Why is reverse osmosis cleaning performed with acid first and then with alkaline? What are the effects of concentration polarization ?

Every reverse osmosis system will be contaminated and need to be cleaned. Since there is usually more than one contaminant when contamination occurs, the typical cleaning process usually includes two steps: low pH cleaning and high pH cleaning.
Different contaminants should be cleaned with different cleaning agents. Alkaline cleaning is mainly used to remove microorganisms or organic contaminants, and acid cleaning is mainly used to remove scaling. The cleaning agent should be selected according to the type of contaminant and membrane type to avoid damaging the membrane.

The order of using cleaning agents should be determined according to the actual problem. The usual cleaning order is to perform acid cleaning first and then alkaline cleaning.
In some cases, if the pollution is mainly organic matter or microorganisms, you can first perform alkaline cleaning, then acid cleaning, and finally perform an alkaline cleaning step.
The important reason for using alkaline cleaning as the last step is that it can fully open the membrane pores after acid cleaning to restore the water production flow.
If you do not use the conventional cleaning procedure of acid cleaning first and then alkaline cleaning, you need to consult a membrane expert to help you make the right choice.

During the reverse osmosis membrane separation process, after water molecules pass through, the salt content in the membrane interface increases, forming a higher concentrated water layer. This layer forms a large concentration gradient with the concentration of the feed water flow. This phenomenon is called concentration polarization of the membrane . Concentration polarization can have a harmful effect on operation.

( 1 ) Since the concentration in the interface layer is very high, the osmotic pressure will increase accordingly. After the osmotic pressure increases, the water production under the original operating conditions will inevitably decrease. In order to achieve the original water production, the water supply pressure must be increased, which increases the energy consumption of the product water.

( 2 ) As the salt concentration in the sub-interface layer increases, the ▲C on both sides of the membrane increases, which increases the permeability of product water and salt. ( 3 ) As the concentration in the interface layer increases, the tendency of scaling substances to precipitate increases, resulting in membrane fouling. In order to restore performance, the scale must be cleaned frequently, which may cause irreversible membrane performance degradation.

( 4 ) Although measures are taken to diffuse salt away from the membrane surface due to the concentration gradient formed, the diffusion rate of colloids is hundreds or thousands of times slower than that of salt. Therefore, concentration polarization is an important cause of colloid contamination on the membrane surface.

The result of concentration polarization is that the osmotic pressure of the brine increases, so the pressure required for reverse osmosis also increases; in addition, it may cause some insoluble salts (such as CaSO4) to precipitate on the membrane surface. Therefore, during operation, the brine side must be kept in a turbulent state to reduce the degree of concentration polarization.