Why is scale inhibitor added to reverse osmosis equipment?

Reverse osmosis membranes are key components of reverse osmosis systems. During prolonged continuous operation, ions such as calcium and magnesium in the water continuously precipitate and adhere to the surface of the reverse osmosis membrane, forming scale that blocks the membrane pores. This affects the water output efficiency of the reverse osmosis system and damages the reverse osmosis membrane. Because reverse osmosis membranes are expensive, a chemical dosing system should be added during system operation to add reverse osmosis antiscalants to the water, delaying the precipitation of calcium and magnesium ions and membrane scaling.

1. Characteristics of Reverse Osmosis Antiscalants:

Effective control of inorganic scaling over a wide concentration range.

Does not condense with iron and aluminum oxides and silicon compounds to form insoluble substances.

Effectively inhibits the polymerization and deposition of silicon; the SiO2 concentration on the concentrate side can reach 290 ppm.

Can be used for reverse osmosis CA and TFC membranes, nanofiltration membranes, and ultrafiltration membranes.

Excellent solubility and stability.

Effective within a water pH range of 5-10.

2. Basic Functions of Reverse Osmosis Antiscalants:

Complexation and solubilization: After dissolving in water, reverse osmosis antiscalants ionize to produce negatively charged molecular chains. These chains form water-soluble complexes or chelates with Ca2+, increasing the solubility of inorganic salts and providing an antiscaling effect.

Lattice distortion: Some functional groups in the reverse osmosis antiscalant molecules occupy certain positions on the inorganic salt nuclei or microcrystals, hindering and disrupting the normal growth of inorganic salt crystals, slowing down the crystal growth rate, and thus reducing the formation of salt scale.

Electrostatic repulsion: After dissolving in water, reverse osmosis antiscalants adsorb onto the microcrystals of inorganic salts, increasing the repulsive force between the particles, hindering their aggregation, and keeping them in a well-dispersed state, thus preventing or reducing the formation of scale.

Reverse osmosis antiscalants are used to improve the performance of reverse osmosis and nanofiltration systems.

Antiscalants and dispersants are a series of chemical agents used to prevent the precipitation and scaling of crystalline mineral salts.

3. Functions of Antiscalants:

Inhibition of precipitation: In systems with antiscalants, the ionic product at which the cations and anions of easily scaling components begin to precipitate is much larger than the critical precipitation ionic product without antiscalants.

Dispersion: With antiscalants, the precipitated particles are smaller and less likely to aggregate than those precipitated without antiscalants, making them harder to settle.

Lattice deformation effect: In systems with antiscalants, the precipitated crystals have amorphous states such as spherical, polyhedral, and snowflake shapes. It is generally believed that amorphous crystals are formed when antiscalants adsorb onto the crystal growth points during crystal growth, causing a sharp decrease in the growth rate of the surface and the growth of crystals with different shapes than the original.

Threshold effect: Even when the amount of antiscalant added is much lower than the amount of scaling components in the water, it can still show an antiscaling effect.

4. Applications of RO Antiscalants:

When using polyacrylic acid antiscalants, be especially careful as they may cause membrane fouling at higher iron concentrations. This fouling increases membrane operating pressure, and effective removal of this fouling requires acid washing.

If cationic coagulants or filter aids are used in pretreatment, special care should be taken when using anionic antiscalants, as they can produce a viscous, sticky pollutant that increases operating pressure and is very difficult to clean.

Antiscalants hinder the growth of salt crystals in RO feed water and concentrate, allowing sparingly soluble salts to exceed their saturation solubility in the concentrate. The use of antiscalants can replace or be used in conjunction with acid addition.

The ideal dosage and the high saturation of scaling substances and pollutants are best determined using specialized software packages provided by chemical suppliers. Excessive addition of antiscalants/dispersants can lead to the formation of deposits on the membrane surface, causing new fouling problems. Antiscalants and dispersants must be thoroughly flushed out when the equipment is shut down, otherwise they will remain on the membrane and cause fouling problems. Antiscalants and dispersants should be stopped from being injected into the system during low-pressure rinsing with RO feed water.

The design of the antiscalant/dispersant injection system should ensure thorough mixing before entering the reverse osmosis components. Static mixers are a very effective mixing method. Most systems have injection points upstream of the RO feed water security filter, promoting mixing through the buffer time in the filter and the stirring action of the RO feed water pump. If the system uses acid addition to adjust pH, it is recommended that the acid addition point be far enough upstream to ensure complete and uniform mixing before reaching the antiscalant/dispersant injection point.

The chemical injection pump for scale inhibitors/dispersants should be set to a higher injection rate; a suggested injection rate is at least once every 5 seconds. The typical dosage of scale inhibitors/dispersants is 2-5 ppm. To allow the chemical injection pump to operate at a higher frequency, dilution of the chemicals is required. Scale inhibitors/dispersants are commercially available as concentrates or solid powders. Diluted scale inhibitors/dispersants in storage tanks are subject to biological contamination, the extent of which depends on the ambient temperature and the dilution factor. The recommended retention time for diluted solutions is approximately 7-10 days. Under normal circumstances, undiluted scale inhibitors/dispersants are not subject to biological contamination. Another major consideration in selecting scale inhibitors/dispersants is ensuring complete compatibility with the reverse osmosis membrane. A reminder from Dio Pure Water engineers: Incompatible chemicals can cause irreversible damage to the membrane.