Phosphate chemicals are among the few recognized substances which can be safely added to potable water to produce a significant improvement in many water quality parameters. Over 200 phosphate-based products are now ANSI/NSF Standard 60 certified for use in potable water treatment. Many of these are virtually identical competitive offerings. In general, all commercially available products can be classified as one of the following phosphate technologies:
- Orthophosphates
- Zinc Orthophosphates
- Zinc Polyphosphates
- Ortho Poly Phosphate Blends
- Linear Chain (Poly) Phosphates
- Silicate Phosphate Blends
Selection of the best corrosion technology and eventual recommendation of a specific treatment product is dependent upon two primary factors:
(1) the intended treatment objectives of the chemical program, and
(2) a thorough understanding of the characteristics of the water being treated
Success of a treatment program depends on choice of the most effective product and technology, and proper application (start up, feed rate, monitoring program).
(1) All phosphate technologies have a record of success in specific applications;
(2) Each phosphate technology also has a record of failure when inappropriately applied.
Treatment Objectives
The objectives of a chemical treatment program are defined by the problems that one wishes to correct or avoid. Examples include:
- excessive lead and copper residuals at the tap
- red water complaints
- scale build-up in the distribution system
- excessive treatment costs
- THM formation, and
- excessive asbestos fiber counts
Identification of these treatment objectives will largely eliminate inappropriate technologies and narrow the field of choices.
Water Characteristics
Water quality, defined by specific levels of contaminants measured in the water, will also eliminate many potential treatment technologies and influence any final treatment recommendation. Examples of important water characteristics include:
- pH
- alkalinity
- hardness
- iron
- manganese
- temperature
- dissolved oxygen
- corrosivity
- secondary factors
Although all waters differ to some degree, the Product Selection Guide can be used to determine the optimum phosphate technology based upon specific water quality parameters.
Zinc Orthophosphates Aggressive Waters
Advantages:
- most comprehensive and effective corrosion control
- effective lead and copper control
- best galvanic corrosion inhibitor
- offer flexibility for system Management
Limitations:
- contribute to zinc load at wastewater treatment plant
- cannot be used above pH 8.1
Zinc Orthophosphates as a technology provide the most effective and comprehensive corrosion inhibition. Zinc orthophosphates have traditionally been used to stop colored water caused by bleeding tuberculation and have a long history of success. This technology is very effective in aggressive waters with no source iron and manganese to sequester and where calcium stabilization and scaling are not a concern. Zinc orthophosphates have demonstrated particular effectiveness in providing effective lead and copper protection. Zinc orthophosphates reduce asbestos fiber counts and protect cement pipe.
These formulations consist of a zinc salt (either chloride or sulfate) combined with orthophosphate formulated in a wide variety of zinc to phosphate ratios ranging from 2:1 to
One of the limitations of zinc orthophosphates is that they contribute to zinc load at the wastewater treatment plant. The trend for this technology is toward "high ortho/low zinc" formulations for this very reason.
Zinc orthophosphates offer flexibility for a water system manager. Zinc residuals can easily be optimized without sacrificing historically-proven protection offered by the orthophosphate component by simply changing to a higher ratio product.
Orthophosphates Aggressive Waters
Advantages:
- provide non-zinc lead protection
- low cost
- modest copper protection
Limitations:
- anodic inhibition only
- limited galvanic protection
Used exclusively in aggressive waters, straight orthophosphates can be considered somewhat an extension of the zinc ortho's in that the blend ratio is so high that zinc has virtually been eliminated.
The benefits of orthophosphates are much more limited in comparison to the zinc orthophosphates. Orthophosphates have proven very useful in situations that require non-zinc lead protection without red water protection. In contrast to the di-polar functionality of the zinc orthophosphates, straight orthophosphates offer only anodic corrosion control. The entire focus is on controlling lead residuals.
Some systems have chosen to eliminate zinc altogether and accept reduced red water control and corrosion protection. For these systems, straight orthophosphates will find wider acceptance.
