How to Prevent Scale Formation in RO Systems

1. Causes of scaling

Within membrane elements, sparingly soluble salts are continuously concentrated. When the concentration exceeds their solubility limit, scaling occurs on the reverse osmosis or nanofiltration membrane surface. The higher the system recovery rate, the greater the risk of scaling. Currently, due to water shortages or environmental concerns, it has become a common practice to install reverse osmosis concentrate recovery systems to improve recovery rates. In this context, it is particularly important to adopt carefully designed and comprehensive scaling control measures and prevent sparingly soluble salts from exceeding their solubility limits, thus preventing precipitation and scaling. In RO/NF systems, common sparingly soluble salts include CaSO₄ _, CaCO₃ _, and SiO₂ _. Other compounds that may cause scaling include CaF₂ _, BaSO₄ _, SrSO₄ _, and Ca₃ ₍ PO₄ ₎₂ _. The table below lists the solubility product data for sparingly soluble inorganic salts.

Table 4.3 Solubility product constant of sparingly soluble inorganic salts (291–298 K)

2. Prevention of scaling

To prevent inorganic salt scaling on the membrane surface, the following measures should be taken:

• Add acid

The occurrence of scaling on the membrane surface follows an inherent pattern. Whether calcium carbonate precipitate will form can be predicted using the Langerile index (LSI).

LSI = ph – phs (TDS < 4000 mg/L)

Where: ph — the actual ph value of the aqueous solution

phs — ph value of an aqueous solution when calcium carbonate is saturated.

Generally, a system with an LSI greater than 0 is considered to have a tendency to scale, while a system with an LSI less than 0 is not. Since the ph of the concentrate is usually higher than that of water in other locations within a typical system, the ph of the concentrate side is generally considered first.

When the TDS content in the water is too high, the Davis Index (SDSI) is needed for prediction.

SDSI=ph－pCa－pAlk－K (TDS＞4000mg/L)

Where: K – empirical coefficient

pCa — the negative logarithm of calcium concentration

pAlk — the negative logarithm of alkalinity

_CaCO3 in water depends on the ph value:

Ca²⁺ + HCO₃ ^– ↔ H ⁺ + CaCO₃

+ ions from the acid , the chemical equilibrium can shift to the left, keeping calcium carbonate in a dissolved state. In most regions, sulfuric acid is easier to use than hydrochloric acid; however, on the other hand, the increased sulfate content in the influent can actually exacerbate scaling caused by certain compounds.

• Add scale inhibitor

Adding scale inhibitors can control carbonate scale, sulfate scale, calcium fluoride scale, etc. There are generally three types of scale inhibitors: sodium hexametaphosphate, organophosphates, and polyacrylates. For the dosage of scale inhibitor, please refer to the scale inhibitor supplier’s recommendations. Over-addition or improper addition must be avoided, as excessive scale inhibitor can also contaminate the reverse osmosis membrane. For example, cationic polymers may undergo a synergistic precipitation reaction with negatively charged scale inhibitors and contaminate the membrane surface.

Table 4.4 Comparison of the performance of commonly used scale inhibitors

In seawater reverse osmosis systems, scaling is not as prominent as in brackish water, but when the recovery rate is higher than 35%, it is recommended to use scale inhibitors for safety reasons.

• Ion exchange

An ion exchange system can be used to replace and remove scale-forming cations such as Ca²⁺, Ba²⁺, and Sr²⁺ from water with Na⁺ ions ^, while ^the ion ^{exchange resin} is regenerated with NaCl after exchange saturation. This process is called water softening treatment. During the treatment, the ph of the influent does not change, so degassing is not required.

Some highly efficient ion exchange resins can remove Ca²⁺ ^, Ba²⁺ ^, and Sr²⁺ ^with an efficiency greater than 99.5%, eliminating the danger of various carbonate and sulfate scales. Softening with strong acid cation exchange resins is a very effective and reliable method for scale inhibition, primarily used in small to medium-sized brackish water systems.

^scale brackish water treatment systems typically employ weak acid cation exchange resins for alkalinity reduction, achieving partial softening to conserve reagents. During the treatment process, some Ca²⁺ ^, Ba²⁺ ^, and Sr²⁺ ^{are replaced and removed} by H⁺ , lowering the ph of the raw water to 4-5.

• Lime softening

The lime softening method removes carbonate hardness by adding calcium hydroxide to water.

Ca(HCO₃ )₂ + Ca(OH)₂ → 2CaCO₃ + 2H₂O

Mg(HCO₃ )₂ + 2 Ca(OH)₂ → Mg(OH)₂ + 2CaCO₃ + 2H₂O

The non-calcium carbonate content can be further reduced by adding sodium carbonate (soda ash).

CaCl₂ + Na₂CO₃ → 2NaCl + CaCO₃

Lime-soda treatment can also reduce the concentration of silica. When sodium aluminate and ferric chloride are added, CaCO3 _and a complex of silicic acid, aluminum oxide and iron will be formed. When lime and magnesium oxide are added, a high-temperature lime desilicification process can be used to significantly reduce the concentration of silicic acid.