Phosphates

 

 

What Are Phosphates?

Phosphates are water treatment chemicals used to solve specific water quality problems resulting from inorganic contaminants (iron, manganese, calcium, etc.) in ground water supplies and also to maintain water quality (inhibit corrosion, scale, biofilm, reduce lead and copper levels) in the distribution system. Orthophosphate and polyphosphate are two general types used in water treatment along with many different phosphate compounds that exist for use in the water treatment process. Ortho and polyphosphates work together, stabilizing water quality and minimizing color, scale, deposits, corrosion, and chlorine demand in drinking water systems.

 

 

Are Phosphates New Chemicals?

Elemental phosphorus was discovered in 1669, but commercial production of orthophosphate did not begin until about 1850 when phosphoric acid was used for fertilizer. Polyphosphate was first described in 1833, but not until 1929 was it used as a sequestering agent that formed soluble complexes with metallic ions in water. In the 1960’s orthophosphate was combined with zinc salts to produce zinc orthophosphate (ZOP) corrosion inhibitor. ZOP was patented and used extensively to control corrosion in water systems. The EPA Lead and Copper Rule (1991) identified ZOP as a ‘best available technology’ (BAT) to minimize the leaching of lead from water lines and brass fixtures into the drinking water. During the period 1970-2000, various blends of phosphates, both orthophosphate and polyphosphate, were recognized as multi-functional chemicals used to sequester metals and inhibit corrosion.

 

 

What Are The Problems That Phosphates Help To Solve?

Phosphates are used in municipal water systems to perform three broad functions: inhibit corrosion of water mains/plumbing (iron, steel, galvanized, asbestos/cement, lead, copper), sequester nuisance metals in the water supply (iron, manganese, calcium, magnesium). They can also improve the quality of water in the distribution system by removing scale deposits & tuberculation, discourage microbial film formation/regrowth, and stabilizing free chlorine disinfectant residuals.

 

 

How Widely Are Phosphates Used In Drinking Water Systems?

Estimates suggest that 15-20% of public and private water systems use some form of phosphate in the treatment of their drinking water. Groundwater supplies use polyphosphate to sequester iron, manganese, calcium, and magnesium, while surface plants use orthophosphates, ZOP, or blends of phosphates to inhibit corrosion in the distribution system. All systems can use phosphates to meet the EPA regulations on Lead and Copper.

 

 

Are Phosphates A Cure-All?

Rarely is a single treatment process or chemical additive a cure-all. Any chemical used in water treatment may have particular advantages or disadvantages. Water quality and treatment methods vary greatly. However, application of phosphates has been considered one of the most cost effective means of controlling a multitude of problems.

The American Water Works Association Research Foundation (AWWARF) and the EPA have reported that corrosion control (phosphate use included) provides numerous health and consumer benefits at a rate of return much greater than the original cost of the additive. EPA Lead & Copper Rule Guidance suggest that annual expenditures of $200 million/year on corrosion inhibition yields approximately $4.3 billion in consumer benefit (20-fold increase).

 

 

How Do Phosphates Work In A Water System?

Orthophosphate based additives are classified as corrosion inhibitors and as such react with dissolved metals (e.g. Ca, Mg. Zn, etc.) in the water to form a very thin metal-phosphate coating or it reacts with metals on a pipe surface to form a microscopic film on the inner surface of the pipe that is exposed to the treated water.

Polyphosphate type chemicals react with soluble metals (iron, manganese, calcium, magnesium, etc.) by sequestering (bind-up) the metals to maintain their solubility in water. The phosphate sequestering process minimizes the risk of discoloration, staining, scaling, taste/odor and other water quality complaints.

 

 

How Effective Are Phosphates At Controlling Color?

Color in a water system may be the result in the precipitation of soluble iron/manganese when they react with dissolved oxygen, chlorine disinfectant, and other oxidizing agents during water treatment. Polyphosphates bind-up the Fe/Mn, keeping them in solution and preventing the color from initially forming. Another source of color is the natural release of soluble iron by-products that appear to be ‘bleeding’ from the scale deposits (tuberculation) inside water pipes. Polyphosphates bind with soluble iron before it turns color (precipitates), while orthophosphates react with the pipe surface to slow down pipe corrosion and the release of corrosion particulates. Blended mixtures of ortho/ polyphosphates control both sources of potential color at the water supply and in the distribution system.

 

 

Do Phosphates Affect Trihalomethane (THM) Formation?

