Introduction to Reverse Osmosis Pretreatment Processes

Introduction to Reverse Osmosis Pretreatment Processes
(I) Coagulation Sedimentation and Dissolved Air Flotation Processes
① Coagulation Sedimentation: The principle is identical to that used in wastewater treatment's physicochemical coagulation sedimentation processes. It primarily employs aluminum salts, iron salts, or polymeric aluminum and iron coagulants, along with other organic polymeric coagulants. Through coagulation and flocculation reactions, suspended solids and colloidal substances in water form precipitates or larger flocs, facilitating interception and adsorption in subsequent processes. The specific selection and effectiveness of these agents depend on various characteristics of the raw water (temperature, pH, ion concentration, etc.). Note: Coagulation sedimentation is typically applied to water sources like rivers and lakes with high suspended solids. Excessive dosing increases aluminum/iron ion concentrations, placing additional strain on subsequent treatment stages. Since municipal water is commonly used as feedstock for pure water production, coagulation sedimentation is rarely employed in such scenarios.
② Air Flotation: Utilizes highly dispersed microbubbles to adhere to suspended solids, causing them to rise to the liquid surface for subsequent skimming and further treatment. This achieves solid-liquid or liquid-liquid separation. Commonly applied when suspended solids are abundant or trace oils are present. Similar to coagulation-sedimentation, air flotation is not widely used today.

(II) Mechanical Filtration Processes (Quartz Sand, Activated Carbon, Multi-Media, Iron/Manganese Removal Filtration) First, note that only mechanical filters exist in industrial contexts. Narrowly defined, mechanical filters specifically refer to multi-media filters. Broadly defined, they encompass any container using screens, filter media, cartridges, etc., to block solid particles. This means that common equipment like quartz sand, activated carbon, multi-media, iron-manganese removal, and precision filtration (PP melt-blown filter cartridges, bag-type screens) are all considered mechanical filters when integrated into a container-and-pipeline system. For clarity, we discuss precision filters separately. Again, note that the term “mechanical filtration” is used for convenience and lacks a strictly defined boundary.
① Quartz Sand Filtration
Retains and adsorbs silt, colloids, metal ions, and organic matter, achieving effluent turbidity <1 NTU and SDI ≤5. Quartz sand filter: Utilizes refined quartz sand as filter media. Smaller-grained quartz sand is placed at the top of the filter bed, while larger-grained quartz sand occupies the lower section. This arrangement maximizes the efficiency of the entire filter layer and enhances contaminant retention capacity. Control valves enable forward and reverse washing to remove surface contaminants, prevent clogging, and restore filtration capacity. Working Principle: Through the physical filtration action of quartz sand, impurities and pollutants in water are trapped within the quartz sand layer. As water flows through this layer, it becomes clear and transparent. This function is analogous to a filter screen, capable of removing most impurities and contaminants from the water.
Operating States Filtration State: During filtration, raw water enters the filter bed via the distributor at near-horizontal flow velocity. Impurities are trapped as water passes through the media layer. A collection manifold at the filter base uniformly gathers and discharges filtered water. This horizontal flow design enables high-velocity filtration while maintaining excellent performance. Backwash Mode: As impurities accumulate within the media bed, head loss progressively increases. When head loss reaches a predetermined threshold (a 0.07 MPa increase in the filter inlet-outlet pressure differential compared to initial operation), the system automatically switches to backwash mode to remove accumulated impurities and prevent quartz sand from compacting into clumps during prolonged operation. Compressed air may be introduced during backwashing to enhance cleaning efficiency. Filter Media Replacement: When effluent turbidity exceeds 1 NTU, investigate the cause to determine if media replacement is necessary. Under continuous operation, media typically requires replacement every 1-2 years depending on conditions.

