It is important to understand the raw or feed water characteristics before design a water treatment system. Realize the potential of scale formation especially in RO system is vital for system reliability and efficiency. Therefore, an analysis of RO feed water should have parameters as following:
The pH of water is a mean of expressing its acidity or alkalinity. It is important to water analysis as the pH will dramatically affect the solubility of a number of slightly soluble salts.
Calcium (Ca2+) and Magnesium (Mg2+) make up the majority of water hardness. Technically, other multivalent cations are also included in what is considered hardness, although their concentrations are normally negligible when compared to those of calcium and magnesium. Hardness is a concern for RO system because of its insolubility in the presence of high bicarbonate, carbonate, sulfate, fluoride, or silicate concentrations, which will caused scale formation on RO membrane. Therefore, some means of controlling the precipitation of calcium and magnesium such as water softener may be necessary to obtain satisfactory performance.
Sodium salts are very soluble in water. It may be a concern in certain high purity water applications. Because sodium is mono-valent, it is not 100% rejected by RO membrane and it is not well held by ion-exchange equipment also. In semiconductor manufacturing, trace sodium ions in process water can cause failures in integrated circuit (IC).
Consists of anions that remove hydrogen ions from solution, thus shifting the ratio between hydrogen and hydroxyl ions. This gives water a higher pH. Due to the ability of water to pick up alkalinity, most naturally occurring water sources are close to saturation in calcium carbonate. If the water is concentrated in RO system, calcium carbonate is likely to precipitate and form scale on membrane.
Limited solubility in water, depending on the concentrations of divalent cations. Such cations include calcium and magnesium, as well as barium and strontium. The prevention of sulfate scale formation in RO system usually performed by reducing the divalent cations in the raw water.
Nearly all chloride salts are soluble in water. Chlorides pose little threat to RO system in term of scale formation, but high concentration of chloride can attack 304 grade stainless steel. If the chloride in RO concentrate exceeds several thousand ppm, 316L grade stainless steel are required for housing, high pressure piping and pump to prevent corrosion.
Iron and Manganese
Iron can be substantial in well water, it is usually in divalent ferrous state (Fe2+) and is soluble. If it is oxidized to its ferric state (Fe3+), it is insoluble and can foul RO membrane. Generally, if iron and manganese concentration is greater than 0.05 mg/l in RO feed water, a pre-treatment iron removal should be considered.
The solubility of silica is dependent on water temperature and pH. At high pH, it can precipitate as a salt with calcium, magnesium, iron or aluminum. If silica is present in RO feed water greater than 20 mg/l, the tendency of RO membrane fouling will be substantial, and thus a pre-treatment silica removal should be considered.