The primary cause of THMs is pre-oxidation, with chlorine, of raw water that contains organic precursors. The presence of biological regrowth (biofilm) inside water pipes is another source of organic precursors. After chlorine reacts with organic material, THMs may form. Alternative disinfectants or treatment adjustments will often reduce THM formation. However, phosphates also contribute to lowering system THMs. Phosphates inhibit corrosion effectively in a lower pH (7-7.5) range. THM formation potential is significantly reduced when water is chlorinated at a lower pH (< 8.0). Phosphate inhibitors/sequestering agents minimize corrosion by-product scale formation inside the pipe, thereby keeping the pipes cleaner and free of the biofilm that may generate additional organic precursors if left uncontrolled.

 

 

What Other Benefits Of Phosphate Treatment Exist?

Phosphates easily adapt to any pre-existing water quality without changing the water chemistry. Referred to as inhibitors (ortho), sequestrants (poly), or blends (ortho/poly), phosphates have a selective function, yet wide range of performance. Primary treatment benefits include: corrosion control, lead/copper control, sequestration of iron/manganese, control of calcium carbonate scale, and water softening, etc.

Many secondary benefits develop, such as: reduced chlorine demand due to corrosion inhibition and sequestration of Fe/Mn, lower color and turbidity in the distribution system, less staining, removal of system scale deposits, control of biofilm regrowth, lower TOC, fewer system coliform violations, increased C-factors and hydraulic flow rates in system, reduced electrical demand, fewer main breaks, better valve operation, improved meter accuracy, increased revenue, reduced hydrant flushing frequency, less wasted water during flushing, less maintenance and service expenditures, fewer complaint calls, and overall improved consumer satisfaction.

 

 

Are Phosphates Safe and Approved For Water Systems?

Various forms and purity grades of phosphates exist. Most dry powders and liquid concentrates are safe to handle and store, except for the standard precautions required for orthophosphate acids and zinc orthophosphate solutions. All Carus phosphate additives are either food quality grade or certified to ANSI/NSF Standard #60 Drinking Water Treatment Chemicals as approved for use in potable drinking water. Material Safety Data Sheets (MSDS) are available for all Carus products.

 

 

How Much Phosphate Is Required to Provide Expected Results?

The most effective dosage rate is determined by running a complete water analysis to determine the total demand of the finished water and the consumption rate of the distribution system. The ANSI/NSF Standard #60 authorized listing of all certified drinking water treatment chemicals has limited the application of inorganic phosphates at 10 mg/L as total phosphate ion. In most cases, this is not a health related or safety limitation, but a practical guideline for the maximum required quantity of phosphate typically applied in drinking water. Most ground and surface water supplies contain naturally occurring phosphate at low levels. However, additional phosphate chemicals (1-5 mg/L) are added to control most Fe/Mn, scale or corrosion concerns. There are 20 distinct phosphate species listed by ANSI/NSF Standard #60 for use in drinking water with many more proprietary phosphate blends available on the market. Carus Corporation is a leading manufacturer of 29 types of generic and blended phosphates for use in food processing and drinking water. Carus Laboratory staff provide analytical services, feasibility studies, and dosage evaluations for your water system.

 

 

How Are Phosphates Fed?

Phosphate based corrosion inhibitors are injected via a chemical metering pump into finished water separate from other chemical additives (chlorine, fluoride, caustic soda, etc.). They selectively react with Iron, Copper, Lead, Zinc, and Calcium to form an insoluble protective film of metallic-phosphate that passivates new or precorroded piping surfaces in the distribution system.

Sequestering agents are injected via a chemical metering pump at the well head prior to other chemical additives (chlorine, fluoride, caustic soda, etc.), or, if permissible, down the well casing to mix with groundwater at the pump intake. Carus polyphosphates selectively react with Fe, Mn, Ca, and Mg ions to maintain a color-less soluble molecule that resists precipitation caused by aeration, disinfection, oxidation, storage and transmission of finished water.

 

 

How Are Phosphate Dosages Controlled?

Since phosphates do not change water chemistry, measuring phosphate in the raw and finished water is necessary to monitor the dosage rate. Orthophosphate ion (PO4 -) is the most common species used to measure the initial and total quantity of phosphate in the water. Orthophosphate can be measured on a coldwater sample, while the total phosphate requires a digestion step to break down all other forms of phosphate to the orthor form. Simple field test kits or laboratory analytical equipment can be used to monitor all forms. Subtracting the initial orthophosphate quantity from the total phosphate yields the quantity of polyphosphate present in the finished water (Total-Ortho=Poly).

 

 

What Happens If Phosphate is Overfed?

