe cases the contamination can reduce the membrane flux by more than 80%.

In system operation, membrane contamination is a very difficult problem, which causes a significant decrease in the removal rate and water permeability of the reverse osmosis unit and membrane flux, as well as increasing the operating pressure of each section and contributing to higher operating and operational costs, which seriously affects the service life of the membrane and the development and utilization of reverse osmosis technology.