Courtesy of Andreas Weingartner, s::can Measurement Systems
Nitrite (NO2-) is an intermediate product of the nitrification process. Historically, nitrite was not a target variable in wastewater treatment, but with stricter effluent limits for total nitrogen and increasing pressure for energy efficient treatment, the spotlight shifted over to nitrite. Many highly efficient processes for Nitrogen removal, that came up recently, use nitrite as a main target parameter, because it has been published to be the best – if not only – control parameter. 
During long-term evaluation the s::can spectro::lyser proved to be the perfect tool for monitoring nitrite. Compared to other measuring methods it has many advantages. For example laboratory measurements of samples only give a snapshot picture and the use of 24 hr-composite samples is limited due to the unstable nitrite concentration. The use of on-line analyzers, which need a high sample preparation time, can lead to similar problems. With the spectro::lyser these problems do not occur as the measurement can be conducted directly in the medium and the results are available online in almost real time. The spectro::lyser also showed to deliver constant accuracy and require only minimal maintenance. 
Why measure Nitrite?
Nitrite (NO2-) is an intermediate product of the (simplified) two-step process of nitrification. It is a powerful poison for fish, reducing the oxygen transfer capability of blood . High nitrite concentrations in the effluent of wastewater treatment plants (WWTPs) can lead to damage to organisms if the dilution of the receiving water body is too low.  In addition, Nitrogen based emissions cause increasing concerns for their role as greenhouse gases.
Historically, nitrite was not one of the target variables measured in wastewater treatment since (i) there was limited interest and (ii) it was believed to be too difficult to measure. With stricter effluent limits for total nitrogen and increasing pressure for energy efficient treatment, the spotlight shifted over to nitrite.
The tracking of NO2-N in the effluent of any WWTP, together with nitrate (NO3) and COD, gives a good indication of the performance of the nitrogen removal process. Since the second step of nitrification is very fast, the nitrite concentration in the effluent of a WWTP is normally low (around 0.1 mg/l). Enrichment of nitrite in the system usually suggests that the microbiological processes are disturbed, i.e. they are inhibited due to toxic substances or to unfavorable conditions for the nitrite oxidizer.  Such distortions will show in the nitrite trends, typically paired to COD and naturally mirroring the NO3 concentration.
Ratios like NO2-/COD or NO2-/NO3 can well picture the performance of the process and deviations thereof. This can be used from a purely analytical perspective or for plant design, but can as well be coupled into process control.
A number of highly efficient processes for Nitrogen removal came up recently. They target typically at no or less addition of external carbon source, less sludge production, less oxygen consumption, less energy demand, and less greenhouse gas production:
- The ANAMMOX process is based on partial nitrification (nitritation), and (anaerobic) conversion of ammonium and nitrite to dinitrogen (N2) gas.
- The DEMON process (developed at the University of Innsbruck, AT) is a nitrification/deammonification process in which ammonia and nitrite are simultaneously converted to nitrogen gas, without the use of organic carbon.
- The pioneer SHARON-ANAMMOX process uses the advantages in two separate steps.
- Veolia’s AnitaTM Shunt stops nitrogen oxidation at the nitrite stage and with this reduces the quantity of sludge and greenhouse gases produced, and also the consumption of oxygen and energy.
Nitriteis a main target parameter for most related processes and has been published as the best – if not only – control parameter. 
Greenhouse Gases Control
In the context of waste water treatment, Nitrogen gases became a major concern next to CO2 and CH4 because of their important role as greenhouse gases . Denitrification involves reduction of nitrate to dinitrogen gas via several aqueous and gaseous intermediates including nitric oxide, nitrous oxide and dinitrogen gas.
The relationship between N2O production and NO2– during the aerobic nitrification process has been widely discussed. The production of high levels of NO during partial nitrification was also found rather alarming.
In any case, the efficient control of nitritetrough all stages of treatment is considered of high importance, and first control approaches have been developed (e.g. AnitaTMShunt) to reduce nitric oxide emissions.
How to Measure Nitrite?
For a long time there was limited interest in nitriteand it was believed to be too difficult to measure. Wet chemistry is known to be quite demanding in terms of operating costs and maintenance requirements. Laboratory measurements of grab samples cannot give a picture of the process dynamics and the analysis of 24 hr-composite samples is critical due to the unstable nitrite concentration, so their meaning is limited. Because of these limitations, a robust UV-spectral method had to be developed.