② Activated Carbon Filtration
Adsorbs electrolytes, organic matter, and microorganisms to remove residual chlorine, color, and odor from water, achieving residual chlorine <0.1 ppm, SDI <5, and TOC <2.0 ppm in effluent. Activated Carbon Filter: The bottom may be filled with 0.15–0.4 meters of quartz sand as a support layer, using 20–40 mm quartz sand particles. Above the quartz sand, 1.0–1.5 meters of granular activated carbon is loaded as the filtration layer. The typical packing thickness is 1000–2000 mm.
Working Principle
① Adsorption Principle: A layer of equilibrium surface concentration forms on the activated carbon particles, adsorbing organic impurities into the particles. Adsorption efficiency is high during initial use. However, over time, the adsorption capacity of activated carbon diminishes to varying degrees, leading to reduced adsorption effectiveness. Therefore, activated carbon should be regularly cleaned, regenerated, or replaced.
② Residual Chlorine Removal Principle: Oxygen-containing functional groups within the activated carbon's porous structure undergo rapid oxidation-reduction reactions with oxidizing hypochlorite ions in water, thereby eliminating these oxidizing hypochlorite ions. Operational State: Filtration State
② Principle of Removing Residual Chlorine: The oxygen-containing functional groups within the porous structure of activated carbon undergo rapid oxidation-reduction reactions with the oxidizing hypochlorite ions in water, thereby eliminating these oxidizing hypochlorite ions. Operating Condition - Filtration Mode: When the system is in filtration mode, untreated water passes through the distributor and enters the filter media bed at a near-horizontal flow rate. As water flows through the media bed, impurities are trapped within the bed. A filter collector at the bottom uniformly collects and discharges the filtered water. This horizontal flow filtration enables the filter to achieve effective filtration even at high flow rates. Backwash Mode: As contaminants accumulate within the media bed, head loss progressively increases. When head loss reaches a predetermined threshold, the system automatically switches to backwash mode to remove accumulated impurities. Filter Media Replacement: Since backwashing only removes surface contaminants from activated carbon, it cannot dislodge pollutants adsorbed within the carbon particles' pores. Therefore, when the carbon pores reach adsorption saturation and the free residual chlorine in the effluent reaches ≥0.1 ppm, replacement should be considered. The recommended replacement cycle for activated carbon is one year.

③ Multi-Media Filtration
This process utilizes two or more filtration media. Under pressure, water with high turbidity passes through a layer of granular or non-granular materials, effectively removing suspended impurities to clarify the water. Common filter media include quartz sand, anthracite, and manganese sand. It is primarily used for water treatment to reduce turbidity, soften water, and as pre-treatment for pure water systems. Multi-media Filter: The top layer consists of the lightest and coarsest grade material, while the heaviest and finest grade material is placed at the bottom of the bed. Its principle is depth filtration—larger particles in the water are removed at the top layer, while smaller particles are removed deeper within the filter media. This achieves the standard for coarse filtration. This process removes solid particles, suspended solids, organic matter, and colloids while reducing COD and BOD levels. Working Principle: As raw water flows downward through the filter media, suspended solids are trapped on the surface layer via adsorption and mechanical obstruction. As water flows into the middle of the filter bed, the tighter arrangement of sand particles within the media layer increases opportunities for collisions with waterborne particles. This causes flocculants, suspended solids, and sand particles to adhere to each other, trapping impurities within the media layer and producing clarified water. Operational Status and Media Replacement: Based on the specific filter media selected, replacement standards should be comparable to or slightly better than those for equivalent materials. Typical replacement cycles are 1-2 years.
④ Iron and Manganese Removal Filtration (essentially a type of multimedia filtration) is primarily used for iron and manganese removal in groundwater from high-iron/high-manganese areas, as well as pre-treatment for industrial water softening and desalination equipment. It employs the principles of iron and manganese removal through aeration oxidation, manganese sand catalysis, adsorption, and filtration. Iron and Manganese Removal Filter: Primarily consists of a filter media layer, an oxidation-reduction layer, and a manifold. The filter media layer is composed of various filtration materials such as quartz sand, activated carbon, and magnetite. The oxidation-reduction layer comprises a composite catalyst and a catalyst diffuser, facilitating oxidation-reduction reactions for iron and manganese. The collection pipe gathers the purified water flow. This equipment treats impurities like iron and manganese in water through physical adsorption and redox reactions, enabling rapid and efficient purification. It typically incorporates a separate aeration oxidation unit, commonly known as an aeration tower or oxidizer. Working Principle: Utilizes an aeration device to dissolve atmospheric oxygen into water, thereby oxidizing Fe²⁺ and Mn²⁺ ions into insoluble Fe³⁺ and MnO₂. Combined with the catalytic, adsorbent, and filtering properties of natural manganese sand, this process removes iron and manganese ions from the water. Media Replacement: The typical replacement cycle is 2-4 years.

⑤ Mechanical Filter Selection: The mechanical filtration process in pure water equipment operates at medium-speed filtration. A recommended filtration velocity of 8-20 m/h is advised, while maintaining balanced water production across upstream and downstream equipment and ensuring overall system integrity. Water output Q (m³/h) = Filter bed cross-sectional area S (m²) × Filtration velocity V (m/h) When using the standard V = 15 m/h, the base area S = Q/15. Typically, the filter diameter D = 2√(Q/15π) Based on the diameter, we can select the corresponding filter model. Filter models are categorized into two types: Chinese national standard (e.g., φ600*2100mm) or corresponding American standard (e.g., 24*83 inches), where 1 inch = 25.3mm for simple conversion.
⑥ Filter Media Fill Height: For medium-speed filtration, we generally recommend a media fill height of approximately 0.7-1.0m, roughly 1/2 to 2/3 of the tank height. Correspondingly, we can calculate the approximate quantity of media required. Using single-media filling as an example: Fill Quantity (bags) = S × 2/3 × H × X / G × 1000 S: Cross-sectional area 2/3: Fill ratio

