As nutrient reduction is being implemented at wastewater plants, NPDES permit limits are changing. You may see phosphorus, nitrite and nitrate popping up on your next permit! If you already had these limits, you may see your limits lowered, or your sampling frequency may increase. Either way, this means more testing, which means more paperwork – and unfortunately, more money. You could always use a contract environmental lab, but as the sampling frequency increases, it may become more cost-effective to run these test in-house.
Let’s discuss total phosphorous first. In my opinion, it is the easiest test to run in-house. Most importantly, in order for the test to be approved for reporting effluent permit limits (see 40 CFR 136), the samples must be digested before testing. Hach has made this test as easy as it can be. Of course you don’t have to do the Hach method; you can use a hot plate and an erlenmeyer flask. (Go look in Standard Methods 4500-P B.) But this requires preparing the reagents and watching the sample as it gently boils, adding distilled water to keep the volume between 25-50 mL.
Back to the Hach Total Phosphorus TNTplus method – this particular test is for Total Phosphorus with a range of 0.05-1.5 mg/L PO4-P. (Think of the dash as the word “as”, so in other words PO4 as P.) The digestion process takes an hour, and then another hour or so for cool-down and color reaction. The second most important step is to make sure your colorimeter or spectrophotometer is set to read in mg/L P, not mg/L PO4. Permit limits are for Total Phosphorus (mg/L, P). PO4 values are 3.065 times higher than just P. Remember this when you purchase standards! If you purchase a 50 mg/L PO4 standard, this will be equal to 16.31 mg/L P (or PO4-P).
Nitrite is also easy to run in-house with one of three Hach methods. You can choose the Powder Pillow method, AccuVac method, or the TNTplus method. Pick whichever one is best for your budget and existing lab equipment.
Nitrate, on the other hand, is not so easy. Hach has colorimetric methods, but they are not EPA approved methods for wastewater. Some of the methods are approved for drinking water – but remember, wastewater is not as clean, and we test for much lower levels than we would for drinking water. If you want to use the Hach Nitrate Powder Pillow method for process control, that would work well. But be aware: nitrite at all levels can interfere with the Powder Pillow method. Page 5 of the method describes how to compensate for the nitrite interference.
The approved methods for Nitrate listed in 40 CFR 136 are Ion Chromatography, Capillary Ion Electrophoresis/UV, Ion Selective Electrode, Colorimetric (Brucine sulfate) or NO2NO3 minus NO2 by Cadmium Reduction. Out of these choices, the Ion Selective Electrode will probably be the most cost-effective method. This test will require daily calibration on test days, but once you get used to that, it should be fairly easy.
Related PDFs:
Hach Total Phosphorus TNTplus method
Hach Nitrite Powder Pillow or AccuVac Ampuls method
Hach Nitrate Powder Pillow method
Related Websites:
