TECHTIP: Interpretation of Phosphate Results

This article is part of a series that focuses on all things related to water and wastewater treatment.  Targeted to those working in the industry, each TechTip article, written by a U.S. Water operations & maintenance expert, presents relevant topics, timely issues, and lessons learned from the field.

The difference between Phosphate and Phosphorus concentrations can be confused, resulting in poor treatment plant performance. The many Hach testing methods measure the phosphate ion concentration rather than the elemental phosphorus concentration. When evaluating nutrient concentrations, misreporting the phosphorus concentration can result in impaired biological growth rates and adversely impact nitrification.

Phosphorus is a critical nutrient in wastewater to support healthy bacteria. Phosphorus is an essential component of Adenosine Triphosphate (ATP), helps activate enzymes used to breakdown complex molecules, and is a key component of cell membranes. The necessary ratio of biological oxygen demand (BOD) to phosphorus to support biological growth and health is 100:1. This means for every 100 lbs. of BOD, 1 lb. of phosphorus is required.

What Is Phosphate?

Elemental phosphorus (P) cannot be directly used by cells but must be in the phosphate ion form (PO4-3).

Phosphate is referred to using many different terms that are interchangeable: phosphate, monophosphate, orthophosphate, and reactive phosphorus.

The several different Hach tests used to measure phosphorus use a spectrophotometer (such as the Hach DR-3900) and displays concentration as mg/L phosphate ion (PO4-3) by default.  Some of these tests are: 1)The reactive phosphorus test measures the concentration of dissolved phosphorus as the phosphate ion; 2) The total phosphorus test uses a combination of a strong acid and heat (digestion) to breakdown phosphorus that is bound up in complex molecules such as that bound up in cells. This bound phosphorus is released and combines with oxygen to become phosphate ions. The total phosphorus test then measures the concentration of dissolved phosphorus as the phosphate ion.

Regardless of the Hach test used, the concentration of reactive phosphorus (the phosphate ion) is ultimately being measured.

Avoiding Confusion Between Phosphorus and Phosphate Concentrations

Where phosphorus limits are present in discharge permits, the actual concentration of phosphorus or total phosphorus must be reported, as opposed to reactive phosphorus.

To avoid confusion, our operators report all phosphate concentrations as phosphorus (P) concentrations. To differentiate between the typical reactive phosphorus test and the total phosphorus test, in our Process Control MonitoringTM (PCM) operations system, we specify phosphate as P (PO4-3-P) for the reactive phosphorus test and Total Phosphorus (P) for the total phosphorus test.

The table below presents a comparison between phosphorus and phosphate.

Characteristics Phosphorus Phosphate
Definition A multi-valent non-metal chemical element of the VA group A chemical derivative of the phosphoric acid, containing the phosphate ion (PO3-4)
Molar Mass 30.97 g/mol 94.97 g/mol
Chemical Formula The symbol of phosphorus is P. The chemical formula of the phosphate ion is PO4.
Structure Phosphorus forms several simple substances, the most widely distributed are white phosphorus, consisting of P4 molecules, and red phosphorus, which has a crystal atomic lattice. The phosphate ion is a polyatomic ion with a tetrahedral arrangement of the atoms. It consists of one phosphorus atom, located in the center and surrounded by four oxygen atoms.
Oxidation Degree -3, +3, or +5 -3
  • White phosphorus
  • Red phosphorus
  • Orthophosphates
  • Pryophosphates
  • Tripolyphosphates
  • Polyphosphates

Converting Phosphate Concentrations to Phosphorus Concentrations

The concentration in mg/L phosphate ion (PO4-3), can be converted to mg/L phosphorus (P) using molar ratios.

A mole is a term used to represent a number, much like the term dozen represents a number. A mole is 6.022×1023 (602,200,000,000,000,000,000,000), though the number itself is not important. We need only understand that the periodic table of elements displays atomic mass as the mass of one mole of atoms of that element.

The atomic mass of phosphorus is approximately 31g per mole and the atomic mass of oxygen is approximately 16g per mole.

The phosphate ion (PO4-3) is made up of 1 phosphorus atom and 4 oxygen atoms. The mass of one mole of the phosphate ion is then 1(31g) + 4(16g) = 95g. To use a molar ratio, you simply need to compare the mass of the phosphorus portion of the ion to the total mass of the ion:

31g / 95g = 0.326

Using this molar ratio, 5mg/L PO4-3 is 5mg/L * 0.326 = 1.63mg/L phosphorus (P).

Programming the Hach Spectrophotometer

The Hach spectrophotometer can be programmed to perform this molar ratio conversion for you.  Follow these steps to program the Hach DR-3900 spectrophotometer:

1. In the results display after reading a sample, select Options–More–Chemical Form;

2. The available chemical forms are displayed, as shown below:

3. Select P to report and display the chemical form in the unit PO4-3-P (phosphate as phosphorus);

4. Select ‘Save as Default’ so this chemical form will be used for the current and all future measurements. This default setting will remain the default chemical form even after a power loss;

5. The analyst must ensure the displayed units in the result screen are PO4-3-P whenever performing a reactive phosphorus or total phosphorus analysis.

Phosphorus Analytical Reporting as P

Successful operation of a biological treatment plant, especially one that removes ammonia, requires testing of the mixed liquor for critical parameters such as phosphorus and the correct interpretation of those analytical results by both the on-site operators and the process engineers monitoring the treatment plant performance.

Misreporting the phosphorus concentration as orthophosphate (the phosphate ion) will result in the process engineer believing the biological system has sufficient phosphorus to perform both the organic matter removal and the ammonia removal through nitrification. The reality is that the biological system may be deficient in this critical nutrient.  The end result may be impaired growth rates, reduced nitrification, and potential permit violations.

Another consequence is the extensive time the operators and/or process engineers must spend investigating the reason for the poor treatment plant performance.  Analytical results must be accurate and must be interpreted correctly.