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Pre-treatment: All types of raw water contain a certain concentration of suspended solids and dissolved substances. Suspended solids mainly consist of inorganic salts, colloids, and biological particles such as microorganisms and algae. Soluble substances mainly include readily soluble salts (such as chlorides) and insoluble salts (such as carbonates, sulfates, and silicates), metal oxides, acids, and bases. During the reverse osmosis process, the volume of the feed water decreases, while the concentrations of suspended particles and soluble substances increase. Suspended particles will deposit on the membrane, blocking the water inlet channels and increasing frictional resistance (pressure drop). When the concentration of insoluble salts exceeds their saturation limit, they precipitate out of the concentrated solution and form scale on the membrane surface, reducing the flux of the RO membrane, increasing operating pressure and pressure drop, and leading to a decline in the quality of the produced water. This phenomenon of a deposition layer forming on the membrane surface is called membrane fouling, and the result of membrane fouling is a degradation in system performance. Pre-treatment is necessary before the raw water enters the reverse osmosis membrane system, in order to remove suspended solids, soluble organic substances, and excessive insoluble salt components that could contaminate the reverse osmosis membranes, thereby reducing the tendency for membrane fouling. The purpose of pre-treating the feed water is to improve its quality, thereby ensuring reliable operation of the RO membrane. The effect of pretreatment on the raw water is reflected in a decrease in the absolute values of water quality parameters for pollutants such as TSS, TOC, COD, BOD, LSI, as well as iron, manganese, aluminum, silicon, barium, strontium, etc. A detailed description of these water quality parameters for pollutants is provided in the previous chapter. Another important water quality parameter that characterizes the tendency of membrane fouling is SDI. Through pretreatment, in addition to reducing the aforementioned parameters to within the requirements for water fed into the reverse osmosis membrane system, it is also important to minimize SDI; the ideal SDI (15-minute) value should be less than 3. 5.1 Chemical pretreatment: To improve the operational performance of the reverse osmosis system, certain chemicals can be added to the feed water, such as acids, bases, disinfectants, scale inhibitors, and dispersants. 1 Adding acid – preventing scaling: Hydrochloric acid (HCl) or sulfuric acid (H2SO4) can be added to the feed water to lower the pH. Sulfuric acid is inexpensive, does not emit fumes that could corrode surrounding metal components, and its membrane has a higher removal rate for sulfate ions compared to chloride ions; therefore, sulfuric acid is more commonly used than hydrochloric acid. Industrial-grade sulfuric acid without any other additives is suitable for use in reverse osmosis; commercial sulfuric acid is available in two concentrations: 20% and 93%. 93% sulfuric acid is also known as 66 Baume sulfuric acid. Be very careful when diluting 93% sulfuric acid; heating can occur during dilution to 66%, raising the solution’s temperature to 138°C. The acid must be added to the water slowly while stirring, to prevent localized heating and boiling of the aqueous solution. Hydrochloric acid is mainly used when calcium sulfate or strontium sulfate scaling may occur. The use of sulfuric acid increases the sulfate ion concentration in the feed water for reverse osmosis, directly leading to an increased tendency for calcium sulfate scaling. Industrial-grade hydrochloric acid (without additives) is easily available to purchase, with commercial hydrochloric acid typically having a concentration of 30–37%. The primary purpose of lowering pH is to reduce the tendency for calcium carbonate scaling in RO concentrate, that is, to lower the Langley-Stein index (LSI). LSI is the saturation of calcium carbonate in low-salinity brackish water, indicating the likelihood of calcium carbonate scaling or corrosion. In reverse osmosis water chemistry, LSI is an important indicator for determining whether calcium carbonate scaling will occur. When LSI is negative, water will corrode metal pipes, but no calcium carbonate scaling will form. If the LSI is positive, water is non-corrosive, but calcium carbonate scaling will occur. LSI is the pH saturated with calcium carbonate minus the actual pH of the water. The solubility of calcium carbonate decreases as temperature rises (this is how scale forms in kettles), and it also decreases as pH and the concentration of calcium ions, that is, alkalinity, increase. The LSI value can be reduced by lowering the pH by adding acid (usually sulfuric acid or hydrochloric acid) to the reverse osmosis feed water. The recommended LSI value for reverse osmosis concentrate is 0.2 (indicating that the concentration is 0.2 pH units below the saturation concentration of calcium carbonate). 