Working principle of reverse osmosis dosing

01

Various raw water contains certain concentrations of suspended solids and soluble substances. The suspended solids are mainly inorganic salts, colloids, and biological particles such as microorganisms and algae. Soluble substances mainly include soluble salts (such as chlorides) and insoluble salts (such as carbonates, sulfates, and silicates), metal oxides, acids, and bases. During reverse osmosis, the volume of influent water decreases and the concentration of suspended particles and dissolved substances increases. Suspended particles can deposit on the membrane, blocking the inlet flow path, increasing frictional resistance (pressure drop). When the insoluble salt exceeds its saturation limit, it will precipitate from the concentrated water, forming scale on the membrane surface, reducing the flux of the RO membrane, increasing the operating pressure and pressure drop, and leading to a decline in product water quality. This phenomenon of forming a deposition layer on the membrane surface is called membrane fouling, and the result of membrane fouling is degradation of system performance. It is necessary to conduct pre-treatment before raw water enters the reverse osmosis membrane system to remove suspended solids, dissolved organic matter, and excessive insoluble salt components that may cause pollution to the reverse osmosis membrane, and reduce the tendency of membrane pollution. The purpose of pre-treatment of incoming water is to improve the quality of incoming water and ensure reliable operation of the RO membrane.

The effect of pre-treatment of raw water is reflected in the decrease in the absolute values of water quality indicators for TSS, TOC, COD, BOD, LSI, and iron, manganese, aluminum, silicon, barium, and strontium pollutants, which are described in detail in the previous chapter. Another important water quality indicator that characterizes membrane fouling tendencies is SDI. Through pre-treatment, in addition to reducing the above indicators to the range required for water inflow into the reverse osmosis membrane system, it is also important to minimize SDI. The ideal SDI (15 minutes) value should be less than 3.

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Chemical pretreatment

02

In order to improve the operational performance of the reverse osmosis system, the following agents can be added to the influent: acids, alkalis, fungicides, scale inhibitors, and dispersants.

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Adding acid to prevent scaling

03

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

Sulfuric acid is cheaper, does not emit smoke and corrodes surrounding metal components, and the membrane has a higher removal rate of sulfate ions than 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 concentration specifications: 20% and 93%. 93% sulfuric acid is also known as 66 Baume sulfuric acid. Be careful when diluting 93% sulfuric acid. When diluting to 66%, heating can raise the temperature of the solution to 138 ℃. It is necessary to slowly add acid to the water under stirring to avoid local heating and boiling of the aqueous solution. Hydrochloric acid is mainly used when scaling of calcium sulfate or strontium sulfate may occur. The use of sulfuric acid will increase the concentration of sulfate ions in reverse osmosis influent water, directly leading 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 reducing pH is to reduce the tendency of calcium carbonate scaling in RO concentrated water, that is, to reduce the Languerre index (LSI). LSI is the saturation of calcium carbonate in low salinity brackish water, indicating the possibility of calcium carbonate scaling or corrosion. In reverse osmosis water chemistry, LSI is an important indicator for determining whether calcium carbonate scaling occurs. When the LSI is negative, water will corrode metal pipes, but it will not form calcium carbonate scaling. If the LSI is positive, the water is not corrosive, but calcium carbonate scaling occurs. The LSI subtracts the actual pH of water from the pH saturated with calcium carbonate. The solubility of calcium carbonate decreases with increasing temperature (scale in a kettle is formed in this way), and decreases with increasing pH, calcium ion concentration, or alkalinity. The LSI value can be lowered by injecting acid solution (usually sulfuric acid or hydrochloric acid) into the reverse osmosis influent, i.e. lowering the pH value. The recommended LSI value for reverse osmosis concentrated water is 0.2 (indicating that the concentration is 0.2 pH units below the saturated calcium carbonate concentration). Polymer scale inhibitors can also be used to prevent calcium carbonate precipitation. Some scale inhibitor suppliers claim that their products can achieve an LSI of up to+2.5 for reverse osmosis concentrated water (a more conservative design is a LSI of+1.8).