⑥ Filter Media Fill Volume: For medium-speed filtration, we generally recommend filling with a media height of approximately 0.7–1.0 m, roughly 1/2 to 2/3 of the tank height. Correspondingly, we can calculate the approximate quantity of media required. Using single-media filling as an example: Fill Quantity (bags) = S × 2/3 × H × X / G × 1000 S: Cross-sectional area 2/3: Filling ratio H: Container height X: Compaction factor (Sand = 1, Carbon = 0.6, Resin = 0.8) G: Standard unit bag weight. Quartz sand = 50KG, Activated carbon = 25KG, Resin: 20KG/25L Example for φ600*2100mm tank: Quartz Sand: S*2/3*H/50*1000=3.14*0.3*0.3*(2/3)*2.1*1/50*1000≈7.9 (≈8 bags) Activated Carbon: S*2/3*H/50*1000 = 3.14*0.3*0.3*(2/3)*2.1*0.6/25*1000 = 9.5≈10 (bags) Similarly, if the softener is filled with resin, its usage is calculated as follows: Softening resin: S*2/3*H/50*1000=3.14*0.3*0.3*(2/3)*2.1*0.8/20*1000=15.8≈16 (bags) Market resin is sometimes sold solely by volume (L). In this case, simply calculate the required container volume. 25L of resin weighs approximately 20KG.
(3) Temperature Regulation (Plate Heat Exchanger)
The primary reason for temperature regulation in pure water preparation relates to the core treatment process: reverse osmosis (RO). As the mainstream core technology for pure water production, RO system output is closely tied to temperature. Under constant feedwater pressure, a 1°C decrease in system water temperature reduces output by approximately 3%.

The rated water production/coefficient serves as a reference for actual water output at specific temperatures: at 20°C, production is 83.0% of the rated output at 25°C, and at 5°C, it drops to 46.3% of the rated output. This demonstrates that maintaining an optimal feed water temperature is critical for the stable operation of the entire system. Note: Considering the impact of winter temperatures on system water production, designers typically base system capacity on 60-80% of the RO membrane's rated 25°C output (e.g., 8040 membrane ~1 T/H, 4040 membrane ~0.25 T/H). This prevents situations where the designed water output fails to meet production demands.
Plate Heat Exchanger: A novel, highly efficient heat exchanger composed of stacked corrugated metal plates. Thin rectangular channels formed between the plates facilitate heat transfer. Ideal for liquid-liquid or liquid-vapor heat exchange, it offers high thermal efficiency, minimal heat loss, compact and lightweight construction, small footprint, easy installation and cleaning, broad applicability, and extended service life. Limitations of Plate Heat Exchangers: Operation requires a high-temperature liquid or steam source from the heat supply side. Electric heating is often impractical due to cost-inefficiency. Consequently, these units are typically deployed in cold northern regions during winter, or in settings with surplus heat sources like thermal power plants and large chemical facilities. They are primarily used in large-scale industrial projects for pure water production, reclaimed water reuse, and centralized industrial park water systems. In southern regions and for most small-scale equipment, the impact on system water production during winter or ambient temperatures is limited, making it unnecessary to overly prioritize optimal water temperatures.
(IV) Resin Adsorption Process (Water Softeners and Specialized Resin Adsorption)
Resin adsorption: A separation and purification technology that leverages the chemical properties and structural characteristics of resins to adsorb specific solutes from solutions. The principle of resin adsorption involves the chemical properties, physical structure, and interactions during the adsorption process. Resin adsorption processes in pure water preparation systems are primarily categorized into four types: resin softening, specialty resin adsorption, mixed-bed (dual-bed) systems, and polishing mixed-bed processes. Resin softening and specialty resin adsorption are typically employed as pretreatment steps. The dual-bed process is increasingly being replaced by reverse osmosis technology, while the polishing mixed-bed process remains an essential core technology for producing ultrapure water at 18MΩ·cm and above. Resin adsorption in pure water preparation primarily relies on ion exchange principles, supplemented by other resin adsorption mechanisms. Common resin adsorption principles are as follows:
1. Ion Exchange Adsorption Principle: Ion exchange adsorption is the most common principle in resin adsorption. Resin surfaces typically carry cation exchange groups (e.g., -NH₂, -NH₃⁺) or anion exchange groups (e.g., -OH, -COO⁻), enabling ion exchange reactions with ions in solution. This process adsorbs target ions from the solution onto the resin surface. Suitable resins can be selected based on the type and properties of the ion exchange groups.
2. Coordination Adsorption Principle: Coordination adsorption occurs when coordination groups on the resin surface form coordination bonds with target substances. Common coordination groups include acidic functional groups (e.g., hydroxyl, phenol) and basic functional groups (e.g., amine, imine), which can form complexes with metal ions in solution for adsorption.
3. Hydrogen Bonding Adsorption Principle: Hydrogen bonding adsorption occurs when hydrogen bond donors on the resin surface form hydrogen bonds with hydrogen bond acceptors in the solution. Common hydrogen bond donors include hydroxyl (-OH) and amino (-NH₂) groups, while hydrogen bond acceptors can be inert bonds or atoms like nitrogen and oxygen within solute molecules.
4. Electrostatic Adsorption Principle: Electrostatic adsorption occurs when charged solid particles on the resin surface interact electrostatically with oppositely charged ions or polar molecules in the solution. The charge properties on the resin surface can be modified by controlling pH or adding charge shells to regulate adsorption performance.
5. Van der Waals Adsorption Principle: Van der Waals adsorption occurs when nonpolar regions on the resin surface interact with nonpolar solutes in solution via van der Waals forces. Resin surfaces typically possess hydrophobic groups capable of adsorbing hydrophobic substances.
6. Porous Adsorption Principle: Porous adsorption involves the adsorption of molecules within the pores of the resin. Resin materials generally possess specific pore structures. Pore size and distribution can be controlled by adjusting resin preparation methods and conditions to accommodate adsorption requirements for different molecular sizes. Beyond these common adsorption principles, specialized mechanisms exist—such as photoadsorption and synergistic adsorption—enabling selective binding to target substances through distinct interactions.

