How to Prevent Fouling and Scaling in Reverse Osmosis Systems? Understanding the Impact of Concentration Polarization

Concentration Polarization (CP)

Concentration polarization refers to the adverse effects caused by the continuous accumulation of solutes on the membrane surface, which impairs membrane performance. As water permeates through the membrane, the feed solution (containing water and solutes) is transported to the membrane surface. When purified water passes through the membrane, solutes accumulate near the membrane surface. ① In membrane filtration, particles contact the membrane and form a filter cake layer. ② Due to the distinct removal mechanism of reverse osmosis (RO), solutes in the solution form a high-concentration boundary layer on the membrane surface. This results in concentration polarization, making the solute concentration at the membrane surface higher than that in the bulk solution within the feed channel.

Adverse Effects of Concentration Polarization on RO Performance
① The high solute concentration at the membrane surface increases the osmotic pressure gradient, reducing water flux.
② Elevated concentration gradients and reduced water flux enhance solute mass transfer across the membrane, lowering rejection rates.
③ Solubility limits of solutes may be exceeded, leading to precipitation and scaling.

Fouling and Scaling in Reverse Osmosis

Nanofiltration (NF) and RO membranes are susceptible to fouling through various mechanisms. Primary sources of fouling and scaling include particulate matter, precipitation of insoluble inorganic salts, oxidation of soluble metals, and biological substances.

1.Particulate Fouling

RO operation cycles do not include backwashing to remove accumulated particulate matter (in fact, backwashing may cause delamination of the active layer from the support layer in thin-film composite membranes). Particulate fouling is a major concern in RO systems. Nearly all RO systems require pretreatment to minimize particulate fouling, as residual particles impair cleaning efficiency.

Inorganic and organic substances, including microbial components and biological debris, can cause particulate fouling, leading to blockage and filter cake formation. Blockage occurs when large particles in the feed solution are trapped in the feed channels and piping. Pretreatment of the feed solution using pre-filtration can reduce blockage. RO membrane manufacturers recommend using 5μm cartridge filters as a minimum pretreatment step to protect membrane modules.

Particulate matter forms a filter cake layer on the membrane surface, increasing hydraulic resistance and affecting system performance. Feed water prone to particulate fouling requires advanced pretreatment to reduce particulate concentrations to acceptable levels. Coagulation, filtration (using sand, carbon, or other media), and sometimes microfiltration (MF) or ultrafiltration (UF) are employed as pretreatment methods.

                                               

2.Precipitation and Scaling of Inorganic Salts
Inorganic scaling occurs when salts in the solution exceed their solubility limits and precipitate. Precipitation happens when ions constituting these salts are concentrated beyond their solubility products, particularly in high-concentration areas near the membrane surface, exacerbating concentration polarization. Inorganic scaling on the membrane surface reduces water permeability or causes irreversible membrane damage.

In the absence of pretreatment, precipitation must be avoided by minimizing concentration polarization, limiting the salt rejection rate, or recovery rate. Concentration polarization can be reduced by enhancing turbulent flow in the feed channels and maintaining minimum flow velocities specified by equipment manufacturers. Limiting salt rejection rates is impractical due to conflicting engineering goals, but restricting recovery rates is often necessary to prevent precipitation. The maximum allowable recovery rate before salt precipitation occurs is defined as the permissible recovery rate, with the salt initiating precipitation termed the "critical salt." Common scales in water treatment applications include calcium carbonate (CaCO₃) and calcium sulfate (CaSO₄).

Pretreatment is essential for all practical RO systems to prevent scaling from sparingly soluble salts. Calcium carbonate precipitation is prevalent, so most systems require pretreatment for this compound. Acidification of the feed solution to adjust pH converts carbonate ions into bicarbonate and carbon dioxide, preventing CaCO₃ precipitation. Sulfuric and hydrochloric acids are commonly used, though sulfuric acid may increase sulfate concentrations, leading to sulfate scaling. Most RO feed solutions are adjusted to pH 5.5–6.0, where most carbonates exist as CO₂ and permeate through the membrane.

Scaling of other critical salts is typically prevented using scale inhibitors. These inhibitors prevent crystal formation and growth, suppressing precipitation even under supersaturated conditions. The allowable degree of supersaturation depends on the inhibitor's properties, often proprietary and specific to equipment configurations. Selection of appropriate inhibitors should follow equipment and inhibitor manufacturer recommendations, with site-specific feed water analysis and recovery rate design.

Beyond acidification and inhibitors, modern installations incorporate measures to reduce concentrate wastewater volumes and enhance water recovery, further mitigating scaling.

3. Metal Oxide Fouling
Groundwater, a common RO/NF feed source, is often anaerobic. Dissolved iron and manganese compounds are oxidized and precipitate when oxidants enter the feed solution, fouling membranes. Iron fouling is more frequent and occurs rapidly upon air ingress. Oxidation or removal of oxidized iron/manganese can prevent fouling. For low iron concentrations, preventing air ingress suffices; scale inhibitors often include additives to mitigate low-concentration iron fouling. Iron pretreatment involves oxidation with oxygen or chlorine, followed by mixing, adequate hydraulic retention time, and oxidation-filtration in granular media or membrane filters. When using oxidants, contact with membranes—especially polyamide or oxidation-sensitive materials—must be avoided. Commercial cleaners and cleaning protocols can remove iron deposits from RO membranes.

Another component in anaerobic groundwater is hydrogen sulfide (H₂S). Air ingress oxidizes H₂S to colloidal sulfur, fouling membranes. As with iron oxidation, preventing air ingress is critical to avoid sulfur fouling. Sulfur deposits on membranes are often irreversible.

4. Biological Fouling
Biological fouling refers to the attachment or growth of microorganisms or extracellular soluble substances on the membrane surface or within feed channels. Common in RO systems, it degrades performance by reducing flux, lowering rejection rates, increasing pressure drop across modules, contaminating permeate, degrading membrane materials, and shortening membrane lifespan.

Biological fouling can be prevented by maintaining optimal operating conditions, applying biocides, and periodically flushing idle membrane modules. Many RO/NF feed solutions (typically groundwater) have low microbial loads. Proper operation ensures shear forces in feed channels prevent excessive bacterial accumulation. However, microbes proliferate rapidly during idle periods. To mitigate this, periodic flushing with permeate or adding biocides is necessary during shutdowns. Chlorine solutions within recommended limits serve as biocides for cellulose acetate membranes, but polyamide membranes—susceptible to chlorine degradation—require alternatives like sodium bisulfite.

For cellulose acetate membranes, continuous chlorination at controlled concentrations may. For polyamide membranes, ultraviolet irradiation, chloramination, or post-chlorination dechlorination may be employed.

                                       

Conclusion
Pretreatment is critical to preventing scaling and fouling. Common methods include acidification and scale inhibitors to prevent salt precipitation and filtration to block particulate matter. Clean feed water sources (e.g., groundwater) may only require cartridge filtration before membrane units, while surface water intakes necessitate advanced filtration methods, including coagulation, flocculation, sedimentation, and granular or membrane filtration. Since membrane performance depends on pretreatment efficacy, proper selection and design of pretreatment trains are essential.