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Increase desalting rate by adding alkali

04

Alkali addition is less used in primary reverse osmosis. Alkali solution is injected into the reverse osmosis feed water to increase the pH. The only commonly used alkali agent is sodium hydroxide (NaOH), which is convenient to purchase and easily soluble in water. Generally, industrial grade sodium hydroxide without other additives can meet the needs. Commercial sodium hydroxide consists of 100% flake alkali, as well as 20% and 50% liquid alkali. When adding alkali to adjust the pH, it must be noted that increasing the pH will increase LSI and reduce the solubility of calcium carbonate, iron, and manganese. The most common alkali dosing application is a secondary RO system. In the secondary reverse osmosis system, the primary RO production water is supplied to the secondary RO as raw water. The secondary RO process "polishes" the primary RO water, and the water quality of the secondary RO water can reach 4 megaohms.

There are four reasons for adding alkali to the secondary RO influent: a. At pH 8.2 or higher, carbon dioxide is completely converted into carbonate ions, which can be removed by reverse osmosis. Carbon dioxide itself is a gas that can enter the RO production water freely with the permeate liquid, resulting in improper loading on the downstream ion exchange bed polishing process. B. Some TOC components are more easily removed at high pH. The solubility and removal rate of silicon dioxide are higher at high pH (especially above 9). D. The boron removal rate is also high at high pH (especially above 9). There is a special case for alkali dosing applications, commonly referred to as the HERO (High Efficiency Reverse Osmosis System) process, which adjusts the pH of the influent water to 9 or 10. Primary reverse osmosis is used to treat brackish water, which can have pollution problems (such as hardness, alkalinity, iron, manganese, etc.) at high pH. Pretreatment typically uses weakly acidic cationic resin systems and degassing devices to remove these contaminants.

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Dechlorination agent - elimination of residual chlorine

05

In order to meet the requirements of polyamide composite membranes, the free chlorine in RO and NF influents must be reduced to below 0.05 ppm. 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 (50-100 gpm), with a reasonable investment cost. It is recommended to use high-quality activated carbon that has been pickled to remove hardness and metal ions. The content of fine powder should be very low, otherwise it may cause pollution to the membrane. Newly installed carbon filter media must be fully rinsed until the carbon powder is completely removed, which typically takes several hours or even days. We can't rely on 5 μ M security filter to protect the reverse osmosis membrane from carbon contamination. The advantage of carbon filters is that they can remove organic substances that can cause membrane pollution, making it more reliable to treat all incoming water than adding chemicals. However, its disadvantage is that carbon can become a feed for microorganisms and breed bacteria in the carbon filter, resulting in biological pollution of the reverse osmosis membrane.

