Permeate flux and salt removal rate are key performance parameters of reverse osmosis and nanofiltration membranes. In addition to the characteristics of the membrane itself, the water flux and salt removal rate of the membrane system are mainly affected by pressure, temperature, recovery rate, feed water concentration and ph value.
Influencing factors:
(1) The effects of stress
The feed water pressure is mainly used to overcome the natural osmotic pressure. As shown in Figure 3.2, there is a positive relationship between the permeate flux and the feed water pressure; increasing the feed water pressure will also increase the desalination rate, but the relationship between the two is non-linear.
Flow rate refers to the rate of influent entering the membrane element, usually expressed in cubic meters per hour (m³ / h) or gallons per day (gpd). The effect of influent flow rate on membrane performance is relatively mild. As the influent flow rate increases, the membrane surface pressure rises, concentration polarization is reduced, and the desalination rate is improved.
Figure 3.2 Effect of influent pressure on flux and desalination rate
(2) The effect of temperature
As shown in Figure 3.3, the permeate flux of the membrane system is highly sensitive to changes in influent temperature. With increasing water temperature, the flux increases almost linearly, primarily due to the decrease in water molecule viscosity and the increase in diffusion capacity. Increased water temperature leads to a decrease in desalination rate, mainly because the diffusion rate of salt through the membrane accelerates with increasing temperature.
Figure 3.3 Effect of influent temperature on flux and desalination rate
(3) Effect of salt concentration
Feed water concentration refers to the salt concentration in the feed water of the membrane system. Osmotic pressure is a function of the concentration and type of salt or organic matter in the water. As salt concentration increases, osmotic pressure also increases. Therefore, the magnitude of the feed water driving pressure mainly depends on the salt content in the feed water. Figure 3.4 shows that if the pressure remains constant, the higher the salt content, the lower the flux. The increase in osmotic pressure offsets the feed water driving force. Simultaneously, as shown in Figure 3.4, the decrease in water flux increases the salt flux permeating the membrane, thus reducing the desalination rate.
Figure 3.4 shows that if the pressure remains constant, the higher the salt content, the lower the flux. The increase in osmotic pressure offsets the feed water driving force. Simultaneously, as shown in Figure 3.4, the decrease in water flux increases the salt flux permeating the membrane, thus reducing the desalination rate.
(4) Impact of recovery rate
Recovery rate refers to the ratio of permeate flow to feed water flow. Reverse osmosis is achieved by applying pressure to the feed water, reversing the natural flow direction between the concentrated and dilute solutions. If the recovery rate increases (with constant feed water pressure), the residual salt content in the raw water will be higher, and the natural osmotic pressure will continue to increase until it equals the applied pressure. This will counteract the driving effect of the feed water pressure, slowing down or stopping the reverse osmosis process, reducing or even stopping the permeate flux (see Figure 3.5), and decreasing the desalination rate.
Figure 3.5 Effect of recovery rate on flux and desalination rate
(5) The effect of ph
ph value refers to the acidity or alkalinity of the feed water to the membrane system. The desalination performance of membrane elements is significantly affected by ph value, and water flux is also affected to some extent. When the ph value is less than 8, CO2 in the water cannot be removed and will permeate through the membrane element, resulting in a decrease in desalination rate. Conversely, continuously increasing the ph value will reduce the solubility of carbonates, leading to scaling. Therefore, controlling the appropriate ph value is a prerequisite for the normal operation of reverse osmosis and nanofiltration membrane elements.
Figure 3.6 shows that the JOZZON reverse osmosis membrane exhibits relatively stable water flux and desalination rate over a wide ph range.
Figure 3.6 Effect of ph on flux and desalination rate
