Nitrogen is an essential ingredient in the formation of proteins for cell growth. From complex organisms like animals to the simple bacteria used to treat wastes in an activated sludge treatment facility, every living thing needs some form of nitrogen to survive.
But too much nitrogen freely available in the environment can be a bad thing. Excess nitrogen discharged into our waterways can contribute to eutrophication, the gradual change of water bodies into marshes, meadows, then forests. It can also contribute to massive algae blooms leading to oxygen depletion in water and its associated problems. Certain forms of nitrogen can cause specific problems too. Ammonia is toxic to fish, and nitrates at high enough dosages in the drinking water cause methemoglobinemia in infants (Nitrates convert to nitrites in the stomach. These nitrites then interfere with the oxygen-carrying capacity of the hemoglobin in blood).
In the wastewater field we are concerned with several forms of nitrogen: ammonia, organic, nitrate, and nitrite. Under the right conditions, each of these forms is biologically convertible to one of the other forms. This creates certain challenges in the treatment of nitrogen in wastewater. Because of these challenges, it is important to properly collect, preserve, and analyze samples for the specific forms of nitrogen so that the appropriate treatment of these wastes can be made. I’ll forgo the explanations of how the biological processes converts one form of nitrogen to another, leaving that to the wastewater treatment books. Instead, I will focus on the lab aspect of nitrogen analyses – specifically on ammonia-nitrogen.
As with any sample, the accuracy of the ammonia result starts with sample collection. To help keep the sample from biologically or chemically degrading, you must add sulfuric acid to a pH of less than 2 and to cool the sample to 4oC. A sample for ammonia analysis preserved this way will keep up to 28 days. If a sample is analyzed after that amount of time, its results cannot be reported for NPDES purposes. Also, residual chlorine reacts with ammonia. If residual chlorine is present, immediately dechlorinate using sodium thiosulfate. Always neutralize samples with potassium hydroxide or sodium hydroxide before analyses.
Prior to ammonia analysis a sample must first be distilled to remove anything that may interfere with the test method. The sample’s pH is adjusted to pH 9.5 using a borate buffer. This solution is then distilled into a receiving solution of either boric acid (for the nesslerization and titration test methods) or into sulfuric acid (for the electrode and phenate methods).
However, distillation may not be needed if certain conditions can be met. According to Chapter 40 of the Code of Federal Regulations, part 136, Table1B, “distillation is not required if comparability data on representative effluent samples are on company file to show that this preliminary distillation step is not necessary…” Unnecessary distillation will decrease the accuracy of the overall ammonia analysis procedure. If you chose the option of not distilling samples for ammonia analyses, make sure you have the documentation demonstrating that the results of distilled versus nondistilled samples are statistically the same. For more information on performing a comparability study, refer to an earlier column, “A Practical Application of the Student’s t-Test.”
Like many of the constituents in wastewater, ammonia concentrations can be measured using many different test methods. But for NPDES reporting purposes, only certain approved test methods can be used. The list of approved methods is found in 40 CFR 136 Table 1B.
The ammonia concentration of a sample can be measured colorimetrically, by titration, or by an ammonia-selective electrode.
Colorimetrically, the ammonia concentration can be determined two ways. The nesslerization procedure uses a potassium/mercury/iodine chemical that reacts with ammonia, creating a yellow to brown-colored compound. The phenate method reacts phenol and hypochlorite with ammonia to create a blue-colored compound. In both methods, the color intensity is proportional to the ammonia concentration.
In the titration procedure, a color indicator is added to a sample. This sample is titrated using 0.02N sulfuric acid until the indicator turns to a pale lavender color. The amount of acid used to the color change is proportional to the ammonia present.
The ammonia-selective electrode method is probably the easiest to perform. After a pH adjustment to 11, the ammonia in solution diffuses through a special membrane at the tip of the electrode. The change in electrical potential at the electrode is proportional to the ammonia concentration.
Finally, before reporting the results for NPDES purposes, make sure the measured values are in the proper units. Sometimes the ammonia concentration may be measured as ammonia and other times it may be measured as ammonia-nitrogen. One milligram per liter of ammonia-nitrogen is equivalent to 1.22 mg/L of ammonia. Confusion on this can result in reporting an ammonia violation when, in fact, there may not be a violation at all.
The next article will deal with the other forms of nitrogen that we are concerned with in the wastewater field: nitrate, nitrite, and organic nitrogen.
The information in this article is very general. As usual, check your federal, state, and local regulations. You may have additional 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.