The last article covered nitrogen in
wastewater and in the environment, focusing specifically on ammonia-nitrogen,
its collection, preservation, and analysis. This article will review some of the
important aspects of the other forms of nitrogen in wastewater – organic
nitrogen, nitrate-nitrogen, and nitrite-nitrogen.
Organic nitrogen is somewhat of a
misnomer. The term “organic nitrogen” in wastewater does not represent all the
nitrogen found in organic matter; it is only a portion of it, mostly consisting
of protein, urea, and certain components of cells (like nucleic acids). So when
you review organic nitrogen results, you are looking at the results of a
test-defined method. Only when analysts all follow the same methods for organic
nitrogen analyses can the results be compared.
Fortunately chemists have developed
a standardized test to do just that. It is called Total Kjeldahl Nitrogen (TKN)
analysis, where the result is the total of the organic nitrogen plus any
ammonia-nitrogen in a sample. For TKN, a sample is digested using a particular
cocktail of chemicals to convert the organic nitrogen portion of the sample into
ammonia. The sample is further distilled and the ammonia concentration is
determined using one of the accepted EPA procedures. From a replicate sample,
the ammonia-only concentration is determined. The difference between the TKN
value (organic and ammonia combined) and the ammonia value will be the organic
nitrogen.
Samples for TKN can be collected in
either glass or plastic bottles. After a pH adjustment to less than 2 using
sulfuric acid and cooled to 4 degrees C, a sample will have a holding time of up
to twenty-eight days before analysis.
Typically, less than one percent of
the nitrogen in raw wastewater is in the form of nitrates (NO3 -) or nitrites
(NO2 -). So why be concerned about this type of nitrogen? Most of the nitrates
and nitrites found in a treatment facility are a result of biological action in
the mixed liquor. Under the right conditions in biological treatment, a lot of
the organic nitrogen is decomposed into ammonia. Certain bacteria then oxidize
the ammonia to nitrite. Other bacteria further oxidize the nitrite to nitrate.
As a result, nitrates are very often the most predominant nitrogen compound
leaving a biological treatment facility.
Under low oxygen conditions in a
wastewater treatment facility or in the environment, some types of
microorganisms that require oxygen to survive can scavenge the needed oxygen
from nitrate molecules, converting the nitrate to nitrogen gas in the process.
Uncontrolled, this can cause a lot of matter to float, either in a wastewater
treatment facility clarifier designed for matter to sink, or in the environment
where sediment therefore becomes mobile and may locate to undesirable locations
downstream.
For NPDES purposes, the only
approved procedure for nitrite analysis is a fairly straightforward colorimetric
method. After the sample is pH adjusted, the nitrite is reacted with another
chemical creating a reddish-purple solution. The intensity of this color is
related to the concentration of nitrite. Samples for nitrite analyses can be
collected in either glass or plastic containers. Holding time is only
forty-eight hours at 4 degrees C.
The most common way to analyze for
nitrate in a sample is to convert the nitrate to nitrite either by a cadmium
reduction column or by using hydrazine sulfate. The resulting solution is then
analyzed for nitrite. A replicate sample with no nitrate-to-nitrite conversion
is analyzed for nitrite only and the difference in value between the two samples
is the nitrate value. Samples for nitrate-nitrite analyses can be collected in
either glass or plastic containers. After a pH adjustment to less than 2 using
sulfuric acid and cooled to 4 degrees C, a sample will have a holding time of up
to twenty-eight days before analysis.
The analysis of nitrogen is
important in assessing the impact of this nutrient on our wastewater facilities
and in our local waterways. Knowing which form of nitrogen is prevalent in a
system will help in determining the appropriate treatment or remedial actions,
if needed.
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. Approved methods for NPDES
reporting purposes are listed in Chapter 40 of the Code of Federal Regulations,
part 136.
