Effectiveness of Near Infrared Reflectance Spectroscopy in the Quick Evaluation of Nitrogen Content in Sewage Sludge

One of the main uses of sewage sludge is the incorporation into agricultural soils, the application dose being based on the specific composition of the waste according to the established normative. However, a full analytical characterization of sewage sludge is time-consuming and expensive. Thus, many times, sewage sludge is applied to soils without this information. Therefore, the use of near infrared reflectance spectroscopy (NIRS) to estimate nitrogen (N) contents in sewage sludge could be interesting because this technique is quick and a sample treatment is not required. In this study, more than 300 sewage sludge samples were used to calibrate and validate the estimation of total N in these residues using NIRS. The NIRS analysis was performed using a monochromator spectrometer (InfraAlyser 500, Bran+Luebbe GmbH, Norderstedt, Germany) in the range from 1100 to 2500nm with a step of 2nm. Elemental nitrogen (TN) (TruSpec, LECO, St. Joseph, Mich.) and Kjeldahl nitrogen (TKN) were used for this estimation. The results showed that NIRS can be used to estimate the content of total N in sewage-sludge samples, with the best estimation obtained with multiple linear regression calibration using the absorbance signal removing the outliers' spectra from the calibration set (R = 0.97, SEE = 0.34, and RMSEP = 0.72).