Resin Softening: Utilizes sodium-ion resin to replace Ca²⁺ and Mg²⁺ in raw water with Na⁺, thereby reducing the hardness of water entering downstream systems and mitigating structural risks from insoluble salts (calcium carbonate, calcium sulfate, magnesium sulfate, etc.). Water Softener: Water flows downward through the softening resin bed. Ca²⁺ and Mg²⁺ ions in the raw water undergo ion exchange with Na⁺ ions in the resin, causing them to adsorb onto the resin and thereby softening the water. Typically, the inlet water hardness must be ≤200 mg/L to ensure the treated water meets the standard for soft water (<50 mg/L). Regeneration of softening resin typically involves four steps: backwashing, regeneration, rinsing, and final rinsing.
Step 1: Backwashing
After prolonged use, accumulated impurities in the raw water are trapped in the upper layer of softening resin. These impurities must be removed to fully expose the ion-exchange resin, ensuring effective regeneration. This process typically takes 10-15 minutes at a backwash flow rate of 10 m/h.
Step 2: Brine Absorption (Regeneration) Brine absorption refers to the gradual addition of brine solution into the softening resin tank. For optimal results in daily operation, surpassing those achieved by soaking the resin in brine, allow the brine solution (5-10% concentration) to flow slowly (6-8 m/h) through the resin bed. This yields superior regeneration outcomes. This step generally takes approximately 30 minutes, with salt dosage adhering to the principle of equal substance exchange: 1 mol of NaCl replaces 1 mol of softening resin. 
Step 3: Slow Rinse (Exchange) Following regeneration, rinse the resin thoroughly with raw water at the same flow rate as the brine to remove residual salts. This process is critical for softening resin regeneration, as numerous calcium and magnesium ions chemically react with sodium ions through exchange reactions. It is also referred to as “exchange.” This step typically lasts about 30 minutes, matching the salt absorption duration. 
Step 4: Fast Rinse (Forward Flush) Rinse the softening resin at a flow rate close to normal operation, using raw water. This step thoroughly removes residual salt to ensure cleanliness. The final effluent from this process is compliant softened water. This stage generally requires 5-15 minutes.
Special Resin Adsorption: With the continuous advancement of resin adsorption technology and the stringent requirements for trace element concentrations in semiconductor ultrapure water, specialty resins—represented by boron removal resins—are increasingly playing a unique role in pure water preparation, wastewater treatment, and other applications.
Boron Removal Resin: This material possesses highly selective adsorption properties, effectively removing boron from water. Its adsorption principle primarily relies on the resin material's unique chemical structure and interactions between the adsorbent and solutes in the solution (distinct from traditional ion exchange resin adsorption principles). Boron removal resins typically consist of functional groups with adsorption properties and a porous structure. These functional groups can form chemical bonds or electrostatic forces with boron elements, thereby adsorbing boron ions onto the resin surface. The porous structure of boron removal resin provides a larger surface area, increasing adsorption sites and thereby enhancing adsorption efficiency. Regeneration of boron removal resin: Desorbing adsorbed boron ions from the resin restores it to its initial adsorption state, enabling the next adsorption cycle. The regeneration principle primarily relies on controlling competing adsorbents and pH levels in the solution. Altering the solution's pH modifies the resin surface's charge properties, thereby changing the interaction between boron ions and the resin. When the solution's pH increases, the resin surface becomes negatively charged, weakening the electrostatic forces between boron ions and the resin, thus desorbing the boron ions. The regeneration process of boron removal resin can also be achieved by adding appropriate competitive adsorbents. The competitive adsorbent interacts with the functional groups on the resin surface, displacing boron ions from the resin. The interaction force between the competitive adsorbent and the boron ions is stronger than that between the resin and the boron ions, thereby achieving desorption of the boron ions. The regeneration process of boron removal resin can be cycled multiple times until the adsorption capacity of the resin decreases to a certain level, at which point the resin needs to be replaced or regenerated.

