Potable Water

The Use of Phosphates For Potable Water Treatment

The Phosphate Forum of the Americas has prepared this document as an educational resource for the general public.

I. Introduction

Phosphates have many uses in the treatment of potable (drinking) water. They are used to prevent “red” (from iron) and “black” (from manganese) water; to prevent and/or retard scale formation (from minerals depositing) and corrosion (from low pH and/or dissimilar metals) in the water distribution system; and to reduce soluble lead and copper in potable water delivered to the consumer’s tap.

The Environmental Protection Agency (EPA) administers the Safe Drinking Water Act (SDWA), which provides for the enhancement of the safety of public drinking water supplies through the establishment and enforcement of nationwide drinking water regulations. Congress gave the primary responsibility for establishing regulations to the U. S. Environmental Protection Agency (EPA). Until 1990, EPA administered a certification process for chemicals, including phosphates, to be used for potable water treatment. In 1990, the National Sanitation Foundation International (NSF) assumed responsibility for the total certification process. The process involves several steps. The toxicology database and impurity profiles are thoroughly reviewed by NSF’s toxicology staff. NSF then audits all manufacturing locations. Samples are taken and analyzed to confirm impurity data submitted on certification applications. Raw materials used in the process are verified against submitted lists and any gaps must be filled. The raw material suppliers are also required to submit detailed information similar to the product application.

II. Selected Properties of Phosphates for Potable Water Treatment
This section discusses the selected properties of phosphates that make them suitable as additives for potable water treatment.

A. Sequestration

Sequestration is a chemical combination of a chelating agent and metal ions in which soluble complexes are formed. Hardness ions are metal ions commonly found in water and include calcium and magnesium. Sequestration is dependent upon pH; a given sequestrant works best in a particular pH range. Sodium hexametaphosphate (SHMP) performs very well at neutral pH ranges, while pyrophosphates and polyphosphates work best under alkaline conditions.

B. Threshold Activity

Many polyphosphates can accomplish the desired effect at levels far below that which would seem to be required for a stoichiometric (molar equivalent) reaction. For example, water containing 200 parts per million (ppm) hardness (as calcium carbonate, or CaCO3) would theoretically require about 500 ppm of SHMP to sequester the available calcium. Actually, only 2-4 ppm of SHMP is typically used to inhibit scale formation. This “threshold effect” of SHMP apparently occurs by interfering with early crystal growth.

C. Deflocculation

Flocculation occurs because small dissolved particles, typically those less than 10 microns insize, tend to attract one another due to the presence of regions of positive and negative charges on each particle. These groups of particles, which clump together due to the interaction of opposite charges, form hard deposits out of a water solution. To prevent this process, threshold levels of polyphosphates are added to the water. These threshold levels of polyphosphates tend to coat the small particles and reduce their attraction to each other by changing the surface charge distribution. These coated particles tend to repel rather than attract one another – hence deflocculation. Deflocculated particles are suspended in water and show little or no tendency to settle in standing water. This property is important for the removal of existing hardness scale deposits (CaCO3) and iron oxides.

D. Chlorine Stability

Ortho- and polyphosphates are stable in the presence of chlorine at the levels found in chlorinated potable water. There are no interactions that reduce the levels or effectiveness of either the chlorine or polyphosphate. In addition, iron and manganese sequestered as colorless complexes before chlorination will remain colorless after chlorination.

E. Hydrolytic Stability

In solution, linear polyphosphates undergo slow hydrolysis. This process continues as the shorter chain phosphates break down further to yield still shorter chain polyphosphates, metaphosphates and orthophosphates. Under neutral pH and normal room temperatures, this hydrolysis is relatively slow. At 20o C and a pH of 7, about 50% of pyrophosphate (the shortest polyphosphate) will revert to orthophosphate in 12 years. At 50o C, the half-life is reduced to two months. Generally, lower pH and higher temperatures will increase the rate of hydrolysis. Because long chain polyphosphates will break down into shorter, but still functional chains, the overall step-by-step process should be considered in estimating shelf life and product stability.

F. Safety

In 1990, the NSF assumed responsibility for a certification program involving plant inspection, raw material certification and product labeling. The NSF certification process applies to all of a company’s plants and products that could be used in treatment of potable water.

