In the fields of industrial water treatment and domestic pure water preparation, reverse osmosis (RO) systems have become core equipment for achieving deep water purification due to their highly efficient impurity retention capabilities. However, the stable operation of RO membranes is highly dependent on the precise dosing of various functional chemicals, where the dosing positions directly determine treatment effectiveness and system safety. This article systematically reviews the dosing positions of major chemicals in RO systems and focuses on analyzing the compatibility characteristics between coagulants and anti-scalants, providing technical reference for engineering practice.
I. Core Chemicals and Dosing Position Distribution in RO Systems
Chemical dosing in RO systems must follow the principle of "staged treatment with targeted action." Based on raw water quality characteristics and treatment unit functions, different chemicals are precisely dosed at corresponding process nodes to achieve core objectives including pretreatment enhancement, membrane fouling prevention, and system corrosion inhibition. Currently, common chemicals and their typical dosing positions in mainstream RO systems are as follows:
(I) Pretreatment Stage: Coagulants and Coagulant Aids
The core task of the pretreatment stage is to remove suspended particles, colloidal impurities, and some organic matter from raw water, providing qualified feed water quality for subsequent RO membranes and avoiding physical blockage of membrane surfaces. The most critical chemical in this stage is the coagulant, whose dosing position is typically set upstream of multi-media filters (or quartz sand filters), specifically at the raw water pump outlet pipeline or the inlet of the pretreatment tank.
The mechanism of coagulants involves adsorption and bridging effects, causing dispersed microparticles in raw water to aggregate into larger flocs (commonly called "alum flowers"), which can be retained and removed in subsequent multi-media filters. If raw water turbidity is high or colloid stability is strong, coagulant aids (such as polyacrylamide) can be added. Coagulant aids are typically dosed in the pipeline after coagulants but before multi-media filters, further promoting floc growth and densification to improve filtration efficiency.
(II) Pre-membrane Protection Stage: Anti-scalants and Biocides
Before pretreated water enters the RO membrane system, dual protection of "scale prevention" and "bacterial inhibition" must be achieved through chemical dosing. The core chemicals in this stage are anti-scalants and biocides, both positioned around security filters (also called cartridge filters). Security filters serve as the "last line of defense" for RO membranes, intercepting fine impurities generated by chemical reactions or undissolved chemical particles to prevent membrane element damage.
Anti-scalants: Dosing position is fixed upstream of security filters, typically 1-2 meters from the security filter inlet in the pipeline. This design allows sufficient time for anti-scalants to fully mix with scale-forming ions like calcium, magnesium, barium, and strontium in water before entering security filters. Through mechanisms like "crystal lattice distortion" and "complexation solubilization," they inhibit the growth and deposition of scale crystals, preventing scale formation on RO membrane surfaces. If anti-scalant dosing is too late (after security filters), insufficient mixing with scale-forming ions will fail to achieve effective scale prevention; if dosed too early (before multi-media filters), it may react with pretreatment coagulants or be retained by multi-media filters, causing chemical waste.
Biocides: Dosing positions must be adjusted according to biocide type and system operation mode. For oxidizing biocides (such as sodium hypochlorite), dosing is typically before multi-media filters or at raw water pump outlets to kill microorganisms in raw water during pretreatment, preventing microbial growth and biofilm formation in multi-media filters. For non-oxidizing biocides (such as isothiazolinone), dosing should be upstream of security filters (offset from anti-scalant dosing points by approximately 0.5-1 meter) to kill residual microorganisms after pretreatment and prevent biofilm formation on RO membrane surfaces. Note that when using oxidizing biocides, reducing agents (such as sodium bisulfite) must be dosed after security filters but before RO membranes to remove residual oxidizing substances in water, preventing oxidative damage to RO membranes (especially composite membranes). Reducing agent dosing positions are typically in pipelines between security filter outlets and RO membrane inlets.
(III) System Maintenance Stage: Cleaning Agents and Reducing Agents
In addition to routine operational chemicals, RO systems require periodic dosing of cleaning agents and reducing agents for maintenance. Cleaning agents (divided into acidic and alkaline cleaning agents) are not dosed at fixed pipeline positions but injected through "offline cleaning" or "online cleaning" modes: During offline cleaning, cleaning agents are prepared in dedicated cleaning tanks and injected into RO membrane assemblies through cleaning pumps; during online cleaning, cleaning agents are injected through system-reserved "cleaning chemical dosing ports" (typically located between security filter outlets and RO membrane inlets). Besides neutralizing oxidizing biocides, reducing agents can also be dosed through pretreatment tanks or cleaning tanks during system shutdown maintenance to remove residual oxidizing substances in the system and protect equipment materials.
II. Compatibility Characteristics of Coagulants and Anti-scalants: Why "Stock Solutions Don't Mix, but Dilute Solutions Can Coexist"
One of the most concerning questions for users of RO system chemicals is "whether coagulants and anti-scalants will react with each other." From a chemical perspective, if stock solutions of coagulants (mostly cationic or anionic polymers such as polyaluminum chloride and polyferric sulfate) and anti-scalants (mostly organic phosphonates or polycarboxylic acid polymers) are directly mixed, there are indeed clear reaction risks — cationic coagulants and anionic anti-scalants will undergo "charge neutralization" reactions due to opposite charges, forming water-insoluble flocculent precipitates; even chemicals of the same charge type may cause chemical deactivation due to intermolecular interactions when stock solution concentrations are extremely high (typically 10%-30%).
