There are two aspects of the influence of pH value on reverse osmosis membrane elements: (1) the effect on the desalination rate during normal operation; (2) The cleaning effect of different pH values during cleaning and the range of pH value during cleaning. For the first point, the pH value during normal operation should be close to neutral, that is, the pH value is about 7, which is determined by two factors: 1. The reverse osmosis membrane has the highest desalination rate at pH value of 7.5-7.8, and the following figure shows the change curve of salt permeability with pH value.
2. The equilibrium relationship of carbonate system, we all know that in aqueous solution, carbonate, bicarbonate and carbon dioxide have the following balance:
This equilibrium moves with the change in pH. When the pH value is less than 8, the CO32- and HCO3 in the water begin to be partially converted to CO2, and when the pH value is less than 4, all CO32- and HCO3- in the water are converted to CO2. The reverse osmosis membrane element cannot remove the CO2 dissolved in water, and after the CO2 passes through the membrane element to the water production side, it will be converted into HCO3- in the water, which increases the conductivity of the water producer, so the reverse osmosis membrane element does not exhibit a high desalination rate when operating under low pH conditions.
However, it is not possible to raise the pH unrestricted in order to exclude CO2 interference, as the increase in pH reduces the solubility of carbonates and leads to scaling. Therefore, controlling the appropriate pH range is necessary to ensure the proper operation of the reverse osmosis membrane elements. From the second point of view, the typical pH range of reverse osmosis membrane elements for cleaning is 2.0-12.0. Usually an acidic solution is used to clean inorganic salt scale, and an alkaline solution is used to clean organic pollution. Of course, in addition to separating the skin, the reverse osmosis membrane element has many components, such as: water inlet separator, water production separator, non-woven fabric, adhesive, etc. In addition to the influence of the cleaning agent on the separation layer of the reverse osmosis membrane, it is also necessary to consider the influence of the cleaning agent on these auxiliary materials. The pH range that a reverse osmosis membrane element can tolerate in cleaning is also temperature-dependent. In conclusion, too high or too low pH will accelerate the aging of the separation layer of the membrane element and reduce the lifetime of the membrane element.