1. Introduction

In power plant boiler makeup water, cooling tower blowdown reuse, and industrial wastewater advanced treatment, reverse osmosis (RO) has become a core desalination unit. However, membrane scaling is one of the most common causes of reduced water production, increased operating costs, and even premature membrane replacement. Statistics show that more than 60% of RO system performance decline is related to inorganic scaling. Once scale forms, at a minimum it increases cleaning frequency; at worst it causes irreversible flux loss.

To achieve long‑term, low‑energy, high‑recovery operation of an RO system, it is essential first to understand the true nature of several major scale types and then deploy targeted prevention strategies. 

2. Three Common Scale Types: Characteristics and Scaling Tendency

2.1 Calcium Carbonate Scale – The Most Common “Rapid‑Forming” Scale

Calcium carbonate (CaCO₃) is the most common sparingly soluble salt in surface water and groundwater. Its scaling tendency is closely related to water alkalinity, hardness, and pH. CaCO₃ exhibits inverse solubility – its solubility decreases with increasing temperature or pH, making it very prone to supersaturation in the concentrate stream and at the membrane surface.

Key characteristics:

• Solubility product (Ksp) ≈ 3.36×10⁻⁹ (25°C), relatively easy to control.
• Rapid scaling rate; initial signs include a slight rise in inter‑stage pressure drop and a slow decline in permeate flux.
• Prevention advantage: by adding acid to adjust pH to 6.0–6.5, bicarbonate is converted to CO₂, effectively inhibiting scale formation. For this reason it is often called the “easiest scale to handle”.

2.2 Calcium Sulfate Scale – The Most Stubborn “Hard‑to‑Remove” Scale

Calcium sulfate (CaSO₄, mainly gypsum dihydrate) is one of the most troublesome inorganic scales in RO systems. Unlike calcium carbonate, calcium sulfate remains stable under acidic conditions and cannot be prevented by simple acid addition. Its crystals are hard, dense, and strongly adherent to the membrane surface. Once formed, conventional alkaline or acidic cleaning (pH 2–3) is rarely able to remove it completely, often leading to irreversible flux loss.

According to the author’s statistics on high‑hardness, high‑sulfate water, calcium sulfate scaling can reduce membrane flux by more than 30%, increase cleaning costs by 20%–50%, and shorten membrane life by 1–3 years. Its scaling mechanism follows a two‑step “nucleation – crystal growth” process. Concentration polarization at the membrane surface exacerbates local supersaturation, accelerating the scaling rate by 3–5 times. Detailed methods for scaling tendency calculation, the allowable saturation limit for antiscalants (typically ≤400% of Ksp), and the alkaline STPP+EDTA cleaning strategy can be found in the article “Scaling of Calcium Sulfate in Reverse Osmosis Systems: Calculation, Prevention, and Cleaning Strategies”, and will not be repeated here. Key conclusion: calcium sulfate requires “calculation first, combined prevention, and timely cleaning”.

2.3 Silica Scale – The Most Insidious “Difficult‑to‑Treat” Scale

Silica scale mainly refers to amorphous silica (SiO₂) or metal silicates. Its special features are:

• Low solubility with limited pH dependence; conventional pH adjustment is ineffective.
• Once polymerized (colloidal silica) or deposited, conventional acid/alkali cleaning is almost useless; often aggressive fluoride‑based cleaners or high‑temperature strong caustic soaking are required.
• Silica scale often forms complex silicates with iron, aluminum, magnesium, etc., making it even denser and more difficult to remove than calcium sulfate scale.

Scaling tendency judgment: When the silica concentration in the concentrate exceeds 150–200 mg/L (depending on temperature, pH, and coexisting ions), the risk of polymerization and deposition increases significantly. In power plant cooling tower blowdown reuse projects, because the makeup water undergoes multiple concentration cycles, silica levels can easily exceed the limit.

3. Core Roles and Boundaries of Three Major Prevention Strategies

3.1 Antiscalants – The Most Common “On‑Line Safeguard”

Antiscalants work through threshold effects (inhibiting nucleation at low concentrations), crystal distortion (interfering with normal crystal growth), and dispersion (preventing micro‑crystal agglomeration and deposition), allowing RO systems to operate safely at supersaturation levels far exceeding the solubility product of sparingly soluble salts.

• For calcium carbonate: antiscalants combined with acid can extend the allowable Langelier Saturation Index (LSI) to +1.5 to +2.0.
• For calcium sulfate: high‑quality antiscalants allow the concentrate calcium sulfate saturation (IP/Ksp) to reach 4–6 times (i.e., 400%–600%), but this has an upper limit. As shown in the calculation example of the previously mentioned article, when the IP at 75% recovery reaches 13.9 times Ksp, antiscalants alone can no longer prevent scaling.
• For silica scale: specialty silica antiscalants can raise the allowable concentrate SiO₂ concentration from 150 mg/L to 300–400 mg/L, but this still requires a reasonable recovery design.

Limitations and risks: Antiscalant effectiveness depends on proper product selection, continuous dosing (typically 2–6 mg/L), and thorough mixing. Over‑reliance on antiscalants while ignoring recovery control can lead to local supersaturation at the membrane surface exceeding the chemical threshold, resulting in sudden scaling.

