Circular 65 STATE OF ILLINOIS WILLIAM G. STRATTON, Governor DEPARTMENT OF REGISTRATION AND EDUCATION VERA M. BINKS, Director BY T. E. LARSON ILLINOIS STATE WATER SURVEY WILLIAM C ACKERMANN, Chief URBANA 1957 (89920—12-58) Printed by authority of the State of Iuinois REPRINTED FROM AND COPYRIGHTED AS A PART OF JOURNAL AMERICAN WATER WORKS ASSOCIATION Vol. 49, No. 12, December, 1957 Printed in U. S A Evaluation of the Use of Polyphosphates in the Water Industry Thurston E. Larson A contribution to the Journal by Thurston E. Larson, Head, Chem• istry Section, Illinois State Water Survey, Urbana, Iu. HE statements in this attempt to favorable data acquired in one appli• T separate fact from fancy in the cation are not necessarily applicable to sale and use of polyphosphates in water another. The first need in any applica• treatment derive from findings which tion is a statement of the end to be have been described in reliable scien• accomplished. tific literature. Several comments are contrary to the implications of sales Softening of Household Water representatives who lack either ade• Data are available to show that about quate training or the capacity to resist 5-10 ppm polyphosphate are required the temptation to oversell. Such defi• to counteract 1 ppm hardness. Thus, ciencies have resulted in misapplica• for example, water with a hardness tions which have not benefited the repu• 300 ppm will require an application of, tation of polphosphates in water 1,500-3,000 ppm of polyphosphate to treatment. soften it, or 6-12 tons per million gal• In a broad general sense, all poly• lons. On the other hand, considering phosphates show the same characteris• smaller quantities, 1 oz of polyphos• tics—with some notable exceptions phate would be required to soften 10 which are of minor consequence in this gal of water of 300 ppm hardness. discussion. These polyphosphates in• Thus, a household job such as dish• clude the crystalline pyrophosphate, washing represents a practical applica• metaphosphate, and tripolyphosphate, tion. Such softening is accomplished as well as a number of glassy polyphos• by a process called sequestration, or by phate blends. Polyphosphates behave the formation of soluble calcium poly• chemically in a manner quite different phosphate and magnesium polyphos• from the well known orthophosphates. phate complexes. In many ways the polyphosphates are This should not be confused with remarkable as a class of chemical what is normally considered as thresh• compounds. old treatment—a stabilizing or inhibit• Many claims have been made; some ing treatment which is designed for are adequately supported by data, but another purpose when treating mu• others are often misinterpretations or nicipal or industrial water supplies. inferences which cannot be substanti• Threshold treatment refers to the ap• ated by suitable records. In each case plication of polyphosphate in a concen• the circumstances of the application tration of about 2-4 ppm. This con• must be considered. Inferences from centration theoretically reduces the 1581 1582 THURSTON E. LARSON Jour. AW WA hardness of the water by about 0.5 the point where this nondestructive ppm—for example, from 300 ppm to type of treatment can no longer be 299.5 ppm. applied with success. Polyphosphates are normally an inte• gral ingredient in dishwashing com• Red Water pounds for restaurants and household Rusty water may occur as the result use, as well as in many synthetic deter• of the natural presence of iron in the gents. A typical dishwashing ma• water pumped from ground water wells chine compound contains about 40 per or it may result from iron dissolved in cent polyphosphate, 40 per cent sodium the water as a product of corrosion. silicate, and 20 per cent soda ash. Iron in well water is normally present Thus, the use of polyphosphate for in the soluble (ferrous) state. The softening water for domestic purposes water is clear until, upon exposure to is practicable, but exceeds the bounds air or to chlorination, it is converted to of ridiculousness when it is implied the insoluble (ferric) state which re• that the same applies to municipal acts with water to form ferric hydrox• softening of hard-water supplies. ide, or rust. Polyphosphates have the property of Cleaning of Wells being able to combine with or seques• As cleaning and wetting agents, poly• ter soluble iron. They do not combine phosphates have achieved frequent no• with or sequester insoluble ferric hy• table success when used for cleaning droxide. In view of this property, the screened sand and gravel wells. When more effective use of polyphosphate confronted with a well problem, it is requires the application of this treat• important to be sure, first, that the ment to the water before it is exposed problem results neither from pump to air or before chlorination. Subse• inefficiency, lack of water in the aqui• quent