Other factors affecting the performance of reverse osmosis and nanofiltration membranes


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The influence of temperature

The conductivity of the produced water in the membrane system is very sensitive to changes in the inlet temperature. As the water temperature increases, the viscosity of the water molecules passing through the membrane decreases and the diffusion ability increases, resulting in a linear increase in water flux. Increasing the water temperature will lead to a decrease in desalination rate or an increase in salt permeability, mainly because the diffusion rate of salt through the membrane will accelerate due to the increase in temperature.




Impact of pH

The pH value has a certain impact on the conductivity of product water, as most reverse osmosis membranes themselves contain some active groups. The pH value can affect the electric field on the membrane surface and thus affect the migration of ions. In addition, the pH value has a direct impact on the form of impurities in the influent. For example, for dissociatable organic compounds, their retention rate decreases with the decrease of pH value. CO2 is greatly affected by pH value. When the pH value is low, it exists in the form of gaseous CO2 and is easy to penetrate the reverse osmosis membrane. When the pH is low, it is difficult to remove CO2 dissolved in water, so the desalination rate is also low; As the pH increases, gaseous CO2 is converted into HC03- and C032- ions, and the desalination rate gradually increases. When the pH is between 7.5 and 8.5, the desalination rate is the highest.


The effect of salt concentration

Due to the presence of a boundary layer on the membrane surface with a higher concentration than the main solution, concentration polarization exists within the solution. Concentration polarization increases the osmotic pressure of the membrane surface solution, which is a function of the concentration and type of salt or organic matter contained in the water. As the salt concentration increases, the osmotic pressure also increases, which leads to a decrease in water flux and an increase in salt migration through the membrane. Therefore, the driving pressure of the inflow that needs to reverse the direction of natural infiltration flow mainly depends on the salt content in the inflow. If the pressure remains constant, the higher the salt content, the lower the flux. The increase in osmotic pressure offsets the driving force of the inflow, resulting in a decrease in water flux and an increase in salt flux through the membrane. If the concentration of dissolved solute in the boundary layer exceeds its solubility, precipitation or scaling will occur on the membrane surface.


The impact of recovery rate

By applying pressure to the inlet water, the reverse osmosis process is achieved when the natural osmotic flow direction between the concentrated solution and the dilute solution is reversed. If the recovery rate increases (the inlet 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 applied pressure. This will offset the driving effect of the inlet pressure, slow down or stop the reverse osmosis process, and cause the permeation flux to decrease or even stop.

The discharged concentrated water must have a sufficient flow rate to remove impurities and prevent mechanical blockage or sedimentation on the inlet side of the membrane. For the convenience of system operation, the ratio between produced water and inlet water is usually used as an important operating parameter. This ratio is called the "recovery rate" and is usually expressed as a percentage. For example, the inlet flow rate of the RO device is 100m ³/ Hr, product water flow rate is 75m ³/ Hr, with a recovery rate of 75%. The remaining 25m without passing the membrane ³/ HR, which means that concentrated water will usually be discharged.

The recovery rate is mathematically defined as: recovery rate (%)=production water flow rate x100/inflow flow rate The sum of production water flow rate and concentrated water flow rate is equal to the inflow flow rate, which can also be expressed as: inflow flow rate=production water flow rate+concentrated water flow rate

According to specific applications, the recovery rate of reverse osmosis systems is usually between 70% -80%. If the inlet TDS is high, a lower recovery rate is required; On the contrary, if the TDS of the influent is low, a higher recovery rate can be used, but its limit value should be that there will be no precipitation due to supersaturation of salts and other substances in the membrane system.


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