Phosphorus (2nd of 2 articles)
How Do I Get That Pretty Blue Color?
The first part of these phosphorus articles (Why Be Concerned About Phosphorus?) addressed the role of phosphorus in the environment and the reasons why this nutrient needs to be treated by wastewater treatment plants. The article also described the different forms of phosphorus and how one needs to convert organic and condensed forms of phosphorus to orthophosphate, an easily measurable form for analysis.
As with all chemical analyses, accurate results start with the cleanliness of the sampling equipment and with proper sample protocol. These are especially important when a low level of detection must be achieved, a level at which many wastewater treatment plants that remove phosphorus must meet.
Samples for phosphorus analyses can be collected in either glass or plastic containers. Prior to sample collection, wash containers with a non-phosphate detergent, rinse with hot dilute hydrochloric acid, then follow with several rinses of reagent water. You should also follow this procedure in cleaning all of the other glassware associated with phosphorus analyses: digestion glassware, the spectrophotometer’s cells, etc. It’s even better if you dedicate this glassware only for phosphorus testing.
For total phosphorus analysis, the sample must be immediately acidified to a pH below 2 using sulfuric acid and then cooled to 4 degrees C. Samples preserved this way can be stored up to 28 days. However, if only the simpler form of phosphorus (orthophosphate) needs to be determined, filter the sample through a 0.45-micron pore diameter membrane filter (this is the same type of filter you would use for coliform analyses), then cool to 4 C. Analyze orthophosphate within 48 hours.
In the wastewater field, it is generally believed that a 0.45 um pore diameter membrane filter separates suspended phosphorus from dissolved, and that this dissolved phosphorus is mostly orthophosphate. While not totally accurate, you must follow this test-defined orthophosphate filtration procedure to be able to compare other orthophosphate values within the wastewater field.
All samples for total phosphorus must be digested with acid and heat to convert any phosphorus compounds to dissolved orthophosphate. There are a number of digestion procedures that will break down the different forms of phosphorus into orthophosphate. However, the only procedure that is EPA acceptable for NPDES reporting is a persulfate digestion.
Once the sample has been digested it can be analyzed for orthophosphate. There are a number of ways to do this, but like the digestion procedure, EPA accepts only one way to do this. Only the ascorbic method is approved for NPDES purposes. This is done colorimetrically on a spectrophotometer. Certain chemicals are reacted with orthophosphate to form a blue-colored compound. The intensity of this blue color indicates the amount of orthophosphate present in the sample. The details of both the persulfate digestion procedure and the ascorbic acid procedure are found in Standard Methods for the Examination of Water and Wastewater and in EPA’s Methods for Chemical Analyses of Water and Wastes.
Reporting phosphorus values has long been a source of headaches for technicians and operators. Phosphorus is reported as a chemical species, most commonly being “as PO4” or “as P.” Think of “as PO4” and “as P” in a similar way as measuring an item in both pounds and in kilograms. The weight is the same, but there are two ways to express it. Know which phosphorus species your spectrophotometer is calibrated to, what your standards are measured in, and what you should report on you NPDES permit.
As you can see, if you report phosphorus “as PO4” and your permit requires you to report “as P,” you may be reporting a violation, when in fact, you may be meeting your NPDES permit limit.
If you have measured a sample “as PO4,” you can easily convert this to “as P” by dividing the PO4 value by 3.07. For example, a value of 6.21 mg/L as PO4 converts to a value of 2.00 mg/L as P (6.21/3.07 = 2.00). The amount of phosphorus hasn’t changed, only the way this amount is reported.
The procedures for analyzing environmental samples for phosphorus are easy and straightforward. The main problems you will most likely encounter are interferences and contamination, especially at the low levels that are now required in many of the recent NPDES permits. Using clean equipment, following proper sampling protocol, and following only EPA approved methods and procedures will help assure NPDES compliance.
The information in this article is very general. As usual, check your federal, state, and local regulations. You may have additional regulations or requirements that you must meet.
If you have any questions, suggestions, or comments, contact NEWEA Lab Practices Committee Chair Tim Loftus at (508) 949-3865 firstname.lastname@example.org. For more information on the NEWEA Laboratory Practices Committee, please contact Tim Loftus or Elizabeth Cutone, NEWEA Executive Director, 100 Tower Office Park, Woburn, MA 01801, (781) 939-0908, email@example.com.