An overdose of phosphate is difficult to detect immediately unless orthophosphate is being monitored in the finished water. Too much orthophosphate typically will not result in a water quality problem unless calcium hardness reacting with the phosphate begins to form a slight turbidity during the film formation process inside the system. Excessive polyphosphate dosage may result in an accelerated cleaning of scale and tuberculation from the pipe surface, resulting in colored water, turbidity or suspended solids.

 

 

Is Phosphate Expensive To Use?

Compared to other technologies of corrosion inhibition or iron/manganese removal, application of orthophosphate, polyphosphate, or blended phosphates is considered economical or at least comparable in cost. Phosphates may range in cost from $0.30/lb for generic orthophosphates to more than $2.00/lb for specialty phosphate blends. At a dosage rate of 1-5 mg/L, the chemical cost may range from less than 1 cent to 10 cents per thousand gallons of water treated, depending on the treatment criteria and water quality. In most every case, the cost of the phosphate is offset by the direct operational and maintenance savings or benefits to the utility and consumer. Case studies and reports record savings from a break-even point up to a 20-fold return above the chemical investment.

 

 

How Is Phosphate Packaged and Shipped?

Dry powder, granular or crystalline phosphates are packaged in plastic lined paper bags (50 lb.) or containers to minimize caking. Carus liquid phosphate concentrates are packaged in medium density polyethylene pails (5 gallon) and drums (15, 30, 55 gallon) or shipped in food quality stainless steel tanker trucks (45,000 lbs).

 

 

Test Procedure

 

 

 

 

 

 

 

Interference

 

Interfering Substances and Levels

Interfering Substance Interference Levels and Treatments
Aluminum Greater than 200 mg/L
Arsenate Interferes at any level
Chromium Greater than 100 mg/L
Copper Greater than 10 mg/L
Hydrogen Sulfide Interferes at any level
Iron Greater than 100 mg/L
Nickel Greater than 300 mg/L
pH, excess buffering Highly buffered samples or extreme sample pH may exceed the buffering capacity of the reagents and require sample pretreatment. pH 2-10 is recommended.
Silica Greater than 50 mg/L
Silicate Greater than 10 mg/L
Turbidity (large amounts) or color May cause inconsistent results because the acid in the powder pillow may dissolve some of the suspended particles and because of variable desorption of orthophosphate from the particles. For highly turbid or colored samples, add the contents of one Phosphate Pretreatment1 Powder Pillow to 25 mL of sample. Mix well. Use this solution to zero the instrument.
Zinc Greater than 80 mg/L

 

 

 

Sample Collection, Storage, and Preservation

Collect sample in plastic or glass bottles that have been cleaned with 1:1 Hydrochloric Acid Solution* and rinsed with deionized water. Do not use commercial detergents containing phosphate for cleaning glassware used in phosphate analysis.

Analyze samples immediately for best results. If prompt analysis is not possible, preserve samples by filtering immediately and storing at 4°C (39°F) for up to 48 hours. The sample should be at room temperature before analysis.

 

 

 

Accuracy Check

Standard Additions Method (Sample Spike)

  1. After reading test results, leave the sample cell (unspiked sample) in the instrument. Verify the chemical form.
  2. Press OPTIONS > MORE. Press STANDARD ADDITIONS. A summary of the standard additions procedure will appear.
  3. Press OK to accept the default values for standard concentration, sample volume, and spike volumes. Press EDIT to change these values. After values are accepted, the unspiked sample reading will appear in the top row. See the user manual for more information.
  4. Open a Phosphate 10 mL Ampule Standard, 50 mg/L PO43-
  5. Prepare a 0.1 mL sample spike by adding 0.1 mL of standard to the unspiked sample. Press the timer icon. After the timer expires, read the result.
  6. Prepare a 0.2 mL sample spike by adding 0.1 mL of standard to the 0.1 mL sample spike. Press the timer icon. After the timer expires, read the result.
  7. Prepare a 0.3 mL sample spike by adding 0.1 mL of standard to the 0.2 mL sample spike. Press the timer icon. After the timer expires, read the result. Each addition should reflect approximately 100% recovery.


    Note: For AccuVac Ampuls, fill three Mixing Cylinders with 50 mL of sample and spike with 0.2 mL, 0.4 mL, and 0.6 mL of standard. Transfer 40 mL from each of the three mixing cylinders to three 50 mL beakers. Analyze each standard addition sample as described in the procedure above. Accept each standard additions reading by pressing READ. Each addition should reflect approximately 100% recovery.


  8. After completing the sequence, press GRAPH to view the best-fit line through the standard additions data points, accounting for matrix interferences. Press IDEAL LINE to view relationships between the sample spikes and the "Ideal Line" of 100% recovery.