THE USE OF PHOSPHATE IN WATER TREATMENT FOR SEQUESTERING AND CORROSION CONTROL By Richard E. DeBlois Carus Chemical Company 315 5th Street Peru, Illinois 61354 (800) 435-6856
Phosphate chemicals are among the few recognized chemicals that can be safely added to potable water in an attempt to improve water quality. Over 200 phosphate based products are certified for potable water treatment for corrosion control and metal sequestering. In general, all of these commercially available products can be classified into a specific family of phosphate species. This paper will review the basic role of phosphate species and how they are applied to water treatment. Abstract for 12th Annual South Carolina Environmental Conference, 17-20 March 2002.
Use of Phosphate in Water Treatment for Sequestering and Corrosion Control By Richard E. DeBlois Carus Chemical Company
Phosphate corrosion inhibitors and sequestering agents are among the few recognized chemicals that can be safely added to potable water to produce a significant improvement in distribution system corrosion, colored water and scale control. In fact, many phosphate containing derivatives play a role in food technology and are present in many modern food formulations such as taste providers, buffering agents, dough conditions, etc. These derivatives include phosphoric acid, which can be found in soda in concentrations as high as 7000 mg/L. Over 200 phosphate-based products are ANSI/NSF Standard 60 certified for potable water treatment for corrosion control and metal sequestering. Many of these products are virtually similar competitive offerings. In general, all commercially available products can be classified into several phosphate-based technologies: Orthophosphates Zinc Orthophosphates Zinc Polyphosphates Ortho/Poly Blends Poly (Condensed) Phosphates Silicate Phosphate Blends
All phosphate technologies have a record of success, but each also has a record of failure when inappropriately applied. Because the distinction between the application of orthophosphate and polyphosphate for water treatment is very important, an overview of each will be given. Orthophosphate Orthophosphate is widely accepted as a corrosion inhibitor. Orthophosphate precipitates with divalent metal ions forming a thin film on the inner surface of the pipe. These metal salts of orthophosphate are very insoluble (i.e. lead and calcium). For example, lead can form several orthophosphate solids that are much less soluble than the lead carbonate salts over a wide pH range.1 Orthophosphate does not sequester and the presence of dissolved inorganic carbon (DIC) and alkalinity affect its performance. Orthophosphate can be obtained through several different phosphate raw materials. Monosodium phosphate (MSP) can be used to produce orthophosphate for corrosion
1
Schock, M.R. J. Am. Water Works Assoc., 81(7):88 (1989)
control. This is a dry, neutral, and safe product but requires mixing and handling. Orthophosphate liquids are also available. These liquid products include MSP, disodium phosphate (DSP) and dipotassium phosphate (DKP). The most common source of orthophosphate is phosphoric acid (H3PO4). Phosphoric acid is available as technical grade acid in solution strengths of 36% or 75% - the most common being 36%. Unlike the phosphate salts of sodium and potassium, phosphoric acid is a hazardous material and requires special handling procedures for both shipping and applying. Zinc Orthophosphate Zinc orthophosphates were recognized for potable water treatment when in 1970, Murray2 described a treatment system consisting of a 1:1 weight ratio of zinc to orthophosphate for corrosion protection. Today, zinc orthophosphate products with zinc to phosphate ratios vary from 2:1 to 1:15, depending on specific requirements defined by the end-user, engineering firm, and/or vendor. These corrosion inhibitors are typically formulated from the zinc salt, for example zinc chloride or zinc sulfate and blended with phosphoric acid. Because these products have a solution pH <1.0, proper personal protective equipment (PPE) and special shipping and handling procedures are required. There are pros and cons related to the two types of zinc orthophosphates, the chloride based products and the sulfate based products. Zinc orthophosphate manufactured from zinc chloride has very good solubility; however, the chloride is corrosive to stainless steel. It is important to make sure that transfer equipment (shipping), storage tanks, and feed equipment are compatible to the chloride ion. In addition, there is the potential to generate hydrochloric acid (HCl) during the blending of zinc chloride with phosphoric acid. There is evidence that also suggests the chloride-based products may also contribute to atmospheric corrosion.3 The sulfate based zinc orthophosphates have limited solubility and can experience precipitation problems, especially when manufactured with water containing high calcium. Unlike the chloride ion, the sulfate ion is non-corrosive to stainless steel and does not require special transfer equipment (shipping), storage tanks, and feed equipment. Zinc orthophosphate has been demonstrated to have good efficacy in aggressive waters (low to moderate hardness and alkalinity). These waters are usually soft and slightly acidic. The optimum pH for zinc orthophosphate is in the range from 7.3 – 7.8. Above pH 8.0 there is a potential for precipitation of the zinc phosphate. The only limitation of the zinc orthophosphate technology is the contribution to the zinc load at the wastewater treatment plant. Zinc Polyphosphate These products are appropriate for less aggressive waters that require some level of sequestering and/or calcium stabilization, as well as corrosion control. Most of the
2 3
W.B. Murray, J. Am. Water Works Assoc., 62 (10) (1970) Pedersen, E, Internal Research, Carus Chemical Company, LaSalle, IL