Nitriteexhibits a distinct absorption peak close to 210 nm, and with this, widely overlaps with NO3, but also with a number of organics (see Figure 2). Thus the stable distinction between nitrate and nitrite is not an easy task. For optical nitrate measurements, nitrite even used to be a residual on the nitrate signal, noise at best.
A stable, drift-free, dual beam spectrometer with better than 1 nm optical resolution is the necessary hardware basis for successfully measuring nitrite.
Using mathematical algorithms the spectral absorption data delivered by the hardware then has to be turned into practical measurement results. For distinction and quantification, there are two pathways to accomplish:
- One is the use of classical chemometric methods, typically a combination of principal component analysis with partial least square fit . Starting point is always a specific mathematical process that identifies the principal components that most contribute to the identification and quantification of nitrite, in all occurring matrices and under variation of all potentially cross-sensitive parameters, especially nitrate. The resulting algorithms typically are rather complex and use all provided spectral information. Limitations are noise, stray light and other sources of non-linearity, and cross-sensitivities to not considered substances.
- Another method is the analytical identification of the NO2-polynom in an unknown matrix by comparison of the measured spectrum with a library of known spectra, followed by a fitting procedure for quantification.
By best use of modern mathematical tools, and validation with many years of measured data from all kind of applications, s::can was able to find algorithms that are extremely stable and fit to be used in almost any type of water. Sometimes the found algorithm has to be adapted to special water composition, by conventional slope/intercept calibration.
spectro::lyser for Nitrite Measurement
The spectrometer is built as a compact submersible sensor enabling measurement of optical spectra directly in liquid media with an accuracy approaching laboratory analysis quality. Its sensitivity can be adapted to the application demands by selecting the optical pathlength within a range of 1 – 100 mm.
In a WWTP the optical equipment is exposed to difficult environmental conditions. The system has to deal with interferences stemming from organic matter and other disturbing ions which show an absorption in the same wavelength range or particles that block the optical measuring path. Since the interferences are of different magnitude for different water matrices, the question is also if the calibration is stable over a prolonged time period.
The spectro::lyser is able to deal with all these challenges and has demonstrated its function in diverse applications from drinking water to highly concentrated wastewaters like SBRs. [see 1, 2, 4 for examples]. It is equipped with an auto-cleaning system using pressurized air which keeps the measurement windows free of fouling and has been proved to be extremely reliable. 
Long-term evaluation of the spectro::lyser showed, that it delivers constant accuracy and requires only minimal maintenance:
“The UV sensor showed good results for NO2-in the low concentration range and very accurate results for higher concentrations (up to 10 mgN/l). This allows using the sensor for alarm systems as well as for control concepts at WWTPs.” 
During this long-term evaluation the spectro::lyser also provided better results than another on-line analyzer system, which required a higher demand in terms of consumption of chemicals and maintenance.
All rights including copyright: Andreas Weingartner, s::can Mess-technik GmbH
 “Advanced Control System to Reduce N2O Emission and Improve Performance of an SBR Treating N-Rich Effluent Via NO2 Pathway”; R. Lemaire, J. Chauzy, F. Veuillet, R. DiMassimo, K. Sorensen and S. Deleris, Water Environment Federation, 6480-6493, WEFTEC, 2011
 “Long-term evaluation of a spectral sensor for NO2 and NO3”; L. Rieger, G. Langergraber, D. Kaelin, H. Siegrist, P.A. Vanrolleghem, Water Sci. Technol., Vol. 57, 1563-1569, IWA Publishing, 2008
 “Siedlungswasserwirtschaft”; Gujer W., Springer-Verlag Berlin, Heidelberg, New York, 1999 [in German]
 “Spectral in-situ analysis of NO2, NO3, COD, DOC and TSS in the effluent of a WWTP”; L. Rieger, G. Langergraber, M. Thomann, N. Fleischmann and H. Siegrist, Water Sci. Technol., Vol. 50, 143–152, IWA Publishing, 2004
 “N2O emissions from activated sludge 2008-2009: Results of a nationwide monitoring survey in the United States”; Ahn, J.-H., S. Kim, H. Park, K. Pagilla and K. Chandran*, Environmental Science and Technology, 44(12), 4505-4511, 2010
 “Practical aspects, experiences and strategies by using UV/VIS sensors for long-term sewer monitoring “; Gruber, G., Bertrand-Krajewski, J.-L., De Beneditis, J., Hochedlinger, M. & Lettl, W. 2005. 10th International Conference on Urban Drainage, Copenhagen/Denmark