Polyphosphates and Threshold Treatment

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Polyphosphates are polymers of the phosphate ion, PO4-3. The smallest one is pyro-phosphate, P2O7-4; the next is called tripolyphosphate, P3O10-5, and so on, up to large molecules of 20 or 30 phosphates linked together in chains, having very large electrical charges. They are useful for inhibiting lime scale in two ways. The first is direct, mechanical interference with the growth of calcium carbonate crystals. The shape of PO4-3 group is somewhat similar to the shape of CO3-2, and this similarity enables a phosphate to take the place of carbonate on the surface of a crystal of growing calcium carbonate. However, it is just enough unlike carbonate to prevent any further crystal growth on top of it. Chemists call this type of mechanism “competitive inhibition:” lime scale crystal growth is inhibited when polyphosphate ions compete with carbonate ions for the same Ca++ sites on surface of growing scale; when the polyphosphate wins, scaling is stopped at the point for several hours or days, depending on temperature. At boiling water temperature, polyphosphates break apart or “revert” to single PO4 unties, or “ortho-phosphate” in less than an hour. If the hardness level is very high, it may cause precipitation of calcium ortho-phosphate, thus negating the original reason for using a, polyphosphate. The other mechanism of lime scale inhibition by polyphosphate feed is called “electro-static dispersion.” There are millions of tiny particles of all kinds in water – not just dirt, but also tiny scale particles that use a speck of dirt as a “nucleus” to crystallize upon and grow-and the vast majority of them carry a negative electrical charge. That means they all naturally repel one another to some extent, because, just as in magnets, opposites attract and likes repel. But there is a lot of violent activity in water on a microscopic scale: molecules and particles in water are involved in some million-billion-trillion or an estimated 1027 collisions per second. Sometimes these particles collide with enough momentum to overcome the repulsion, and then they stick together to form a larger particle. That is one of the ways scale and other kinds of sediment grow: by agglomeration. Fortunately, the repelling force can be magnified by dosing the water with dissolved ions having many negative charges, and that is exactly what polyphosphates do. The smallest polyphosphate has a -4 charge; many have charges of -20 or -30 or even more. They cluster around particles and add their charges to make them repel each other thousands of times more strongly than before, with the result that very few are able to overcome the repelling force and become agglomerated into larger masses that deposit as scale or sludge.

The concentration of polyphosphates needed to be effective is less than 10 ppm, and it does not depend on the hardness level. Neither the competitive inhibition mechanism nor the electrostatic dispersion mechanism operates “chemically,” with one molecule reacting with another molecule one-on-one. Instead, the presence of small amounts of polyphosphate creates a non-scaling environment by physical means, and the overall effect is called threshold treatment. Higher polyphosphate concentrations of 20-100 ppm usually exceed the “threshold” of calcium polyphosphate precipitation, producing massive amounts of phosphate scale and sludge. Still higher concentrations – up to 500 ppm or more – create a “soft” water by sequestering every hardness ion with a polyphosphate molecule as a “complex ion” which remains dissolved. Examples of this approach are high-phosphate laundry detergents and “bath salts,” but it is never used for drinking water because such high levels of polyphosphates can cause diarrhea in sensitive persons.

If the water has been pre-filtered by an efficient fine-filter, .there will be very few particles present at all, and the enhanced repelling force will be even more effective. Everpure research has shown that fine-filtration alone can reduce scale by half or more, just because scale includes a lot of dirt, iron floc, and other particulate matter in addition to calcium carbonate crystals. Pairing efficient fine-filtration with threshold polyphosphate treatment can reliably reduce scaling, therefore maintenance costs for ice-makers and coffee brewers, by more than 80%. This patented technology is embodied in Everpure InsuriceTM products for ice makers and is effective for water supplies having up to about 15 grains per gallon (GPG) or 250 ppm hardness as CaCO3. Above 20 GPG hardness, dealkalization usually gives better results.