(V) Chemical Dosage Process (Oxidizing Agents, Reducing Agents, Scale Inhibitors, pH Adjustment) During pure water production, to ensure stable system operation, we often need to mitigate potential risks by adding chemical agents. Commonly used agents include oxidizing agents/sterilizers, reducing agents, scale inhibitors, and acids/alkalis (pH adjustment). These are typically integrated with the main system via dedicated dosing equipment, mixed with raw water at specific ratios to achieve the desired treatment effect.
Dosing System: Typically consists of a solution tank, a metering pump (pneumatic diaphragm pump), and associated piping components. Select an appropriately sized solution tank, pre-configure the solution to the required concentration, and set the desired dosing rate.

A. Oxidizing Agents/Sterilizers
Oxidizing agents are classified by their mechanism of action, while sterilizers are categorized by their effect. There is significant overlap between the two in water treatment processes, though they are not entirely equivalent. Most oxidizing agents function as disinfectants, and most disinfectants operate on the principle of oxidative disinfection. Below is a brief distinction: Oxidizing agents in water treatment primarily fall into three categories:
① Neutral atoms that reduce to negatively charged ions upon electron acceptance, such as O₂, Cl₂, O₃, etc.
② Positive ions that reduce to negatively charged ions upon electron acceptance. For example, under alkaline conditions, Cl⁺ from chlorite ions (ClO⁻) in bleaching powder and sodium hypochlorite, and Cl⁴⁺ from chlorine dioxide, reduce to Cl⁻ upon electron acceptance.
③ Atoms bearing high-valent positive charges that, upon accepting electrons, reduce to atoms with lower-valent positive charges. Examples include Fe³⁺ in ferric chloride and Mn⁷⁺ in potassium permanganate, which reduce to Fe²⁺ and Mn²⁺ respectively upon electron acceptance.
Among these, ① and ② are primarily employed for system sterilization, targeting organic microorganisms within the system. ③ is generally employed in the pretreatment stage for chemical ion oxidation, such as oxidizing Fe²⁺ and Mn²⁺ into insoluble Fe³⁺ and MnO₂, thereby achieving removal. Note: When the primary purpose of ① and ② substances is oxidative sterilization, the presence of reductive substances like Fe²⁺ must be considered simultaneously. Such reductive substances also consume corresponding chemical dosages, potentially leading to suboptimal sterilization outcomes.
Sterilants in water treatment processes can be categorized into oxidative and non-oxidative types:
① Oxidative Sterilants: Primarily composed of oxidizing compounds such as peroxides, chlorine-containing compounds, and bromine-containing compounds. These compounds generally offer advantages including rapid bactericidal and algicidal action, broad-spectrum efficacy, low treatment costs, relatively minor environmental impact, and reduced likelihood of microbial resistance development.
A drawback is that it is significantly affected by organic matter and reducing substances in water, has a short residual time, and is also highly influenced by water pH. Halogen elements chlorine, bromine, and iodine are all highly effective oxidizing disinfectants. Chlorine is widely available, inexpensive, easy to use, and provides excellent disinfection. It can be used alongside many water treatment chemicals with little to no interference, causes minimal environmental pollution, and is extensively employed as a microbial disinfectant for industrial and domestic water. Chlorine's disinfection mechanism involves forming molecular hypochlorous acid in water. Hypochlorous acid molecules penetrate microbial cell membranes, forming stable N-Cl bonds with proteins. This process weakens or inactivates reductase enzymes essential for respiration. At higher concentrations, it also disrupts cell walls. The higher the proportion of chlorine present in water as hypochlorous acid, the more effective the disinfection.

Note: Oxidizing disinfectants should be continuously dosed at the system inlet, with controlled dosage rates. When used in membrane systems, oxidizing disinfectants require reduction treatment prior to RO to prevent membrane oxidation failure.
② Non-Oxidizing Disinfectants
Non-oxidizing disinfectants exhibit varying mechanisms of action depending on their type. However, they all function as toxicants by targeting specific sites within microorganisms, thereby destroying their cells or vital components to achieve disinfection. Consequently, they are unaffected by reducing substances in water. Non-oxidizing bactericides and algicides exhibit persistent biocidal effects, penetrate and strip deposits or slime, and are minimally affected by reducing substances like hydrogen sulfide or ammonia, as well as pH variations in water.

Non-oxidizing disinfectants offer versatile dosing methods, including continuous online addition, online shock dosing, or use during system cleaning.