3.2 Softening Pretreatment – Removing the “Culprits at Source”

Softening directly reduces the concentration of scale‑forming ions in the feed water, making it a fundamental measure, especially for high‑hardness, high‑sulfate, or high‑silica waters.

• Ion exchange softening: replaces Ca²⁺ and Mg²⁺ with Na⁺. It can completely eliminate calcium carbonate and calcium sulfate scaling risks, but is ineffective against silica. Suitable for low‑TDS, low‑turbidity freshwater feeds.
• Chemical softening (lime‑soda ash process) : adding Ca(OH)₂ and Na₂CO₃ precipitates Ca²⁺ as CaCO₃ and Mg²⁺ as Mg(OH)₂, while also partially co‑precipitating silica (as magnesium silicate or aluminum silicate). This method has relatively low chemical costs but produces large amounts of chemical sludge. It is suitable for large power plants using a clarifier + filtration as RO pretreatment.
• Auxiliary silica removal: for silica scale, magnesium hydroxide adsorption co‑precipitation (raising pH above 10 and adding MgO or MgCl₂) or adding aluminate to form aluminum silicate precipitate can be used.

Selection principle: When feed total hardness > 300 mg/L (as CaCO₃) or sulfate > 400 mg/L, and the design recovery > 70%, softening pretreatment is recommended. Otherwise, relying solely on antiscalants and reduced recovery may not ensure long‑term stable operation.

3.3 Recovery Control – The Most Direct “Concentration Reduction” Measure

Recovery (R = permeate flow / feed flow) directly determines the concentration factor (CF = 1/(1‑R)) on the concentrate side. The higher the recovery, the higher the ion concentration at the membrane surface, and the greater the driving force for scaling.

Cost of reducing recovery: lowering recovery means increased concentrate discharge and reduced system water efficiency. For water‑scarce regions or where concentrate volume reduction is required, recovery cannot be reduced without limit. The usual approach is: set a target recovery, calculate the ratio of concentrate ion product to Ksp, and if it exceeds the maximum allowable saturation for the antiscalant (e.g., 400% for calcium sulfate), then recovery must be reduced or softening pretreatment added.

Best practice: during the design phase, perform scaling simulations based on the worst‑case water quality. For calcium carbonate, use LSI or Stiff‑Davis index; for calcium sulfate, use the ion product (IP) method and compare with the corrected Ksp; for silica, limit concentrate SiO₂ ≤ 150 mg/L (without antiscalant) or ≤ 350 mg/L (with specialty antiscalant). During operation, measure feed conductivity, hardness, sulfate, and silica daily, and adjust the recovery setpoint dynamically.

4. Integrated Prevention Strategy Framework

For typical power plant RO systems (e.g., cooling tower blowdown reuse, boiler makeup water), a multi‑barrier prevention approach is recommended:

1. First barrier – Pretreatment softening: when feed hardness > 300 mg/L or sulfate > 400 mg/L, prioritize lime‑soda ash softening + clarifier. If silica exceeds the limit (feed SiO₂ > 50 mg/L), add a magnesium‑based silica removal step.
2. Second barrier – Acid / Antiscalant dosing: after softening, if residual hardness remains, dose antiscalant combined with acid to adjust pH to 6.5–7.0 (to control calcium carbonate). For calcium sulfate and silica, select a composite antiscalant that provides both dispersion and crystal distortion; maintain the dosage at the upper end of the manufacturer’s recommendation.
3. Third barrier – Dynamic recovery management: set maximum recovery alarm limits. For example, when feed hardness > 200 mg/L or sulfate > 300 mg/L, limit recovery to ≤65%; when feed SiO₂ > 30 mg/L, keep recovery ≤70%. Install on‑line conductivity, pH, and ORP meters, and manually measure concentrate ion composition weekly to validate calculations.
4. Monitoring and early warning: establish baseline trends for normalized permeate flow, inter‑stage pressure drop, and salt rejection. When inter‑stage pressure drop increases by 15% or permeate flow decreases by 10%, immediately investigate whether scaling is occurring. If calcium sulfate or silica scale is confirmed, perform a targeted cleaning within 7 days to avoid irreversible damage.

5. Conclusion

Preventing RO membrane scaling is not a “one‑man show” by a single chemical; it is a systematic engineering task that integrates water quality analysis, process selection, operating parameters, and on‑line monitoring. Calcium carbonate is relatively mild and can be handled by acid or conventional antiscalants. Calcium sulfate is hard and stubborn, requiring precise calculation and combined strategies. Silica scale is insidious and difficult to remove, and the emphasis must be on pretreatment silica reduction and conservative recovery control.

For power plant technical personnel, scaling tendency calculations should be a basic task during RO system commissioning and operation. Adjust antiscalant dosage and recovery setpoints dynamically according to seasonal changes in feed water quality. Only by organically combining softening, antiscalants, and recovery reduction can an RO system achieve efficient, long‑term, scale‑free operation.

Finally, a reminder: once calcium sulfate or silica scale is detected, do not blindly perform intense acid cleaning (which may aggravate silica polymerization). Instead, give priority to alkaline chelating cleaning (STPP+EDTA) or a professional silica‑scale cleaner. After cleaning, thoroughly identify the root cause of scaling and adjust the preventive measures to avoid recurrent scaling.