In RO system, the control of water quality is very important. In order to be able to operate the RO system well, Daivietprotech recommends that customers operate the system with the input water quality within the allowable limits as recommended by the manufacturer and maintain the membranes by the following methods. technically correct.
In this article, we will continue to learn about the parameters to keep in mind in an RO system. Additional notes:
1. Sulfates
Sulfates are present in relatively large concentrations in most raw waters. Their concentration can be rapidly increased when sulfuric acid is added to water to adjust the pH. In this case, Ba2+ and Sr2+ must be analyzed accurately at 1 μg/L (ppb) and 1 mg/L (ppm) detection levels, as BaSO4 and SrSO4 are less soluble in water than CaSO4 and more. Moreover, barium and strontium sulfate flakes are extremely difficult to decompose, causing hard-to-treat deposits of RO membranes. Therefore, using anti-scale chemicals for RO membranes is very important.
2. Alkalinity
Alkalinity consists of negative ions including bicarbonates, carbonates, and hydroxides. Most of the alkalinity in natural water sources is in the form of bicarbonate (HCO3-). At pH below 8.3, bicarbonate alkalinity will be in equilibrium with a certain concentration of dissolved carbon dioxide. At pH greater than 8.3, HCO3- will be converted to carbonate form (CO32-). With water with a pH above 11.3, hydroxide (OH-). Alkalinity is important in RO systems because it can increase the likelihood of carbonate deposits. At pH above 8.3 where carbonate salts exist in significant concentrations, the potential for carbonate deposit formation is significantly increased, as well as an increased rate of carbonate deposition which severely affects the RO membrane. As a result, most RO systems need to use an RO anti-scale chemical or reduce the pH to below 8 to control scale in the RO system.
3. Nitrates
Nitrates are very soluble in water and therefore will not precipitate in an RO system. However, nitrates are a health concern because when ingested by mammals including humans, they are converted to nitrite which interferes with hemoglobins for oxygen exchange in the blood. This can cause serious problems, especially for unborn babies and children. For this reason, it is recommended to maintain nitrate levels below 40 mg/L in drinking water. Typical nitrate removal rate by RO membrane is in the range of 90~96%.
4. Iron and Manganese
Iron and Manganese are present in water in a divalent state, soluble in water or in a trivalent state, forming insoluble hydroxides. Dissolved iron (Fe2+) can come from well water or rust from pumps, pipes and tanks, especially if acid is pumped upstream. If the iron or manganese concentrations are greater than 0.05 mg/L in the RO feed water and they are oxidized by air or oxidants to a trivalent state, the hydroxides are insoluble Fe(OH)3 and Mn (OH)3 will precipitate in the system, when the water pH is neutral or higher. They can also catalyze the oxidizing effects of residual oxidants, which can accelerate RO membrane degradation. Therefore, iron and manganese must be removed in the pretreatment step.
5. Aluminum
Aluminum is not normally found in natural water sources. With iron-like 3+ valence (Fe3+), aluminum will form a very insoluble hydroxide [Al(OH)3] at the normal operating pH range of 5.3 to 8.5 in an RO system. Due to their highly charged properties, alum [Al2(SO4)3] or sodium aluminate (NaAlO2) are used as negatively charged coagulants in surface water pretreatment. Care must be taken not to use excessive flocculants to result in the carrying of residual aluminum across the RO membrane.
6. Copper and Zinc
Copper and Zinc are not found in abundance in natural water sources. Occasionally, small amounts can be obtained from the piping material. Their hydroxides Cu(OH)2 and Zn(OH)2 will come out of solution in the operating pH range of 5.3 to 8.5.
Due to the low concentration of copper and zinc, their precipitates will damage the RO system only if allowed to precipitate for a long time without cleaning the system. However, a more serious situation can develop, when an oxidant such as hydrogen peroxide is present along with copper or zinc, which rapidly degrades the film.
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