Bimetallic Zinc Polyphosphates Less Aggressive, Moderately Hard Waters
Advantages:
- low zinc
- sequesters iron & manganese
- stabilizes hard water & controls scaling
- limited lead protection
- effective copper control in hard, high TDS water
Limitations:
- excess polyphosphate can increase lead solubility
- contributes to zinc load at wastewater treatment plant
- available in dry formulations only
- modest galvanic corrosion inhibition
This technology is appropriate for harder, less aggressive waters that require some level of sequestering and/or calcium stabilization.
Zinc polyphosphates offer a variety of benefits. In addition to some of the traditional benefits offered by zinc orthophosphates, the polyphosphate component prevents scale and stabilizes soluble iron, manganese and calcium by sequestering these ions. Limited lead protection is obtained through hydrolysis of the polyphosphate to orthophosphate ion. Products in this category consist of a zinc salt combined with hexametaphosphate in a dry state.
Like the zinc orthophosphates, this group of products contributes to the zinc load at the wastewater treatment facility and offers modest corrosion control in comparison with the zinc orthophosphates. These formulations provide effective copper control and are the technology of choice in harder waters where calcium stabilization is a necessary objective. The trend for this technology is toward formulations with orthophosphate combined with zinc and polyphosphate to provide a higher measured dose of orthophosphate for lead control.
Ortho Polyphosphate Blends Aggressive and Scaling Waters
Advantages:
- broad pH range of application
- stabilizes calcium and prevents scaling
- effective copper control in hard, high TDS water
- modest lead protection
Limitations:
- limited effectiveness in aggressive (low hardness) water
- excess polyphosphate can increase lead solubility
- modest galvanic corrosion inhibition
Bridging both corrosive and scaling waters, orthophosphate polyphosphate blends provide sequestering, moderate corrosion control, and limited lead and copper protection without zinc.
These formulations consist of orthophosphates and polyphosphates blended together with approximately 30 to 70 percent orthophosphate content on a typical basis. The products are stabilized to ensure that the product maintains the ortho poly blend ratio as manufactured and require a somewhat lower treatment dosage compared to traditional polyphosphate applications used primarily for colored water and scale control.
The ortho polyphosphate blends offer limited effectiveness in aggressive or low hardness waters. They can increase lead residuals at the tap if the phosphate dosage is too high, and offer only modest galvanic corrosion control. However, these products are effective over a broad pH range and provide effective copper control in high hardness waters. The trend for this group of products is toward much wider product proliferation both in blend ratio and phosphate species used.
Linear Chain Polyphosphates Scaling Waters
Advantages:
- most effective sequestering agents
- stabilize hard waters
- prevents and removes scale deposits
- prevents "mud balling" (filter aid)
Limitations:
- poor galvanic corrosion inhibition
- can increase lead residuals
- high dosages required
Use of this technology is appropriate in waters that are very hard and/or have very high levels of source iron and manganese that demand sequestering.
The linear chain polyphosphates are the most effective sequestering agents and at sufficient dosages can "scour" a distribution line and remove existing deposition. Fed prior to filtration, linear chain polyphosphates will prevent the calcium deposition referred to as "mud balls".
Manufactured entirely using condensed polyphosphate, these products are typically relied upon to sequester very high levels of iron or manganese and stabilize very hard waters. They are therefore used at higher threshold dosages as compared to other technologies. At sufficient dosages, the polyphosphate in these formulations can increase lead concentrations at the tap by exposing contaminating surfaces as scale is gradually removed.
Silicate Phosphate Blends Aggressive and Scaling Waters
Advantages:
- wide range of application (corrosive and scaling waters)
- effective lead protection
- appropriate for open finished water reservoirs
- moderate galvanic corrosion inhibition
- non-zinc
Limitations:
- high dosage typically required
- long passivation required
- long-term storage concerns
- industrial customer economic concerns
Primarily used in corrosive
The silicates possess limited sequestering capabilities and inhibit corrosion by means of film formation. Field experience has shown that silicates must be administered at a very high dosage to provide equivalent treatment results as compared to the other technologies. In addition, silicate additives present particular economic difficulties to industrial customers using municipal water to operate boiler and cooling water equipment.
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