Sodium bisulfite (SBS) is a typical reducing agent selected for larger RO units. Dissolve solid sodium metabisulfite in water to prepare a solution. The purity of commercial sodium metabisulfite is 97.5-99%, and the dry storage period is 6 months. The SBS solution is unstable in the air and can react with oxygen. Therefore, it is recommended that the service life of 2% of the solution be 3-7 days, and that of solutions below 10% be 7-14 days. Theoretically, 1.47 ppm SBS (or 0.70 ppm sodium metabisulfite) can reduce 1.0 ppm of chlorine. In the design, considering the safety factor of the industrial brackish water system, the addition amount of SBS is set to be 1.8-3.0 ppm per 1.0 ppm of chlorine. The injection port of the SBS should be located upstream of the membrane element, and the distance should be set to ensure a reaction time of 29 seconds before entering the membrane element. It is recommended to use an appropriate online mixing device (static mixer). SBS dechlorination reaction:? Na2S2O5 (sodium metasulfite)+H2O=2 NaHSO3 (sodium bisulfite)? NaHSO3+HOCl=NaHSO4 (sodium bisulfate)+HCl (hydrochloric acid)? The advantage of using SBS for dechlorination of NaHSO3+Cl2+H2O=NaHSO4+2 HCl is that it requires less investment than carbon filters in large systems, and reaction by-products and residual SBS are easily removed by RO. The disadvantage of SBS dechlorination is that it requires manual mixing of small volumes of reagents, which increases the threat of chlorine to the membrane when the dechlorination system is not designed with sufficient monitoring and control instruments. In addition, in a few cases, sulfur reducing bacteria (SBR) exist in the influent water, and sulfite can become bacterial nutrients to help bacteria reproduce. SBR is commonly found in shallow well water anaerobic environments, where hydrogen sulfide (H2S), as a metabolite of SBR, occurs simultaneously. The dechlorination process can be monitored using a free chlorine monitor to monitor the concentration of residual sulfite, as well as an ORP monitor. The recommended method is to monitor the concentration of residual sulfite to ensure that there is sufficient sulfite to reduce chlorine. Most commercial chlorine monitors have a detectable concentration of 0.1 ppm, which is the upper limit for residual chlorine in the CPA membrane. The method of directly using ORP monitors to monitor the concentration of sulfite is not reliable, and it is difficult to predict the baseline changes of such instruments for measuring the redox potential in water. The chlorine resistance of the CPA membrane is approximately between 1000 and 2000 ppm per hour (double the salt penetration rate), and 1000 ppm per hour is equivalent to 3 years of operation with 0.038 ppm residual chlorine. It should be noted that in some cases, it has been found that the resistance to chlorine can significantly decrease due to increased temperature (above 90 degrees Fahrenheit), increased pH (above 7 degrees Fahrenheit), and the presence of transition metals (such as iron, manganese, zinc, copper, aluminum, etc.). The chloramine resistance of the CPA membrane is about 50000 to 200000 ppm per hour (with a significant increase in salt penetration rate), which is equivalent to 1.9 to 7.6 ppm of chloramine in the RO influent. The membrane can operate for 3 years. Similarly, the chloramine resistance of the membrane varies with increasing temperature, decreasing pH, and the presence of transition metals. At a tertiary wastewater treatment plant in California, it was found that the desalination rate of the membrane decreased from 98% to 96% within 2-3 years under influent conditions with a chloramine concentration of 6-8 ppm. Designers should note that dechlorination after chloramination is still necessary. Chloramine is a product of mixed chlorine and ammonia, and free chlorine has a much stronger degradation effect on the membrane than chloramine. If the amount of ammonia is insufficient, free chlorine will exist. Therefore, the use of excessive ammonia is critical, and system monitoring should ensure this.

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Scale inhibitors and dispersants

06

Many scale inhibitor manufacturers can provide a variety of scale inhibitors and dispersants for improving the performance of reverse osmosis and nanofiltration systems. Scale inhibitors are a series of chemical agents used to prevent the precipitation and scaling formation of crystalline mineral salts. Most scale inhibitors are specialized organic synthetic polymers (such as polyacrylic acid, carboxylic acid, polymaleic acid, organometallic phosphates, polyphosphates, phosphonates, anionic polymers, etc.), with molecular weights ranging from 2000 to 10000 Daltons. The scale inhibitor technology for reverse osmosis systems has evolved from the chemistry of cooling circulating water and boiler water. The effectiveness and efficiency of a wide variety of scale inhibitors vary greatly in different applications and organic compounds used.

When using polyacrylic acid scale inhibitors, special care should be taken, as high iron content may cause membrane fouling, which can increase the operating pressure of the membrane. To effectively remove this type of fouling, acid cleaning is necessary.

If cationic coagulants or filter aids are used in pre-treatment, special attention should be paid when using anionic scale inhibitors. It produces a viscous contaminant that increases operating pressure and is very difficult to clean.

Sodium hexametaphosphate (SHMP) is a common scale inhibitor used in reverse osmosis in the early stage, but with the emergence of dedicated scale inhibitors, the dosage has been greatly reduced. There are some limitations to the use of SHMP. The solution should be prepared every 2-3 days, as exposure to air will cause hydrolysis, which not only reduces the scale inhibition effect, but also causes the possibility of calcium phosphate scaling. Using SHMP can reduce calcium carbonate scaling, and the LSI can reach+1.0. Scale inhibitors inhibit the growth of salt crystals in RO influent and concentrated water, allowing insoluble salts to exceed their saturated solubility in concentrated water. The use of scale inhibitors can be used instead of or in conjunction with acid addition. There are many factors that can affect the formation of mineral scaling. A decrease in temperature will reduce the solubility of scaling minerals (except calcium carbonate, which, in contrast to most substances, decreases as temperature increases), while an increase in TDS will increase the solubility of insoluble salts (this is because high ionic strength interferes with the formation of crystal seeds).