In the primary reverse osmosis process, the amount of alkali used is comparatively smaller. By injecting lye into the reverse osmosis influent water, the pH of the water can be effectively raised. In practice, sodium hydroxide (NaOH) is commonly used as an alkali agent, which is easy to purchase and highly soluble in water. In general, no additional additives are required for an application with industrial-grade sodium hydroxide. On the market, sodium hydroxide products are available in a variety of forms, including 100% pure flake soda and 20% and 50% concentration liquid soda.
It is important to exercise caution when adjusting the pH level. This is because the increase in pH leads to an increase in LSI and a decrease in the solubility of calcium carbonate and elements such as iron and manganese. In general, the application of alkali addition is more common in secondary RO systems. In this system, the water produced by the primary RO is used as the raw water for the secondary RO. The secondary reverse osmosis further "polishes" the permeate of the primary reverse osmosis, bringing its water quality to the standard of 4 trillion ohms.
There are four reasons for adding alkali to the secondary RO influent water: first, when the pH value reaches 82 or more, carbon dioxide will be completely converted into carbonate ions, which can be effectively removed by reverse osmosis. Carbon dioxide, on the other hand, as a gas, may freely enter the RO permeate with the permeate and adversely affect the downstream ion exchange bed polishing process. Second, some TOC components are easier to remove in a high pH environment. Third, the solubility and removal rate of silica will be improved at high pH values, especially when the pH is higher than 9. Fourth, the boron removal rate is also relatively high at high pH (especially when pH is higher than 9).
It is important to note that there is a special case in alkali dosing applications, the so-called HERO (High Efficiency Reverse Osmosis System) process. In this process, the pH of the influent water needs to be adjusted to 9 or 10. However, when treating brackish water, the high pH of primary reverse osmosis may cause contamination problems (such as hardness, alkalinity, accumulation of iron, manganese, etc.). To address these issues, the pretreatment stage typically employs a weakly acidic cationic resin system and a degassing unit to remove these contaminants.