Fungal growth in the antiscalant – some antiscalants contain impurities that can provide the necessary nutrients and carbon for biogrowth. It is not uncommon to find fungal growth in polymer-based antiscalants. The growth in the drum can seed growth on the membrane surface. Reputable manufacturers manufacture higher purity products and use NSF approved anti-fungal preservatives to prevent these issues.
The dosing point too close to acid addition point – pH is typically adjusted using concentrated sulfuric acid. In the absence of good mixing, the acid will temporarily form a separate phase due to its very high density relative to water. If the antiscalant is dosed immediately after the acid dosing point, it can be hydrolyzed by the strong acid. Certain polymer-based antiscalants are very susceptible to this degradation.
Incompatibility with water quality – some antiscalants do not have good tolerance to iron and aluminum. Such antiscalants will form an insoluble complex with these metals and precipitate on the RO membrane surface.
Very high hardness or very high antiscalant dosage – antiscalants carry a higher negative charge than typical scale forming anions such as carbonate or phosphate; so they have a strong attraction to calcium and magnesium. During threshold inhibition, this attraction to calcium/magnesium causes adsorption to forming clusters, and interferes with the formation of a stable nucleus. However, when hardness concentrations are very high, the antiscalants itself can form an insoluble calcium-antiscalant salt. This is problematic because once the active inhibitor precipitates, other scales will also precipitate. Hardness impacts all antiscalants, regardless of whether they are polymer or phosphonate-based. AWC’s Proton antiscalant projection software can predict this salt formation, and can therefore predict maximum recovery at a given temperature and pH.
Quality control issues – in some cases, manufacturing issues can cause certain polymer-based antiscalants to form insoluble gels on the membrane surface. It is therefore imperative to use antiscalants only from reputable suppliers.