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 concentrate (a more conservative value is +1.8 for LSI). 2 Adding alkali – improving removal efficiency. The use of alkali in primary reverse osmosis is relatively rare. Alkali solution is added to the reverse osmosis feed water to raise the pH. The commonly used base is sodium hydroxide (NaOH), which is easily available and soluble in water. Industrial-grade sodium hydroxide, generally free of other additives, is sufficient to meet the requirements. Commercial sodium hydroxide includes 100% solid caustic soda, as well as liquid caustic soda at 20% and 50% concentrations. When raising the pH by adding alkali, it is important to note that an increase in pH raises LSI and reduces the solubility of calcium carbonate as well as iron and manganese. The most common application of alkali addition is in secondary RO systems. In a two-stage reverse osmosis system, the water produced by the first-stage RO is supplied to the second-stage RO as feed water. Secondary reverse osmosis performs a \"polishing\" treatment on the water produced by primary reverse osmosis, and the quality of the water from secondary RO can reach 4 megohms. There are four reasons for adding alkali to the secondary RO feed water: a. At a pH above 8.2, carbon dioxide is completely converted into carbonate ions, which can be removed by reverse osmosis. Carbon dioxide itself is a gas that can freely enter the RO product water along with the permeate fluid, placing undue stress on the ion exchange bed used for polishing in the subsequent stage. b. Certain TOC components are more easily removed at high pH. c. The solubility and removal rate of silica are higher at high pH values (especially above 9). d. The boron removal rate is also high at high pH values (especially above 9). There is a special case for the use of alkalis, commonly known as the HERO (High-Efficiency Reverse Osmosis) process, which involves adjusting the pH of the feed water to 9 or 10. Primary reverse osmosis is used to treat brackish water, which presents contamination issues at high pH levels (such as hardness, alkalinity, iron, manganese, etc.). Pre-treatment typically uses a weakly acidic cation resin system and a degassing device to remove these contaminants. 3 Dechlorination agents – to remove residual chlorine. The free chlorine content in the water fed into RO and NF systems must be reduced to below 0.05 ppm in order to meet the requirements of polyamide composite membranes. There are two pre-treatment methods for dechlorination: granular activated carbon adsorption and the use of reducing agents such as sodium sulfite. In small systems (50–100 gpm), activated carbon filters are generally used, as their investment cost is relatively reasonable. It is recommended to use high-quality activated carbon that has been acid-washed to remove hardness and metal ions; the content of fine particles must be very low, as otherwise it can cause contamination of the membrane. The newly installed carbon filter media must be thoroughly rinsed until all the carbon powder is removed, which usually takes several hours or even days. We cannot rely on 5μm security filters to protect the reverse osmosis membrane from contamination by carbon powder. The advantage of carbon filters is that they can remove organic substances that cause membrane fouling, and they are a more reliable method for treating all incoming water compared to the use of chemicals. However, its drawback is that carbon serves as food for microorganisms, leading to the growth of bacteria in the carbon filter, which in turn causes biological fouling of the reverse osmosis membrane. Sodium bisulfite (SBS) is a typical reducing agent used in larger RO systems. Solid sodium bisulfite is dissolved in water to prepare a solution; the purity of commercial sodium bisulfite is 97.5–99%, and its shelf life when stored dry is 6 months. SBS solutions are unstable in air and react with oxygen; therefore, it is recommended that a 2% solution be used for 3–7 days, while solutions with a concentration of less than 10% can be used for 7–14 days. Theoretically, 1.47 ppm of SBS (or 0.70 ppm of sodium bisulfite) can reduce 1.0 ppm of chlorine. When designing, the safety factor for industrial brackish water systems was taken into account, and the addition amount of SBS was set at 1.8–3.0 ppm per 1.0 ppm of chlorine. The inlet for SBS must be located upstream of the membrane element, at a distance that ensures a 29-second reaction time before it reaches the membrane element. It is recommended to use an appropriate online mixing device (static mixer). Media filtration: The common method for removing suspended solids from water is multi-media filtration. Multi-media filters use layered beds of anthracite, quartz sand, crushed garnet, or other materials. The top layer of the bed is made of materials of lightweight and coarse grades, while the materials of heaviest and finest grades are