Orthophosphates versus Polyphosphates
The selection of a phosphate water treatment chemical additive can be one of the most difficult chemical treatment decisions that many public water systems will make. This is particularly true because the chemistry of orthophosphate and polyphosphate chemical additives is complex, and phosphate water treatment chemical additives are commercially available in an overwhelming number of chemical blends.
Orthophosphates and polyphosphates are salts derived from two different forms of phosphoric acid. Orthophosphates are small molecules, formed from the smallest and most basic form of phosphoric acid. Polyphosphates are larger molecules, formed from a longer chain version of phosphoric acid.
Even though the words orthophosphate and polyphosphate contain the word "phosphate," these two chemical compounds serve radically different water treatment purposes. A public utility system's failure to understand the significant differences between these two treatment compounds could result in serious water-quality problems and possible MCL violations. An incorrect selection of phosphate chemical blends by a public utility system could even create serious public health problems.
In public water systems, orthophosphates are used for lead and copper corrosion-control purposes. Orthophosphates chemically react with lead and copper atoms that have leached off of piping and have entered into the surrounding water. This chemical reaction of orthophosphates with lead and copper atoms forms lead and copper phosphate. The lead and copper phosphate is then electrochemically drawn back down onto the piping surface, where it forms a tough, water-resistant coating on the piping. This tough, water-resistant coating helps prevent further leaching off of lead and copper atoms into the surrounding water. Most public utility systems have experienced far greater success with orthophosphate lead corrosion control than they have experienced with orthophosphate copper corrosion control.
Polyphosphates are sequestering agents that are virtually ineffective against lead and copper corrosion. When a jury in a criminal trial is sequestered, that jury is "held in seclusion." A chemical sequestering agent is a chemical agent that surrounds another molecule or atom and holds that other molecule or atom "in seclusion." By surrounding the other molecule or atom and holding it in seclusion, the chemical sequestering agent hides the molecule or atom from sight and prevents it from entering into various chemical reactions. As a sequestering agent, polyphosphates will only sequester soluble "invisible-in-water" metals that have not been oxidized into their insoluble forms. Polyphosphate applied to water before the water is chlorinated will prevent invisible iron and manganese from becoming visible after the water is chlorinated.
As a sequestering agent, water treatment polyphosphate is used to sequester soluble iron atoms that remain in settled water before it is chlorinated or that leach off of iron piping in water distribution systems. By surrounding and sequestering these soluble iron atoms, they are prevented from displaying the typical reddish colors associated with iron oxides and iron hydroxides. Water treatment polyphosphates also interfere with the crystallization of and formation of calcium and magnesium carbonate scales, but not with the cystallization of and formation of magnesium hydroxide scales. If any soluble manganese atoms are still present in water after the floc has settled out, polyphosphates will also serve to sequester these soluble manganese atoms, preventing them from displaying the typical dark manganese dioxide color.
A gross misconception about polyphosphate is the belief that the use of polyphosphate sequestrants to hide iron and manganese is a casual, routine treatment technique for the removal of excess iron and manganese that was not removed during a water treatment plant's sedimentation and filtration processes. In reality, the use of polyphosphate to sequester iron and manganese that a plant failed to remove during the sedimentation and filtration processes is a desperation maneuver. Undesirable quantites of iron and manganese in raw water should be properly oxidized by aeration, permanganate, or ozone and should be deposited in sedimentation basins as part of the floc. Polyphosphates, which sequester iron and manganese for only a limited period of time, are not the ideal or the preferred solution for any water treatment plant's iron and manganese problems. Polyphosphates should only be used to catch the few particles of iron and manganese that were missed during the initial aeration and oxidation process.
Most phosphate treatment compounds used by public water treatment systems are actually blends of polyphosphates and orthophosphates. Polyphosphates are usually added in their sodium or potassium polyphosphate form. Orthophosphates are added in the sodium or potassium orthophosphate form or in an orthophosphate form that is mixed with zinc chloride or zinc sulfate. The zinc in the mixture plays no role in forming the coatings that prevent lead and copper corrosion. Instead, the zinc plays a significant role in protecting galvinized surfaces (galvanized means "zinc-coated") and in preventing asbestos fibers from eroding off of asbestos-cement piping. It should however be noted, some phosphate blends may also contain zinc polyphosphates, but zinc orthophosphate formulations are much more commonly used in public water treatment operations.
Phosphate water treatment mixtures are available in dozens of different blends. There is no perfect blend that is universally usable in all situations. If a water treatment system uses a blend with more polyphosphate than their system needs, the coatings laid down by the orthophosphate can be stripped away. If a water treatment system uses a phosphate blend with more orthophosphate than their system needs, iron can be stripped away from iron piping. Water treatment plants need to regularly assess the lead, copper, iron, manganese, and asbestos levels in their water and consult with a professional phosphate specialist whenever a change in phosphate blends seems to be warranted.
COMMON POLYPHOSPHATE SALTS: Sodium acid pyrophosphate, Tetrasodium pyrophosphate, Tetrapotassium pyrophosphate, Sodium tripolyphosphate, Potassium tripolyphosphate, Sodium trimetaphosphate, Sodium hexametaphosphate (glassy)
COMMON ORTHOPHOSPHATE SALTS: Monosodium orthophosphate, Monopotassium orthophosphate, Disodium orthophosphate, Dipotassium orthophosphate, Trisodium orthophosphate, Tripotassium orthophosphate, Zinc orthophosphate
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