Forms of Nitrogen in Wastewater
Total Nitrogen (TN) is the sum of nitrate-nitrogen (NO3-N), nitrite-nitrogen (NO2-N), ammonia-nitrogen (NH3-N) and organically bonded nitrogen. Total Nitrogen (TN) should not be confused with TKN (Total Kjeldahl Nitrogen) which is the sum of ammonia-nitrogen plus organically bound nitrogen but does not include nitrate-nitrogen or nitrite-nitrogen.
TN is sometimes regulated as an effluent parameter for municipal and industrial wastewater treatment plants, but it is more common for limits to be placed on an individual nitrogen form, such as ammonia. Treatment plants that have a TN limit will usually need to nitrify and denitrify in order to achieve the TN limit. (See ChemScan Application Summaries #94 “Biological Nutrient Removal”, #96 “Nitrification Process Control” and #59 “Denitrification Process Control”.)
Because nitrogen in wastewater can be found in four major forms (excluding trace amounts of nitrogen gas), each major form is generally analyzed as a separate component, with Total Nitrogen calculated from the sum of the four forms.
Nitrogen in freshly polluted water is originally present in the form of organic nitrogen and ammonia. Natural biochemical processes slowly convert the organic nitrogen into ammonia, which is the form of nitrogen best able to be utilized as a nutrient by microorganisms in the treatment process. (Some waste waters may be nitrogen deficient and require supplemental ammonia for adequate reproduction. See Application Summary #95 “Nutrient Balancing”.) Under aerobic conditions the conversion of organic nitrogen into ammonia reaches a peak and, under the appropriate biological conditions, is biochemically oxidized first into nitrite, then into nitrate. When nitrite and ammonia nitrogen are at minimum concentration (at or near zero) and nitrate is at a maximum value, the wastewater has been fully nitrified. A fully nitrified wastewater will have little or no organic nitrogen, as shown in Figure l.
Laboratory Analysis Methods
Measurement of organic nitrogen is difficult due to the need to digest the sample prior to analysis in order to convert the organic nitrogen into a form that is more amenable to analysis. A Kjeldahl digestion converts the organic nitrogen to ammonia but requires collection of a distillate, which is not easily accomplished by an on-line analyzer in the
field. The persulfate digestion method oxidizes all nitrogenous compounds into nitrate, but requires heating the sample with a digestion reagent for 30 minutes at 110oc, then cooling to room temperatures before analysis. This is also difficult to automate under field conditions.
ChemScan Analysis Methods
Because organic nitrogen is a very small component of Total Nitrogen in fully nitrified wastewater effluent, the standard ChemScan method will use the sum of the individual analysis results for ammonia-nitrogen (See Method #4l), nitrate-nitrogen (See Method #56) and nitrite-nitrogen (See Method #57). Organic nitrogen can be estimated mainly from the nitrite and ammonia concentration. High nitrite-nitrogen is indicative of incomplete nitrification or denitrification.
Samples should be obtained from intermediate points within the nutrient removal process for control applications. The individual values and trends for ammonia, nitrate and nitrite at intermediate and effluent sample points are far more instructive about the state of the process than the TN value.
Measurement of TN in raw influent wastewater will require analysis of the organic fraction. A rapid UV-persulfate oxidation method could be used for this application, but requires additional cycle time and reagent expense. Raw wastewater samples (especially prior to screens, grit removal, FOG removal and primary clarification) are difficult to analyze and are of questionable value. The Total Nitrogen load on the process is the sum of the soluble organic nitrogen and ammonia contributed by the incoming wastewater plus any nitrogen contributed by the Return Activated Sludge plus the nitrogen from any recirculation flow back from the nitrification stages of the treatment process. The ideal initial sample point for process load estimation is after RAS and recirculation addition to the primary effluent. If denitrification is performed in an initial anaerobic zone, separate measurement of nitrate and nitrite in the influent and effluent to the denitrification zone may be a valuable parameter for process control.
The ChemScan Process Analyzer can accommodate samples with up to 150 mg/l of total suspended solids and turbidity of up to 60 NTU. Samples extracted from points in the treatment process ahead of secondary clarification will typically exceed these solids or turbidity specifications. These samples will require filtration or settling prior to analysis to produce a sample meeting ChemScan solids and turbidity requirements. ChemScan has cross flow membrane filters and porous plastic cyclic filters available for use with on-line analyzer systems. Sample points should be selected to assure that fat, oil and grease (FOG) will be low enough not to interfere with the sample filtration method selected for the analyzer system.