Calcium carbonate, calcium phosphate, calcium sulphate, barium sulphate, strontium sulphate, calcium fluoride, iron hydroxide, iron phosphate, silica, and colloidal foulants, are among the scales that are controlled by RO antiscalants.
How can antiscalants cause fouling of RO membranes?
Under the following circumstances, antiscalants in reverse osmosis systems can result in membrane fouling:
· Fungal growth in the antiscalant
Some antiscalants have impurities that can supply the carbon and nutrients for biogrowth. In polymer-based antiscalants, fungus development is not unusual. The membrane surface may provide fertile ground for the growth in the drum. To avoid these problems, reputable manufacturers create higher purity products and employ preservatives.
· pH change
The pH is frequently changed using concentrated sulphuric acid, which brings the dosage point too close to the acid addition point. Due to its extremely high density in comparison to water, acid will momentarily form a distinct phase if there is poor mixing.
The strong acid can hydrolyse the antiscalant if it is dosed right after the acid dosing point. Some antiscalants made of polymers are extremely prone to this breakdown.
· Water quality incompatibility
Some antiscalants do not tolerate iron and aluminium well. Such antiscalants will form an insoluble combination with these metals, and precipitate on the RO membrane surface.
· Very high antiscalant dosages
Antiscalants have a stronger attraction to calcium and magnesium because they have a bigger negative charge, than usual scale-forming anions like carbonate or phosphate. This attraction to calcium and magnesium during threshold inhibition, results in adsorption to cluster formation and prevents the establishment of a stable nucleus.
However, the antiscalants themselves can create an insoluble calcium-antiscalant salt, at very high hardness concentrations. This is a concern since other scales will also precipitate when the active inhibitor does. All antiscalants are affected by hardness, whether they are phosphonate or polymer-based.
Some polymer-based antiscalants may produce insoluble gels on the membrane surface, due to quality control or manufacturing difficulties. Therefore, it is essential to only employ antiscalants from reliable vendors.
What happens if antiscalants are released into the environment?
Different antiscalants come in various varieties. Some are polymer-based, while others are phosphonate-based. Many are mixtures to maximise the benefits of the two chemistries. If antiscalants were harmful, they would not have been approved for use in potable water applications. Antiscalants based on polymer and phosphonate are both biodegradable over time. As the chemical bonds are disrupted by ultraviolet light and some bacteria create enzymes that break the bonds, the rate of disintegration rises.
Conclusion
The highly effective reverse osmosis (RO) antiscalants work at surprisingly low dosages, and can prevent scales even at very high saturations. These antiscalants can minimise cleaning frequency, eliminate the requirement for acid dosing, and let facilities to run at greater recoveries.
If you need any advice on a water treatment problem, Netsol Water can guide you towards the best water purification system for your requirements, including RO Plants.