oxidation by air or chlorine will fer, nor failures in the column pipe. convert the iron to the ferric form, but Where it has been determined that lack it remains dispersed until the polyphos• of water is due to clogged screen or a phate has lost its dispersing property by clogged water-bearing formation in the reversion to orthophosphate. immediate area of the screen, treatment The amount of polyphosphate re• with about 30 lb of polyphosphate per quired for this purpose is about 2-4 100 gal of water in the well bore has ppm per 1 ppm of iron in concentra• been found successful. Such treatment tions up to about 2-4 ppm. At higher is often accompanied by the use of 1-2 concentrations of iron, more than 9 lb of chlorine or chlorinated lime and, ppm polyphosphate is not always on a few occasions, has been in con• beneficial. junction with a wetting agent of the As it has been recognized that poly• non-ionic type. The practice of surg• phosphate is an excellent source of ing the well periodically during the phosphorous for the growth of bacteria, several hours following the application it is normally recommended that the of the treatment is recognized as highly concentrated solution of polyphosphate beneficial. For continued success, it is for injection into the water be treated desirable to repeat the application at with about 50 ppm chlorine each time scheduled intervals dictated by experi• the solution is prepared. This prevents ence so that incrustation does not reach the development of a breeding ground Dec. 1957 POLYPHOSPHATES 1583 for possible coliform organisms in the carbonate. Although either calcium polyphosphate treatment solution. or magnesium may be responsible for In certain areas where sufficient con• the scale, polyphosphate is effective centrations of ammonia, methane, or against calcium carbonate deposits both may be present in the well water, only; it has no effect on redissolving some iron removal water treatment or preventing the magnesium hydrox• plants are rendered ineffective by bac• ide deposits so frequently encountered terial growths in the filter bed. Such where lime softening plants are not growths deplete the oxygen which was operating properly. dissolved in the water by aeration for The first of four points where cal• the purpose of oxidizing the soluble iron cium carbonate deposits can be con• to the insoluble ferric hydroxide prior trolled is the sand filter. Lime soften• to filtration. In fact, even though chlo- ing plants without sufficient recarbo- rination is subsequently applied to the nation to prevent calcium carbonate water effluent, the water mains them• deposition on sand frequently acquire selves may be so infected with such an accumulation of calcium carbonate growths that chlorine is destroyed by on the sand grains. Continuous treat• the growths faster than the reverse can ment with polyphosphate in threshold occur. Subsequently, such insoluble concentrations (2 ppm) has frequently iron as may be deposited in the mains been reported to prevent such deposi• is redissolved as ferrous iron because tions. Magnesium hydroxide deposits of the anaerobic conditions created by which may occur at this point are no the bacterial growths. problem because they do not accumu• Chlorination before filtration has late on the sand grain and are easily proved to be successful in many cases. removed during backwashing. In persistent situations it may be de• Distribution systems constitute a sirable to use copper sulfate in con• second point of scale accumulation be• junction with polyphosphate for the cause lime softening plants with insuf• purpose of inhibiting the growth of ficient recarbonation may also deposit bacteria while the polyphosphate keeps a rather heavy calcium carbonate scale in soluble form such soluble iron as in the main as the water leaves the may be picked up from the mains. plant. Again, threshold treatment with The economics of such treatment is not polyphosphate will prevent this accu• prohibitive for small water plants. mulation at this point. However, such Normally, a treatment with 1 ppm cop• plants frequently also produce a water per (2.5 ppm anhydrous copper sul• at or near saturation with magnesium fate) is adequate when applied with hydroxide. Such lime-softening efflu• more than 2-4 ppm and less than 9 ents are usually high in magnesium ppm polyphosphate. and pH and low in calcium and alka• linity or both. Deposits of magnesium Scale Prevention hydroxide in the water main cannot be Scale is defined here as a deposit removed or prevented by the use of from hard water and does not refer to polyphosphates. There is no record either rust deposits or corrosion prod• in the literature to indicate the success• ucts. Threshold dosage of polyphos• ful use of polyphosphate for this phate in water has the unique power purpose. of preventing crystal growth of calcium 1584 THURSTON E.