B. Reducing Agents
Reducing agents in water treatment are primarily categorized into three types:
① Those oxidized into positively charged neutral atoms after electron donation, such as iron filings or zinc powder. ② Atoms bearing negative charges that, upon electron donation, oxidize into positively charged atoms. For example, boron in sodium borohydride (B₅⁻) can reduce mercury ions to metallic mercury under alkaline conditions while oxidizing itself to B₃⁺. ③ Positively charged atoms of metals or nonmetals that, upon electron donation, oxidize into atoms bearing higher positive charges. For example, the divalent iron ion Fe²⁺ in ferrous sulfate and ferrous chloride is oxidized to the trivalent iron ion Fe³⁺ after donating one electron; the tetravalent sulfur in sulfur dioxide SO₂ and sulfite SO₃^(2−) is oxidized to hexavalent sulfur after donating two electrons, forming SO₄^(2−). During pure water production, oxidizing substances pose a potential risk of damaging reverse osmosis (RO) membranes. Common polyamide composite membranes require residual chlorine levels in feedwater to be <0.05/0.1 ppm. If feedwater quality is favorable, activated carbon adsorption typically suffices to achieve the desired effect. When water quality is poor (high oxidizing agents), a reducing agent dosing system must be added to enhance oxidant removal. Sodium sulfite (SBS) is a common residual chlorine reducing agent, operating on the following principle: 2NaHSO₃ + 2HClO → H₂SO₄ + 2HCl + Na₂SO₄ Simple calculation shows that approximately 1.47 ppm SBS (or 0.70 ppm sodium bisulfite) can reduce 1.0 ppm residual chlorine. In practice, the dosage is typically increased to 1.8–3.0 ppm. The actual dosage is calculated by multiplying the residual chlorine concentration in the raw water by the corresponding mass ratio. SBS acts similarly on ozone, with a corresponding mass ratio of approximately 2.2/1.0.
Note: Excessive chemical addition during pure water preparation must be strictly avoided. Overdosing may introduce new contaminants, posing systemic risks. Excessive SBS addition can serve as a microbial nutrient source, increasing the risk of microbial contamination.

C. Scale Inhibitors (Dispersants)
Scale inhibitors not only prevent the crystallization, scaling, and precipitation of insoluble salts but also disperse insoluble salts, metal oxides, hydroxides, silicate polymers, colloidal substances, and biological contaminants. Most scale inhibitors are specialized synthetic organic polymers (e.g., polyacrylic acid, carboxylic acids, polymaleic acid, organometallic phosphates, polyphosphonates, phosphonates, anionic polymers, etc.), with molecular weights ranging from 2,000 to 10,000 daltons. Scale Inhibition Mechanism
The formation process of inorganic scale can be divided into three steps:
① Formation of a supersaturated solution; ② Generation of crystal nuclei; ③ Growth of the nuclei into crystals. If any of these three steps is disrupted, the scaling process is slowed or inhibited. The function of scale inhibitors is to effectively prevent one or more of these steps, thereby achieving scale inhibition. There are several theories about the mechanism by which scale inhibitors interfere with crystal growth. 1. Chelation and solubilization. Chelation and solubilization refers to the complexation of scale inhibitors with high-valent metal ions such as Ca2+, Mg2+, Sr2+, and Ba2+ in water to form stable, water-soluble chelates, which reduces the concentration of free calcium and magnesium ions in the water. This increases the solubility of substances such as CaCO3, preventing precipitation of substances that would otherwise precipitate out of the solution. Threshold effect scale inhibition refers to the ability of a small amount of scale inhibitor to stabilize a large number of scaling ions in solution. There is no strict stoichiometric relationship between these ions. When the amount of scale inhibitor is increased excessively, its stabilization and inhibition effect are not significantly improved. 2. Lattice Distortion: The normal process of crystal formation involves the highly regular arrangement of microparticles (ions, atoms, or molecules) according to a specific lattice pattern, resulting in a dense, solid structure with a regular shape, a fixed melting point, and a uniform appearance. Crystal distortion refers to the fact that during crystal growth, external factors often cause defects such as vacancies and dislocations, or distortions such as mosaic structures. This results in unequal development of the individual crystal faces within the same crystal. These local compositional differences within the crystal induce internal stresses, as do differences in the descaling coefficients between the crystal itself and the embedded material. These stresses destabilize the crystal. When the environment undergoes certain changes, large crystals break into smaller ones. Scale inhibitor molecules, adsorbed on the lattice points located at the active growth points of the crystal, disrupt the normal growth of the crystal according to the lattice arrangement, causing crystal distortion and increased internal stress, leading to crystal fracture. This prevents the deposition of microcrystals and achieves the purpose of scale inhibition. 3. Adsorption and Dispersion: Scale inhibitors and dispersants are anionic organic compounds that can be adsorbed onto colloidal particles and microcrystalline particles through physical and chemical adsorption, forming a new electrical double layer on the particle surface and altering the original surface charge. Consequently, like charges repel each other, allowing them to be stably dispersed in the water column. The appropriate addition of scale inhibitors prevents the growth of insoluble salts in the feed and concentrate of reverse osmosis (RO) systems, improving the system's recovery rate and reducing the risk of scaling during stable operation. The design of the antiscalant/dispersant injection system should ensure thorough mixing before entering the RO system. Most systems place the dosing point before the RO inlet safety filter, promoting mixing through buffering time in the filter and agitation by the RO high-pressure pump. If the system uses acid addition for pH adjustment, it is recommended that the acid addition point be sufficiently upstream to ensure thorough mixing before reaching the antiscalant/dispersant injection point. The antiscalant/dispersant dosing pump should be operated at a high frequency, with a recommended frequency of at least every five seconds. The typical dosage of scale inhibitors/dispersants is 2-6 ppm, with 3 ppm generally used. The agent can be diluted based on actual site conditions. Diluted scale inhibitors/dispersants can become bio-contaminated in the storage tank, so the dilution should be retained for approximately 7-10 days. Under normal circumstances, undiluted scale inhibitors/dispersants are not susceptible to bio-contamination.

