By Mike Purcell and Brian Wallace

Summary: Failure to control nitrogen effluent can cost thousands of dollars and take days to recover. Total nitrogen (TNb) by high temperature combustion (HTC) oxidation offers a quick and efficient way to track nitrogen loading throughout a manufacturing process. By identifying problems early, real gains in overall plant efficiency can be attained.


Nitrogen monitoring is an important element of process control for treatment wastewater, seawater and a variety of industrial process water applications. New advances in high temperature combustion (HTC) technology with chemiluminescence detection provide a quick and easy way to monitor nitrogen loading by total nitrogen (TN) analysis.

Nitrogen in wastewater
Wastewater treatment facilities monitor their nitrogen for a variety of reasons. First and foremost, the U.S. Environmental Protection Agency (USEPA) regulates organic nitrogen in wastewater discharge. In addition to regulatory requirements, high amounts of ammonia (NH3), nitrate (NO3) or nitrite (NO2) can cause severe problems in the wastewater treatment process, costing tens of thousands of dollars and can take days to recover. For these reasons, constant nitrogen monitoring is performed to minimize breakdowns in the nitrogen cycle as well as help improve plant efficiency.

Total Kjeldahl Nitrogen (TKN) is the standard method for organic nitrogen analysis of wastewater. The objective of the TKN test is to convert nitrogen from biological origins or organic forms into ammonia through a digestion procedure. The ammonia is then determined through a titration procedure. Hence, TKN is the sum of organic nitrogen and ammonia.

Reaction 1: TKN = Organic Nitrogen + NH3

While effective, TKN has several drawbacks.
Nitrogen is analyzed in the trinega-tive state by TKN. Therefore, nitrogen in the form of azide, azine, azo, hydrazone, nitrate, nitrite, nitrile, nitro, nitroso, oxime, and semi-carbazone aren’t fully detected. Other problems that affect the reliability of the TKN results are found in the digestion procedure. During digestion, if there’s a high amount of salt or acid in the sample, then the digestion temperature will rise above the desired temperature resulting in a loss of nitrogen. If the quantity of acid is too low, however, the digestion temperature will be under the desired level resulting in incomplete digestion.1

Another concern with the TKN method is the extensive use of sulfuric acid as part of the sample digestion process. In most cases, the steps required to safely run this test, and the environmental concerns that have to be addressed, are actually more stringent than the care required to handle the samples. This is especially true when samples that contain pesticides are being analyzed. In summary, Kjeldahl nitrogen is a time-consuming, environmentally unfriendly, and labor intensive test for laboratory personnel to perform.

The HTC method
Fortunately, with the latest developments in bound nitrogen (TNb) analysis, laboratories have a faster, more environmentally friendly and efficient analysis option. Bound nitrogen has been defined as TNb and is considered to be the organically and inorganically bound nitrogen, excluding the elemental nitrogen. Using a HTC technique eliminates concerns raised from TKN analysis and can be used to replace or supplement Kjeldahl nitrogen analysis in wastewater applications.

The High Temperature Combustion (HTC) technique addresses concerns raised by TKN analysis and can be used to replace, or supplement, Kjeldahl nitrogen analysis in wastewater applications.

The HTC technology is already being employed in Germany and other European countries where determination of bound nitrogen is required in freshwater, seawater, drinking water, surface water, wastewater and treated sewage effluent. The requirements are defined in European Norms such as EN-12260 and DIN-EN-ISO 11905-2.2 In these methods, the sample is combusted at up to 1,000°C with the nitrogen in the sample converting to nitric oxide. The nitric oxide is reacted with ozone to produce an excited state of nitrogen dioxide (NO2*)—the asterisk represents the compound in an excited state—which when decaying to its ground state, emits light. The light is then measured with a chemiluminescence detector and correlated to a specific amount of nitrogen in the sample.

Faster analysis time is a major advantage for the HTC technique over the TKN method.

