Simple and efficient dual-wavelength spectroscopy for the determination of organic matter in sewage sludge from wastewater treatment

Abstract

In this study, a dual-wavelength spectroscopic method for rapid determination of organic matter in sludge was developed. The contents of the organic matter were calculated by determining the consumption of potassium dichromate (K₂Cr₂O₇) based on the production of trivalent chromium ions (Cr³⁺). Cr³⁺ could be determined by subtracting the absorption at 800 nm (spectral interference) from the absorption at 650 nm (only contributed by Cr³⁺). The results showed that the relative standard deviation in the test was less than 5%. The same set of samples was used and when the content of organic matter was more than 150 g kg^-1, the relative difference between the spectroscopic method and titration method was within 1%. Furthermore, the method does not require calibration based on the standard samples. In conclusion, the present method is simple, reliable, accurate and suitable for application in mass testing for sludge samples.

Fig. 1. Visible spectra of Cr₂O₇²⁻ and Cr³⁺.

Fig. 2. (a) The spectra of Cr³⁺ and (b) the relationship between the absorption at 650 nm vs. the OM content in the samples.

Fig. 3. Effect light scattering from (a) un-oxidized and (b) in this figure oxidized sewage sludge digested samples.