placed at the bottom of the bed. The principle is depth filtration – larger particles in the water are removed at the top layer, while smaller particles are removed deeper within the filter medium. In a single-media filter, the finest particulate material is backwashed to the top of the bed. Most of the filtration occurs within a 5 cm area at the top of the bed, with the remainder serving as a support medium. A layer of mud is formed. Although the filtration rate of a single-media filter is limited to 81.5–163 L/(min·m2) per filtration area, the hydraulic flow rate in a multi-media filter can reach up to 815 L/(min·m2). However, due to the requirements for high water quality, the flow rate is usually limited to 306 L/(min·m2) in RO pre-treatment. Since colloidal suspensions are very fine and due to the repulsion between the charges of the medium, simple filtration is ineffective. In these cases, flocculants or flocculating chemicals must be added before filtration. Commonly used flocculants include ferric chloride, alum, and cationic polymers. Cationic polymers are the most commonly used because they are effective at low doses and do not significantly increase the solid load of the filter media. On the other hand, if cationic polymers enter some of the most commonly used membranes today, they are very strong contaminants. A very small amount of cationic polymer can clog these membranes, and they are often difficult to remove. It must be kept in mind that when using cationic polymers as filtration aids, care must be taken when using them. 2. Removal of iron and manganese – oxidation filtration. Certain well waters with salt concentrations in the brackish range are in a reduced state; their typical characteristic is the presence of divalent iron and manganese, and sometimes hydrogen sulfide and ammonia as well. If such water sources are chlorinated, or when the oxygen content in the water exceeds 5 mg/L, Fe2+ will be converted to Fe3+, forming insoluble colloidal hydroxide particles. The oxidation reactions of iron and manganese are as follows: 4Fe(HCO3)2 + O2 + 2H2O → 4Fe(OH)3 + 8CO2; 4Mn(HCO3)2 + O2 + 2H2O → 4Mn(OH)3 + 8CO2. Since the oxidation of iron occurs at very low pH values, iron contamination is more common than manganese contamination. Even when the SDI is below 5 and the iron content in the RO feedwater is less than 0.1 mg/L, iron contamination can still occur. Water with low alkalinity has a higher iron ion content, as the solubility of FeCO3 limits the concentration of Fe2+. One method for treating such water sources is to prevent contact with air and any oxidizing agents such as chlorine throughout the entire RO process. A low pH value helps to delay the oxidation of Fe2+. When pH is less than 6 and the oxygen content is less than 0.5 mg/L, the maximum allowable concentration of Fe2+ is 4 mg/L. Another approach involves using air, Cl2, or KMnO4 to oxidize iron and manganese, with the resulting oxides being removed through a media filter. However, it is important to note that the colloidal sulfur formed by the oxidation of sulfides may be difficult to remove using filters. By adding an oxidant in the media filter to oxidize Fe2+ through electron transfer, oxidation and filtration can be carried out simultaneously in one step. Chlorite is such a granular filtration medium; when its oxidizing capacity is exhausted, it can be regenerated through oxidation with KMnO4. After regeneration, any remaining KMnO4 must be completely washed away to prevent damage to the membrane. When the amount of Fe2+ in the raw water is less than 2 mg/L, this treatment method can be used. If the amount of Fe2+ is higher but still less than 2 mg/L, this method can still be applied. In cases where there is a higher level of Fe2+ in the water, KMnO4 can be added continuously before the water enters the filter; however, in such situations, measures must be taken, such as installing an activated carbon filter, to ensure that potassium permanganate does not reach the membrane elements. The Birm filter can also be effectively used to remove Fe2+ from RO/NF feed water. Birm is a matrix based on aluminum silicate coated with manganese dioxide to form precipitates, which can be washed out of the filter through backwashing. Since the pH will increase during this process, changes in the LSI value may occur; therefore, it is necessary to prevent the formation of CaCO3 deposits in the filters and RO/NF systems. 