products are a combination of orthophosphate and polyphosphate. Corrosion protection is obtained with the straight zinc polyphosphate as a result of hydrolysis (reversion) of the polyphosphate to orthophosphate.4 They provide effective copper control and have had much success in harder waters where calcium stabilization is an objective. A recent study by AWWA suggested that polyphosphate plays a role in copper control but is detrimental to lead pipe. The study also showed that orthophosphate was good for lead control, but not too beneficial for copper.5 The trend is toward higher ortho to poly ratios for lead control. Like zinc orthophosphates, these products contribute to the zinc loading at wastewater treatment facilities. Ortho/Poly Blends Bridging both corrosive and scaling waters, ortho/poly blends provide sequestering and corrosion control. These formulations consist of orthophosphate and polyphosphate blended together in a variety of ratios. Blends containing high orthophosphate provide more corrosion protection, while higher polyphosphate concentrations enhance sequestering of hardness (calcium) and iron and/or manganese resulting in colored water suppression. The ortho/poly blends are very effective for copper corrosion control in high hardness waters. The ortho/poly blends are very effective in a variety of water chemistries and are applicable over a broad pH range. One limitation is their effectiveness in aggressive, low hardness waters. One caution with these products is that they can increase lead residuals at the tap if the polyphosphate dosage is too high. Linear Chain Polyphosphates This class of phosphate inhibitors is the group consisting of sodium and potassium polyphosphates (i.e. tripolyphosphate, hexametaphosphate, pyrophosphate). Many forms of polyphosphates are available with their differences stemming from the type of phosphate used in the formulation and the type of manufacturing process. This technology is appropriate in waters that are very hard and/or contain high levels of iron and manganese that demand sequestering. These linear chain polyphosphates are effective sequestering agents and can actually be used to control/remove depositions on a pipe wall. These products have been used to effectively reduce tuburculation resulting in improved water flow (C-Factor) and water quality.6
4
a. Bell, R.N., Hydrolysis of Dehydrated Sodium Phosphates, Ind. & Eng. Chem., Vol 39, Page 136 (1947) b McGilvery and Crowther, The Hydrolysis of the Condensed Phosphates, Candian J. of Chem., Vol 32, 174-185, (1954) 5 Edward, M. et al, Role of Phosphate Inhibitors in Mitigating Lead and Copper Corrosion, ISBN I-58321086-5, AWWA Research Foundation and AWWA (2001) 6 Spon, R.S., Scale and Corrosion Inhibitors, The Kansas Lifeline, November 1986.
Under proper control and dosage, these products can be very effective in maintaining proper corrosion control and sequestering of hardness and build-up on pipe walls. As with any polyphosphate, overdosing can promote lead solubility and elevated concentrations at the tap. Therefore, use of these products requires good monitoring and continued optimization. Silicate/Phosphate Blends These products are primarily used in Eastern US waters where use of zinc may be objectionable (or prohibited) and where phosphate dosage is limited. They have had great success in soft waters with low pH and high oxygen content. These products provide successful sequestering, moderate corrosion control, and work in a variety of water qualities. The phosphate combines with metal to form a precipitate that protects the surface of the pipe, while the silicate improves the protection by clogging pores and covering the more acidic, anodic parts of the surface.7 Good corrosion protection has been achieved using the silicate/phosphate blended products. These products typically require a higher dosage to provide equivalent treatment results, as compared to the other technologies. Also, the silicate-based products are one of the more expensive technologies. Overview There are several technologies available for corrosion inhibition and/or sequestering scale and colored water. Of these technologies, several are phosphate-based products. The choice of technology can be as critical as calculating the proper dosage to achieve desired results. There is a general acceptance of orthophosphate for corrosion control. Blended phosphates give similar results for corrosion control, and there is an added benefit for scale control and sequestering of metals to control colored water. There is some controversy with condensed phosphates with regard to corrosion control, yet they have been demonstrated to perform well in many municipal systems involving a broad range of water chemistries. “Which Phosphate Do I Choose?” This is not always an easy question to answer. Water quality dictates product selection, but the true answer is determined by the performance of the product. There are general “rules of thumb” for product selection, but what works in one place may not work at another even when the water quality appears to be similar. The situation may also dictate which technology is selected. For example, if you are implementing a corrosion inhibitor in a point-of-entry type application (i.e. school, apartment complex, etc.), the product selection may be dependent on chemical hazards. Phosphoric acid might be the desired product, but due to the hazards associated with acid, a blended phosphate might be selected.
7
Cooperative AWWA Research Report, Internal Corrosion of Water Distribution Systems, 2nd Ed., AWWA, Denver, CO (1996)
Selection of the best corrosion technology is dependent on the intended treatment objectives, a thorough understanding of the characteristics of the water being treated, and the properties of the products available. Success depends on choice of product, choice of supplier, and proper application.