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Boiler Water
Ingredients Phosphate Function
Sodium or potassium tripolyphosphate;
Sodium hexametaphosphate;
Tetrasodium pyrophosphate

Prevent scale buildup in boilers; remove scale which may have accumulated

Uses & Applications of Phosphates in Cooling & Boiler Water Treatment


Phosphates for the Removal of Heavy Metals in Wastewater

The Phosphate Forum of the Americas has prepared this document as an educational resource for the general public.

As our world grows ever more industrialized, so too grows the amount of pollution in our water. Of particular concern is the presence of heavy metal contaminants in wastewater; if not managed properly, these heavy metals can leach into the environment and threaten the health of both humans and the surrounding ecosystem. Regulatory guidelines concerning pollutants in wastewater all around the world are becoming more strict as the environmental impact of heavy metal contamination becomes more clear, so efficient strategies to mitigate this risk and maintain legal compliance are critical for nearly every industry. While there are a few different methods for removing heavy metal contaminants, phosphate-based technologies offer one of the most efficacious and low-cost solutions for wastewater treatment operations. What are heavy metals and how do they get into wastewater?

Heavy metals are metallic or semi-metallic elements that are toxic to living organisms. These metals are naturally present in the Earth’s crust. While some of these metals like copper, selenium, and zinc are essential to life in trace amounts, they have the potential to accumulate and cause harm. Other heavy metals include lead, mercury, iron, cadmium, and arsenic, which are commonly found in wastewater.

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Water Softener
Ingredients Phosphate Function
Sodium hexametaphosphate; Sodium tripolyphosphate Chelator; reacts with the calcium and magnesium by removing water
Water Treatment
Ingredients Phosphate Function
Phosphoric acid; Sodium acid pyrophosphate;
Tetrasodium pyrophosphate

Forms the coating for the surface of pipes to prevent leaching of lead and isolate minerals, such as iron; click here to learn more.

Phosphates in Pool Water

If you own a swimming pool, you’ve likely heard the myth that you need to regulate your pool’s phosphate concentration in order to prevent algae growth. A recent blog post by The Pool Butler, Phosphate Remover Fact and Fiction, does a great job of explaining why that’s not true. Algae growth in swimming pools is prevented by basic pool maintenance, including chlorination or other chemical sanitization, and debris removal.

What are phosphates and why are they in my pool?
A phosphate is a chemical compound of phosphorus and oxygen, both naturally occurring elements which are present almost ubiquitously in the environment as well as the human body. Phosphates contain essential nutrients that are necessary for human, animal, and plant life. We use phosphates every day in food and industrial applications, so they’re just as likely to be in the sandwich you ate for lunch as they are in your pool water.

Phosphates might be introduced to your pool through water from municipal sources or by the dirt and leaves and other debris that naturally collect in your pool over time. Over the past couple of decades, a popular myth has circulated among swimming pool owners which claims that excessive accumulation of phosphates can feed algae blooms in pools, and that special “phosphate removers” are needed to prevent this from happening. The fact is, swimming pools are carefully maintained bodies of water–as long as you are regularly cleaning your pool and performing basic maintenance, there’s no reason to worry about algae overgrowth.

Why shouldn’t I use phosphate removers in my pool?
There are plenty of other reasons why your pool doesn’t need phosphate removers. Namely, if your pool water comes from a well or otherwise contains excessive copper or iron, you likely use a metal sequestrant to prevent these metals from turning your pool water an unappealing color. The most effective metal sequestrants are formulated with phosphates, which bind these metals and allow them to be filtered out of your pool water. If you were to use a phosphate remover, you would just be taking out the sequestrant and negating its helpful action—not only does that make no sense, it’s also a waste of your money!

In short, there’s no reason to fall for the misinformed hype surrounding phosphate removers. Regular cleaning and maintenance is all you need to ensure that your swimming pool stays safe, clean, and enjoyable. This includes monitoring the pH and chlorine concentration of your pool water, along with effective filtration and regular use of an algaecide. If you have questions about phosphates and their role in our modern world, check out the Phosphate Forum of the Americas for helpful information.

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