Low molecular weight substances or polymers (those whose chains are not long enough to link particles together via the bridging mechanism), tend to be absorbed on individual particles and on growing crystals of scale.

The action of these substances in preventing scale can be classified as threshold inhibition, whereby sub-stoichiometric quantities of inhibitor agents, inhibit scale formation.

Threshold inhibition can act through dispersion, sequestration, or scale crystal distortion, but the most common in water treatment are the substances that act through supersaturation.

If we observe the solubility diagrams, we distinguish three zones. The zone of the solubility, the zone of supersaturated and the heavily supersaturated area.

As the concentration increases starting from point A, deposition is not taking place, even the concentration overpasses the solubility, and the solution becomes supersaturated.

Deposition starts at the point C at the boundary of supersaturated and heavily supersaturated, and will be deposited a sufficient quantity of substance as to drop the concentration at the point D, at the solubility limits.

A threshold inhibitor enlarges the area of the supersaturated liquid, till a new point B, allowing the water to keep in solution higher amounts of substance.

But if the capability of the threshold inhibitor will be over passed, massive deposition will occur, and the concentration will drop at the same point D, like in the untreated liquid.

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