However, in actual RO system operation, not only do they not interfere with each other, but they can also work synergistically. The core reasons lie in "low-concentration dilution" and "dosing point isolation" — two key design features:
(I) Concentration Differences: Essential Change from "High-Concentration Stock Solutions" to "Low-Concentration Dilute Solutions"
Both coagulants and anti-scalants are dosed in "ppm-level" quantities (parts per million) in systems. For example, coagulant dosing is typically 1-5ppm, and anti-scalant dosing is typically 3-8ppm. Taking a system with raw water flow of 100m³/h as an example, if coagulant stock solution concentration is 10% (100,000ppm), the required stock solution volume per hour is only 0.1-0.5L, which is rapidly diluted to working concentrations of 1-5ppm when entering raw water; similarly, anti-scalant stock solution (concentration typically 20%, or 200,000ppm) requires only 0.15-0.4L per hour, diluting to concentrations of 3-8ppm.
At such low working concentrations, chemical molecules are extremely dispersed in water, greatly reducing intermolecular collision probability. Moreover, coagulants have already completed the "coagulation-filtration" process (retained in multi-media filters) by the time anti-scalant dosing begins, leaving extremely low residual coagulant concentrations in water (typically < 0.1ppm), far insufficient to cause significant reactions with anti-scalants. Even if trace coagulant residues enter the pre-membrane system, both chemicals are within "safe compatibility ranges" and will not form retainable flocs or block RO membranes.
(II) Position Isolation: Dosing Point Spacing Ensures "Temporal and Spatial Separation"
As mentioned earlier, coagulants are dosed before multi-media filters, and anti-scalants are dosed before security filters, with the critical isolation unit of "multi-media filters" existing between them in the process flow path. From the water flow perspective, water dosed with coagulants must pass through multi-media filter filtration (retaining flocs and most coagulants) and pipeline transport (further dilution) before reaching anti-scalant dosing points. The dosing points are typically 10-20 meters apart physically (depending on system layout), with water residence time in this pathway of approximately 1-3 minutes, sufficient for complete chemical dilution and separation.
This process sequence of "coagulation and filtration first, then scale prevention protection" fundamentally avoids direct contact between high-concentration chemicals. Even in extreme cases (such as decreased multi-media filter retention efficiency with trace coagulant residues), residual coagulants have been diluted to extremely low concentrations and will not cause significant chemical reactions or form membrane-blocking flocs when encountering subsequently dosed low-concentration anti-scalants.
III. Safety Control Points for Chemical Dosing
Although coagulants and anti-scalants can safely coexist in actual systems, the following safety control points must be strictly observed to ensure stable RO system operation:
Strictly separate dosing points and prohibit stock solution mixing: Regardless of system layout, coagulant and anti-scalant dosing points must be clearly isolated. Mixing stock solutions of both chemicals in the same dosing tank is strictly prohibited, as is directly connecting dosing pipelines of both chemicals to the same node. If dosing point adjustments are needed, ensure separation of at least 5 meters in the water flow path with filtration or dilution units in between.
Control chemical concentrations and dosing rates: Strictly determine chemical dosing rates according to raw water quality reports and membrane manufacturer recommendations, avoiding residual chemicals due to overdosing (excessive coagulant may increase membrane fouling risk, excessive anti-scalant may cause chemical deposition on membrane surfaces). Chemical dilution should follow the "dilute as needed" principle, typically with coagulant dilution concentrations of 0.1%-1% and anti-scalant dilution concentrations of 0.5%-2%. Excessive dilution concentrations may cause decreased chemical stability, while insufficient concentrations may cause inaccurate dosing due to metering pump precision issues.
Regular monitoring and troubleshooting: During operation, regularly monitor multi-media filter outlet turbidity (ensure coagulation effectiveness and avoid floc residues), security filter inlet/outlet pressure differentials (check for particle blockage from chemical reactions), and RO membrane inlet/outlet pressures and water production (determine scaling or fouling trends). If abnormal pressure differential increases are found in security filters, promptly check coagulant and anti-scalant dosing rates, dilution concentrations, and dosing positions to eliminate possibilities of chemical reactions.
Select compatible chemical types: In chemical selection, prioritize "compatible chemicals" certified by membrane manufacturers. For example, when using cationic coagulants, select anionic or non-ionic anti-scalants to avoid charge conflicts. Simultaneously, provide complete system process parameters and raw water quality reports to chemical suppliers to ensure chemical selection meets system requirements.
IV. Conclusion
Chemical dosing in RO systems is technical work where "details determine success or failure." From coagulant "coagulation and precipitation" before multi-media filters to anti-scalant "scale prevention protection" before security filters, each chemical's dosing position must precisely match process unit functions. Although coagulant and anti-scalant stock solutions pose reaction risks, through "low-concentration dilution" and "dosing point isolation" design, both can safely coexist and work synergistically in systems. In actual operation, strict control of chemical dosing rates and concentrations, along with regular monitoring of system operating parameters, is necessary to fully utilize chemical functions, ensure long-term stable RO membrane operation, extend membrane service life, and reduce system operating costs.
