Antiscalants - history and mode of action
The first commercial membranes were made of cellulose acetate. An acidic pH in the feed prevented the membranes from degrading. The acidic pH also prevented calcium carbonate deposits in the concentrate.
Therefore, early treatment methods of the feed water to membrane plants consisted of acid dosing and, if necessary, addition of sodium hexametaphosphate (NHMP).
With the introduction of the polyamide membrane, which is usually operated in the neutral pH range and allows higher yields, much more effective antiscalants were needed.
The formation of crystalline deposits in the concentrate can be divided into 4 stages: Free ions, which form a protocore by accumulation, in which they then arrange themselves into a nucleus and finally solidify into a crystal lattice.
Depending on its formulation and ingredients, the antiscalant can intervene by different mechanisms at different points of crystal formation:
- threshold inhibition: the antiscalant acts at stage2 of the crystal formation process by inhibiting the ordering of protinuclei, thereby preventing nucleation.
- crystal distorsion: The antiscalant acts in stage 3 and interferes with crystal formation, thus preventing crystallisation.
- dispersion: dispersants use electrical repulsion to keep crystals in suspension and prevent crystals from sticking to each other and to the membrane surface.
- complexation: ability to attract ions to create soluble and stable molecules.
Modern antiscalants are subject to the DIN EN 15039 standard for polycarboxylic acids and the DIN EN 15040 standard for phosphonic acids, depending on the requirements profile. From these European standards, the corresponding active substances were included in the German Drinking Water Ordinance following extended efficacy tests conducted in Osnabrück and Tauberbischofsheim between 2003 and 2008 and supervised by Dr Jens Nähring.
The FreeFlow Antiscalants, which are developed by Dr. Nähring Water Treatment GmbH and manufactured at the company's site, are characterised by excellent efficacy and can, in principle, be used in the context of a direct discharge of the concentrate into the receiving water. As a rule, they are compatible with all common membrane types and, together with the membrane cleaners of the Mem and Memobrane series, form a balanced overall package for use in membrane plants.
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