Feeding Polyphosphate to Treat Iron and Hardness in Water
Polyphosphate is used, not to remove iron and hardness from water, but to "sequester" them so that they do not cause damage to fixtures and outdoor surfaces. This page contains some example of sequestering advice from a number of sources.
Rather than a unified "article," this page reports advice on sequestering with polyphosphates from a variety of sources:
Pure Water Products: Feeding Pro Poly with a 250/1 Chemilizer non-electric pump.
1 lb. Of Pro Poly in 1 gallon of water yields an 80,000 ppm. solution. Sequestering of iron or hardness requires about 2 ppm, 4 if the concentration is heavy.
With a Chemilizer 250/1 feeder, mix 1 oz of Pro Poly with 10 gallons of water to achieve a final treatment mixture of 2 ppm. 2 oz per 10 gallons gives a working mixture of 4 ppm.
Polyphosphate is effective at a pH range of 5.0 to 8.0. (Harrison, WQA.)
Polyphosphates provide many functions including the sequestering of iron and manganese and with the alkali earth metals such as calcium and magnesium. Each type of polyphosphate exhibits a different reactive sequestering rate for each of these metals/minerals. Polyphosphate also cleans or dissolves precipitated mineral scale that already exists in water distribution lines.
Hexametaphosphates (polyphosphate) inhibits scale formation caused by calcium and magnesium through sequestration and crystal growth modification at a rate 20 times more effective than the pyrophosphates. However, the pyrophosphates can sequester iron and manganese at a rate 16 times more effective than the hexametaphosphates.
Typically, in order to sequester iron, manganese and calcium it requires a 1:1 mole ratio with polyphosphate. This is generally achievable when dealing with iron and manganese that are usually in levels that are less than 5 mg/L. However, polyphosphates act as crystal modifiers that need only a fraction of that ratio to effectively modify the crystalline structure of calcium/magnesium. In theory, a hexametaphosphate dosage of 500 mg/L is required to actually sequester 200 mg/L of calcium (as calcium carbonate). However, a dosage of only 2 -4 mg/L of hexametaphosphate is all that is required to modify the crystal growth of calcium carbonate. By modifying the crystalline structure, these compounds will not precipitate into scale and actually stay in solution through repelling and suspension. This crystal growth modification function prevents the formation of mineral scale within water distribution systems. Polyphosphates will also dissolve already deposited mineral scale deposits within the system thereby increasing the carrying capacity of the water system. Polyphosphates are cathodic inhibitors that interfere with the cathodic site of the electrochemical corrosion cell formation. Anodic and cathodic electronic reactions are the two components necessary for the development of electrochemical corrosion. See: http://sperchemical.com/html/corrosion_inhibition.html This interference greatly reduces the rate of internal metallic corrosion within water distribution systems.
If the water contains less than 1.0 mg/L iron and less than 0.3 mg/L manganese, usingpolyphosphates followed by chlorination can be an effective and inexpensive method for itigating iron and manganese problems. No sludge is generated in this method. Below these concentrations, the polyphosphates combine with the iron and manganese preventing them from
being oxidized. Any of the three polyphosphates (pyrophosphate, tripolyphosphate, or metaphosphate) can be used.
Sequestering does not remove Fe/Mn from the water. Sequestration binds the Fe/Mn in soluble form preventing the compound from oxidizing on contact with air or chlorine. This is only an option if the iron is in the form of ferrous iron (Fe2+), manganese as manganous (Mn2+), and if the combined concentration is less than 1 to 3 mg/L. Sequestering prevents staining of plumbing fixtures and discoloration of the water, but a slight metallic taste remains. Sequestering agents break down at high temperatures found in water heaters.
Polyphosphates followed by chlorination can be an inexpensive method for sequestering Fe/Mn. Polyphosphates can be added to the water as a dry feed crystal or as a liquid solution with a metering pump. Polyphosphates can be effective in a pH range of 5.0 to 8.0. Because phosphate compounds are nutrients that contribute to the eutrophication of surface waters, appropriate methods of treating the waste water must be used.
Some sources say that combined Fe and Mn must be no more than 1 ppm for sequestrion with polypyosphates, but other sources say that much larger amounts can be treated if the strength of the polyphosphate dosage is increased.
Iron sesquestration by polyphosphate was found favorable to no iron treatment. The presence of calcium in the groundwater models inhibited treatment effectiveness. The polyphosphate sequestrant should be added to the groundwater before the addition of chlorine but simultaneous addition was almost as successful. Other findings of the study are also presented and discussed..
To test for polyphosphates in water, the total phosphate and orthophosphate
concentrations must be known.
The difference between these two values
will be the polyphosphate. A minimum
of 4 ppm polyphosphate is fed into
the water to treat 2 ppm of iron or less.
For every 1 ppm of iron above 2 ppm,
an additional 2 ppm of polyphosphate
must be added. Unfortunately, the
polyphosphate will frequently be underfed
so only a portion of the iron will be
sequestered; the balance will be one of
the other types of iron, which may require a different water treatment solution.