Chemicals be used in Reverse osmosis system

All kinds of raw water contain certain concentrations of suspended solids and dissolved substances. Suspended solids are mainly inorganic salts, colloids, and biological particles such as microorganisms and algae. Soluble substances are mainly soluble salts (such as chlorides) and insoluble salts (such as carbonates, sulfates and silicates), metal oxides, acids and bases, etc. Suspended particles can deposit on the membrane, clog the inlet flow channels, and increase frictional resistance (pressure drop). When the insoluble salt exceeds its saturation limit, it will precipitate out of the concentrated water, form scale on the membrane surface, reduce the flux of the RO membrane, increase the operating pressure and pressure drop, and cause the product water quality to decline. In order to improve the operating performance of the reverse osmosis system, some chemicals can be added to the feed water.

1) Hydrochloric acid (HCl) and sulfuric acid (H2SO4) can be added to the influent to lower the pH.

Sulfuric acid is cheap, does not fume and corrode the surrounding metal components, and the removal rate of sulfate ions by the membrane is higher than that of chloride ions, so sulfuric acid is more commonly used than hydrochloric acid. Industrial-grade sulfuric acid without other additives is suitable for reverse osmosis, and commercial sulfuric acid has two concentrations of 20% and 93%. 93% sulfuric acid is also called 66 Baume sulfuric acid. Care must be taken when diluting the 93% sulfuric acid, as the heat generated at the 66% dilution can raise the temperature of the solution to 138°C. Be sure to add the acid to the water slowly and with stirring so that the solution does not locally heat up and boil. Hydrochloric acid is mainly used when calcium sulfate or strontium sulfate scaling may occur. The use of sulfuric acid will increase the concentration of sulfate ions in the reverse osmosis feed water, which directly leads to an increase in the tendency of calcium sulfate scaling. Industrial grade hydrochloric acid (without additives) is very convenient to purchase, and the general content of commercial hydrochloric acid is 30-37%. The primary purpose of lowering the pH is to reduce the tendency of calcium carbonate scaling in RO concentrated water, i.e. to reduce the Langelier Index (LSI). LSI is the saturation of calcium carbonate in low salinity brackish water and indicates the potential for calcium carbonate scaling or corrosion. In reverse osmosis water chemistry, LSI is an important indicator to determine whether calcium carbonate scaling will occur. When the LSI is negative, the water corrodes metal pipes but does not form calcium carbonate scale. If the LSI is positive, the water is not corrosive, but calcium carbonate scaling occurs. LSI is calculated by subtracting the actual pH of the water from the calcium carbonate saturated pH. The solubility of calcium carbonate decreases with increasing temperature (this is how scale is formed in kettles), and decreases with increasing pH, the concentration of calcium ions, or alkalinity. The LSI value can be adjusted down by injecting acid solution (usually sulfuric acid or hydrochloric acid) into the reverse osmosis feed water, that is, reducing the pH. The recommended LSI value for reverse osmosis brine is 0.2 (meaning the concentration is 0.2 pH units below the saturation concentration of calcium carbonate). Polymeric antiscalants can also be used to prevent calcium carbonate precipitation, and some antiscalant suppliers claim that their products can make the LSI of reverse osmosis concentrated water as high as +2.5 (a more conservative design is an LSI of +1.8).

2) Adding alkali is less used in the primary reverse osmosis system. The only alkaline agent commonly used is sodium hydroxide (NaOH), which is easy to purchase and easily soluble in water. Generally, industrial grade sodium hydroxide without other additives can meet the needs. Commercial sodium hydroxide has 100% caustic soda, and also has 20% and 50% liquid caustic soda. Care must be taken when adding base to increase the pH, as an increase in pH increases LSI and decreases the solubility of calcium carbonate and iron and manganese. The most common caustic application is a secondary RO system. In a two-stage reverse osmosis system, the primary RO product water is supplied to the secondary RO as raw water. The secondary reverse osmosis "polishes" the primary reverse osmosis product water, and the water quality of the secondary RO product water can reach 4 megohms.

3) The free chlorine in RO and NF feed water must be reduced to less than 0.05ppm to meet the requirements of polyamide composite membrane. There are two pretreatment methods for chlorine removal, granular activated carbon adsorption and the use of reducing agents such as sodium sulfite. Activated carbon filters are generally used in small systems (50-100gpm), and the investment cost is relatively reasonable. It is recommended to use high-quality activated carbon that has been pickled to remove hardness and metal ions, and the content of fine powder should be very low, otherwise it will cause pollution to the membrane. The newly installed carbon filter material must be fully rinsed until the carbon powder is completely removed, usually several hours or even days.

4) Many antiscalant manufacturers can provide various antiscalants and dispersants for performance improvement of reverse osmosis and nanofiltration systems. Antiscalants are a family of chemical agents used to prevent the precipitation and scale formation of crystalline mineral salts. Most scale inhibitors are special organic synthetic polymers (such as polyacrylic acid, carboxylic acid, polymaleic acid, organometallic phosphate, polyphosphonate, phosphonate, anionic polymer, etc.), the molecular weight of these polymers It ranges from 2000-10000 Daltons. Antiscalant technology for reverse osmosis systems evolved from cooling circulating water and boiler water chemistry. For the large variety of antiscalants, the effectiveness and efficiency achieved in different applications and organic compounds used vary greatly.

Special care should be taken when using polyacrylic acid scale inhibitors. Higher iron content may cause membrane fouling, which will increase the operating pressure of the membrane. Pickling is required to effectively remove such fouling. If cationic coagulants or filter aids are used in pretreatment, special care should be taken when using anionic antiscalants.

More inquiries related to Reverse osmosis system please contact:

Mob& Wechat& WhatsApp: (+86)13544774483

Email: sales010@water-sy.com

We will provide high-quality, all-round comprehensive professional services for project consulting, system design, manufacturing, installation and commissioning, personnel training, etc