What factors affect the performance of reverse osmosis and nanofiltration membranes

The production water flux and salt rejection rate are the key parameters of reverse osmosis and nanofiltration membranes. In addition to the characteristics of the membrane itself, the water flux and removal rate of the membrane system are mainly affected by pressure, temperature, recovery rate, influent salt concentration and PH Value influence.

1. The stress

Water inlet pressure is mainly used to overcome natural osmotic pressure. The relationship between them is as follows: there is a positive relationship between the production water flux and the inlet pressure; increasing the inlet pressure will increase the salt rejection rate, but the relationship between the two is non-linear.

Flow rate refers to the amount of water entering the effective cross section of a closed pipeline or open channel per unit time. Often m³/h or gallons per day (g/d). The influence of the inlet water flow rate on the membrane performance is relatively mild. With the increase of the inlet water flow rate, the pressure on the membrane surface rises, and the phenomenon of concentration polarization is reduced, and the desalination rate of the equipment is improved.

2. The temperature

The change of inlet water temperature has a great and sensitive influence on the production water flux of the membrane system.

As the temperature of water increases, the water flux increases almost at a linear rate. This is mainly due to the temperature rise, the gravitational force between water molecules decreases, and the diffusion capacity increases.
However, if the temperature of the water increases, the salt rejection rate will decrease, because the diffusion rate of the salt through the membrane will be accelerated by the increase in temperature.

3. The salt concentration

Osmotic pressure refers to the semipermeable membrane between two aqueous solutions of different concentrations to prevent water molecules from penetrating from the low-concentration side to the high-concentration side, and the minimum additional pressure applied on the high-concentration side. The increase in salt concentration will lead to an increase in osmotic pressure, so the driving pressure of influent water is generally determined by the amount of salt in the water.

Generally, if the pressure is kept constant, the higher the salt content, the lower the water flux. The increase in osmotic pressure offsets the driving force of water inflow, while the decrease in water flux increases the salt flux of the permeable membrane, and at the same time reduces desalination rate.

4. The impact of recovery

A certain pressure is applied to the inlet water, and the reverse osmosis process is realized when the normal osmotic flow direction between the dilute solution and the concentrated solution is reversed. If the recovery rate of the membrane element increases (when the inlet water pressure remains constant), the residual salt content in the raw water will be higher, and the natural osmotic pressure will continue to increase until it is the same as the pressure applied to the inlet water. The pushing effect of water pressure can slow down or stop this pushing effect, and the salt rejection rate will be reduced.

5. The PH

The PH value refers to the pH of the influent water of the membrane system. The pH value has a relatively large impact on the salt rejection characteristics of the membrane element. Water flux will also be affected to a certain extent. When the PH value of the influent water is less than 8, the CO2 in the water cannot be removed, and it will permeate through the membrane element, causing the salt rejection rate to decrease.

On the contrary, continuously increasing the pH value will reduce the solubility of carbonate and cause scaling. Therefore, the need to control proper pH is a prerequisite for the normal operation of reverse osmosis and nanofiltration membrane elements.