D. pH Adjustment (Acid-Base Adjustment)
The concept of pH adjustment in pure water preparation is very broad, and it should be added whenever necessary. Its common uses include the following:
① pH adjustment is necessary to facilitate coagulation and sedimentation, and to improve the operation of processes such as carbon removal tower degassing.
② Maintaining the overall quality of the raw water within a neutral to slightly alkaline range facilitates the smooth operation of the primary reverse osmosis process and increases recovery rates. ③ In multi-stage reverse osmosis systems, the pH of the effluent from the front stage needs to be adjusted to a slightly alkaline state before entering the downstream equipment. RO membranes achieve the highest salt rejection rate at a pH of 7.5-7.8. When the output water from the primary RO unit (pH ≈ 6) enters the secondary RO unit directly, the salt rejection rate and water output decrease. Therefore, the pH needs to be adjusted to a slightly alkaline level before the secondary RO unit enters. ④ Appropriately increasing the pH also facilitates TOC removal, improving silica solubility and removal efficiency. Please refer to the article on sparingly soluble salt scaling.
(VI) Precision Filters/Safety Filters
Precision filters are often used as the final stage of pretreatment in pure water equipment (except for ultrafiltration), safeguarding the stable operation of the system's core processing equipment. They are commonly known as safety filters. Filters with a filtration accuracy of 0.1-50μm are considered precision filters, with safety filters typically using a filtration accuracy of 5μm. Safety filter elements commonly used are PP meltblown filter elements, bag filters, and microporous membrane filter elements. ① Bag filters: The filter bag is supported by a metal mesh basket inside the filter. The most significant advantage of bag filters over conventional precision filters is their larger effective filtration area, making them suitable for high-flow systems. Working Principle: Material flows from the bag filter inlet into the filter bag. Liquid larger than the bag's pore size is trapped on the inner surface or in the middle of the bag. The purified liquid then flows through the bag from the inside out and out of the bag's outlet. Bag filters come in several types: single-bag filters, multi-bag filters, rocker-arm bag filters, and high-precision bag filters. Filter filtration ranges from 0.5 to 200 μm, and the filtration rate can be adjusted by adjusting the pore size of the bag. These filters can be used for fine or coarse filtration. A 5 μm filtration rate is generally selected for pretreatment of pure water.

② PP Meltblown Filter Elements: PP filter elements, also known as meltblown filter elements, are made of heat-melted polypropylene microfibers. The fibers form a random three-dimensional microporous structure, with a gradient distribution of pore size along the filtrate flow direction. This filter element combines surface, deep, and fine filtration, intercepting impurities of varying particle sizes. Filter element filtration ranges from 0.5 to 100 μm, with 5 μm being the most common filtration option for pretreatment. Note: Another commonly used filter element is the pleated filter element, commonly used for EDI device protection and ultrapure water effluent filtration. Pleated filter elements utilize polypropylene hot-sprayed fiber membranes and nylon/polytetrafluoroethylene (PTFE) microporous membranes as filter media to create precision filtration devices. They offer advantages such as compact size, large filtration area, and high precision. Filtration accuracy ranges from 0.1μm to 60μm. For EDI device protection and ultrapure water effluent filtration, 0.45/1.0μm and 0.1/0.22μm are commonly used.
(7) Disc Filters: Disc filters, also known as stacked disc filters, utilize a modular design. When the filter is operating normally, water flows through the discs, where the disc walls and grooves collect and capture debris. The composite internal cross-section of the disc grooves provides three-dimensional filtration similar to that produced in a sand and gravel filter. This results in high filtration efficiency. Disc filters offer several advantages: Precise filtration: Filter discs of varying precision can be selected based on water requirements, including 5μm, 20μm, 55μm, 100μm, 130μm, 200μm, and 400μm, achieving a filtration ratio exceeding 85%. Thorough and efficient backwashing: Backwashing fully opens the filter pores, combined with centrifugal jets, achieving a cleaning effect unmatched by other filters. Each filter unit backwashes in just 10 to 20 seconds. Fully automatic operation and continuous water output: Backwash activation is controlled by time and pressure differential. Within the filter system, each filter unit and workstation backwashes sequentially. Automatic switching between operating and backwashing modes ensures continuous water output, minimizes system pressure drop, and maintains filtration and backwash performance over time. Modular design: Users can choose the number of filter units in parallel as needed, offering flexibility and interchangeability. This allows for flexible utilization of site space and allows for customized installation with minimal footprint.

