Sequestration - Polyphosphates Treatment

There are, in addition, several other techniques for the control of soluble iron in the water. These make use of polyphosphates to keep the iron in solution.

Polyphosphates do not remove iron from water. Rather they stabilize and disperse the iron so that the water remains clear and does not produce iron stains.

However, polyphosphate treatment may not prevent iron from precipitating when water is boiled for a time, as in cooking or in the brewing of tea or coffee, as boiling can cause reversion to the orthophosphate which has no equivalent sequestering action.

  A positive displacement pump  

Solution- Pot type feeder


Polyphosphate treatment also exerts a dispersing action on old deposits of iron in tanks and pipes. While this is advantageous to the removal of these deposits, and iron problem may be temporarily intensified, and frequent flushing of pressure tanks and hot water storage tanks may be necessary until the old iron deposits are removed.

Polyphosphate treatment is not suited for treatment of iron in municipal supplies at the point of use. This is due to the fact that such iron may be partially or completely precipitated and insoluble before it enters the home. Polyphosphates are not effective in the control of precipitated iron, organic iron, or iron bacteria.

Solutions of the very soluble sodium polyphosphates may be fed into the water with the various small chemical solution feeders. In general, these units add solutions only when the good pump is operating, so that adjustments can be made to provide a fairly uniform dosage. The original cost of the solution feeder is somewhat of a disadvantage. However, this is sometimes offset by feeding several solutions to control more than one water problem.

A simple pot-type feeder is a less expensive original unit. These feeders utilize polyphosphates which commonly have calcium or magnesium incorporated into them to provide a product that dissolves slowly and evenly.

The feeder is installed in the water line so that all or a part of the water supply passes through the feeder tank. The water picks up some of the polyphosphate solutions and carries it into the water line.

Since water usage in the home is not constant, the feeding from such solution pots sometimes produces slugs or excessive dosages as well as very low dosages. To provide adequate feed at all times, it is usual practice to set the feed rate somewhat higher than the concentration actually required. A pressure tank will help in a degree to equalize the fluctuations in feed rate which might occur under varying flow conditions.

In order to maintain water clarity and prevent the possibility of iron stains, these polyphosphates must be introduced into the water at a point where the iron is still present in dissolved form. This should be before the pressure tank and as close to the discharge point as possible.

If such a feeder must be installed following the pressure tank, it should be installed on a by-pass and should feed into the well or into the suction side of the pump.

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