3 Micro-flocculation: If the colloids in the raw water are flocculated or coagulated before filtration, the efficiency of the media filter can be significantly improved, reducing the SDI of the effluent to around 5. Ferric sulfate and ferric trichloride can be used to destabilize the negative charge on the surface of colloids, thereby trapping them on newly formed microflocs of iron hydroxide. Aluminum-containing coagulants work on a similar principle, but they are not recommended due to the risk of residual aluminum ion contamination, unless polymeric aluminum is used. Rapid dispersion and mixing of the flocculant is very important; it is recommended to use static mixers or locate the injection point in the suction section of the booster pump. Generally, the optimal dosage is 10–30 mg/L, but this amount should be determined based on specific project requirements. To enhance the strength of coagulant flocs and thereby improve their filtration properties, or to promote bridging between colloidal particles, flocculants are used either together with coagulants or alone. Flocculants are soluble high-molecular-weight organic compounds such as linear polyacrylamide; depending on their various functional groups, they can be cationic, anionic, or neutral non-ionic. Coagulants and flocculants can affect RO membranes directly or indirectly; indirect effects include the formation of precipitates from their reaction products, which then cover the membrane surface, for example, when channeling occurs in the filter and allows coagulant flocs to pass through it and settle ； When iron or aluminum coagulants are used without an immediate reduction in pH, precipitation occurs during the RO stage or as a result of supersaturation induced by water concentration. Precipitation can also occur when compounds are added after the multi-media filter; the most common such compound is scale inhibitors. Almost all scale inhibitors carry a negative charge, and they react with cationic coagulants or coagulant aids present in the water, thereby contaminating the RO membrane. When the added polymer itself affects the membrane and leads to a decrease in flux, this is considered a direct effect. To eliminate the direct and indirect effects of RO/NF membranes, anionic and non-ionic flocculants are more suitable than cationic flocculants, and excessive addition must also be avoided. 4 Microfiltration/ultrafiltration: An reverse osmosis/nanofiltration system that uses ultrafiltration/microfiltration pre-treatment processes is called an integrated membrane system (IMS). Compared to reverse osmosis systems that use traditional pretreatment processes, the IMS design has several clear advantages. ● The quality of the MF/UF permeate water is better. SDI and turbidity are lower, significantly reducing the load of colloidal and organic matter as well as microbial contamination on reverse osmosis. ● Since the membrane acts as an absolute barrier to pollutants here, the high quality of the MF/UF filtrate can be maintained stably. This stability remains unchanged even in water sources with extremely frequent fluctuations in quality, such as surface water and wastewater. ● Due to reduced colloidal contamination, the cleaning frequency of reverse osmosis systems has significantly decreased. ● Compared to some traditional filtration processes, MF/UF systems are easier to operate and require less time. ● Compared to traditional processes that use large amounts of chemicals, the disposal of MF/UF-concentrated wastewater is relatively easier. ● It occupies less space; in some large systems, it is sometimes only 1/5 of that required by traditional processes. ● It facilitates the expansion and capacity increase of the system. ● The operating costs are roughly the same, and in some cases they are lower. ● The equipment investment is roughly the same, and in some cases it is lower. (1) Characteristics of MF/UF membranes: The pore size of microfiltration membranes available on the market is generally between 0.1 and 0.35 mm. The cut-off molecular weight of ultrafiltration membranes used for reverse osmosis pretreatment is generally between 20,000 and 750,000 daltons (0.002–0.05 mm). The typical operating transmembrane pressure difference (TMP) is 3–30 psi. Membrane materials include polysulfone, polyolefins, polyethersulfone, polypropylene, cellulosic materials, and other proprietary formulations. Most membrane materials have a fairly wide pH range to facilitate chemical cleaning under low and high pH conditions. Most membranes also possess resistance to free chlorine, allowing for periodic or continuous disinfection. The maximum operating temperature for polymer membranes is 40°C, but ceramic membranes can be used at higher temperatures. MF/UF membranes come in many configurations: spiral-wound flat membranes, tubular, hollow fiber, and plate-and-frame types. Hollow fiber and spiral wound types are commonly used for RO pretreatment, mainly due to considerations regarding investment, energy consumption, resistance to fouling, and flux recovery performance during flushing and chemical cleaning. (2) Operation characteristics of MF/UF: The MF/UF membranes have two different operation modes: full-flow filtration and cross-flow filtration. The full filtration mode (also known as dead-end filtration) is similar to the cartridge filter, meaning there are only a feed stream and a filtrate stream (no concentrate stream). The full filtration method can maximize water recovery rates, reaching 95–98%, but it is generally applicable only when the suspended solid content in the raw water is low (for example, low turbidity)