In the pure water production process, the high flow rate characteristics of disc filters are often combined with ultrafiltration as a pretreatment process to replace traditional sand filtration.
(8) Ultrafiltration: Ultrafiltration is a pressure-driven membrane separation process. Through the micropores on the membrane surface, it can intercept particles and impurities with diameters between 0.001-0.02μm (1-20nm), effectively removing colloids, silica, proteins, microorganisms, and large organic molecules from water. When a liquid mixture flows through the membrane surface under a certain pressure, the solvent and small molecules pass through the membrane, while large molecules are retained, thus achieving the purpose of separation and purification of large and small molecules. The operating pressure is generally 0.1-0.5 MPa. The basic principle of ultrafiltration is that under a certain pressure, small solutes and solvents are forced through a specially designed membrane with a certain pore size, while large solutes are prevented from passing through and retained on one side of the membrane, thereby partially purifying the large molecules. Ultrafiltration is also a membrane separation process. It utilizes a pressure-activated membrane, which, under the influence of an external driving force (pressure), retains colloids, particles, and relatively high-molecular-weight substances in water, while allowing water and small solute particles to pass through the membrane. The membrane's micropores filter out substances with molecular weights between 3x10,000 and 1x10,000. When the treated water passes through the membrane at a constant flow rate, aided by external pressure, water molecules and solutes with molecular weights less than 300-500 pass through the membrane, while particles and macromolecules larger than the membrane pores are retained by the sieving action, resulting in purified water. In other words, after water passes through the ultrafiltration membrane, the majority of the colloidal silica in the water is removed, along with a significant amount of organic matter. In pure water production, ultrafiltration is often used as a pretreatment process before high-flow equipment, acting as a safety filter. In the semiconductor industry, ultrafiltration is also often used as the final effluent filtration step in ultrapure water production, replacing conventional microporous membrane filtration. (IX) Pretreatment Process Introduction Summary: Common pretreatment processes used in pure water production include coagulation, sand filtration, carbon filtration, softening, fine filtration, ultrafiltration, and disinfection. We typically select the necessary and appropriate processes sequentially based on the quality of the raw water. In most cases, these processes are implemented in tandem, generally following the principle of prioritizing low-precision filtration over high-precision filtration and low-cost filtration over high-cost filtration. Let's briefly review the functions of common pretreatment processes: Heat exchanger: Regulates water temperature. pH regulator: A type of dosing device that adjusts pH. Quartz sand filter: Intercepts and adsorbs sediment, colloids, metal ions, and organic matter, reducing water turbidity. Activated carbon filter: Adsorbs electrolyte ions and performs ion exchange adsorption, removing discoloration from water. It is a broad-spectrum adsorbent. Multi-media filter: Filled with two or more filter media, it effectively removes suspended impurities and clarifies water. Bag filter: A type of precision filter, often with a 5μm filter, acting as a safety filter. PP meltblown filter element: A type of precision filter, often with a 5μm filter, acting as a safety filter. Disc filter: A high-efficiency filter, typically with a 50/100μm filter, suitable for high-flow equipment. Ultrafiltration: A hollow fiber membrane filtration technology, typically with a 0.01μm filter, acts as a safety filter, but is relatively expensive. Reducing agent: A type of dosing device. Residual chlorine reducers eliminate residual chlorine. Flocculant: A type of dosing device. It enhances the flocculation and sedimentation of suspended solids and colloids. Bactericide: A type of dosing device. It reduces the impact of microorganisms on the system. Iron and manganese filter: Aeration, oxidation of iron and manganese ions, adsorption, and filtration remove iron and manganese ions. Dispersant: A type of dosing device. Silicon dispersants can reduce SiO2 scaling. Scale inhibitor: A type of dosing device. It reduces the risk of scaling in the system. Water softener: Ion exchange technology, effectively removing calcium and magnesium ions. Degassing tower (decarbonizer): Combined with a water softener, it effectively removes CO2. Flotation: Enhances solid-liquid and liquid-liquid separation in water, reducing turbidity and suspended solids.