The time of HTC analysis is usually between 10 and 15 minutes versus two to three hours with Kjeldahl. This significant difference in time allows firms using the TNb test to make necessary adjustments to their treatment processes faster than before. As a result, these firms are simultaneously minimizing risks to the environment from higher-than-expected levels of organic nitrogen loading as well as costs to treat the effluent, since firms are able to respond much more quickly to situations.

Screening out pesticides
Through the use of chemiluminescence and HTC, the TNb measurement can represent a good approximation of TKN without the interferences of a digestion process. HTC technology is much more environmentally friendly than the Kjeldahl technique. With less volume needed for analysis, the HTC technique provides less risk of exposure to pesticides and dangerous nitrogen compounds.

In addition to the analytical advantages, advances in HTC technology in recent years make implementing TNb analysis easier than before. Many manufacturers of TOC analytical instruments that use HTC technology now offer a total nitrogen module as an option on new instruments, and some manufacturers offer nitrogen module upgrades for existing TOC instrumentation. These modules, which are substantially less expensive than a standalone instrument, can analyze total nitrogen simultaneously with TOC. This dual TOC/TN measurement is only two to five minutes longer than the standard TOC analysis and involves little additional analyst time. As a result, many firms are using these TOC/TN instruments to supplement both chemical oxygen demand (COD) and TKN testing in their facilities.

While TNb analysis does have many advantages, it’s important to note it isn’t the same measurement as TKN. Nitrates and nitrites are included in the analysis and the HTC technique has its own unique set of interferences as any method, including Kjeldahl nitrogen. These differences are the primary reasons that HTC technology isn’t yet recognized by the USEPA for wastewater organic nitrogen testing; however, the improvements to the HTC technology and methodology provide the basis for a new look at using this technique for nitrogen monitoring in wastewater.

Industrial applications and seawater
Low-level nitrogen monitoring is needed in many industrial applications where the process is sensitive to organisms utilizing nitrogen as a food source. There may also be a need to monitor nitrogen-containing compounds that are detrimental to system operations or quality. More difficult matrices, such as brines and particulates, can be analyzed with efficiency. Nitrogen monitoring is also an important application for seawater because the supply of nitrogen is a key factor controlling the nature and diversity of plant life and their ecosystems. Because the deionized water used for reagents and rinsing water in most TOC/TN analyzers has a very low nitrogen background, the chemiluminescence detection technique is extremely sensitive. Therefore, nitrogen detection limits for low-ppb range are possible with TNb analysis.

Conclusion
High temperature combustion offers significant improvements in nitrogen monitoring for wastewater, seawater and a variety of industrial applications. Current advances in HTC technology make implementation both cost effective and environmentally superior to alternative technologies like Kjeldahl. Since total nitrogen analysis can be performed simultaneously with TOC analysis, the use of this dual element detection technology can increase both productivity and monitoring effectiveness as a part of an overall monitoring strategy.

References

  1. Eaton, A.D., L.S. Clesceri and A.E. Greenberg, Standard Methods for the Examination of Water and Wastewater, Vol.2: 19th Edition (American Public Health Association, Washington, D.C.), 1995.
  2. Water quality—Determination of Nitrogen, International Organization for Standardization, Geneve, Switzerland, ISO method 11905-2, 1997.
  3. Water analysis—Determination of Nitrogen—Determination of bound nitrogen, after combustion and oxidation to nitrogen dioxide, using chemiluminescence detection, European Committee for Standardization, Brussels, EN method 12260, 1996.

About the authors
Mike Purcell is the product line manager and Brian Wallace is an applications chemist for TOC products at Cincinnati-based Tekmar-Dohrmann, a division of Emerson Process Management. Purcell can be reached at (513) 229-8240 or email: Mike.Purcell@emersonprocess.com. Wallace can be reached at (513) 229-7068 or email: Brian.Wallace@emersonprocess.com

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