Why does the pH value need to be adjusted in the secondary reverse osmosis process of pure water equipment?
In the RO pure water equipment system, alkaline is added to the secondary RO process of the influent. This is because the RO raw water influent contains CO2, which exists in the form of hydrogen ions and carbonate ions through hydrolysis. The membrane has a higher removal rate for carbonate ions, so the pH value of the produced water decreases and the pH value of the RO produced water is generally lower than that of the influent. When the influent alkalinity and HCO3-ion content are high, its pH decreases more. When the pH is less than 8, HCO3- and CO32- in the water begin to convert into CO2. When the pH is less than 4, HCO3- and CO32- in the water are completely converted into CO2. Therefore, in the secondary RO influent, by adding NaOH, CO2 can be converted into HCO3- and CO32- that can be removed by the RO membrane, thereby increasing the desalination rate of the secondary RO. The pH value of the secondary RO influent should be controlled at around 8.3 to obtain the optimal secondary produced water quality.
Where should NaOH be added?
According to the system design, NaOH can be added to the product water pipeline, and the pH measurement probe can be placed in the product water tank, so that the pH value of the secondary influent can be measured more stably. Another method is to add NaOH to the secondary influent pipeline. However, when adding NaOH at this location, the mixing degree of NaOH and water should be considered, and the pH measurement point should be in front of the high-pressure pump to ensure even mixing. In addition, the intense water reflux of the secondary can effectively adjust the pH value of the secondary influent, which can reduce the amount of NaOH used. However, only partial water reflux should be used, and the other part should be refluxed to the pretreatment water tank. It can be seen that the pH value has a close relationship with the conductivity of the secondary produced water. The pH value needs to be adjusted to an appropriate position to obtain the optimal produced water quality.
Post time: Dec-07-2024