High precision wide range online chemical oxygen demand measurement method based on ultraviolet absorption spectroscopy and full-spectrum data analysis

In this paper, a new method for high precision and wide range measurement of chemical oxygen demand (COD) based on ultraviolet absorption spectroscopy without reagent is proposed. The reasons for limiting measurement range and the main factors affecting the measurement accuracy are analyzed. A novel method of selecting different calibration wavelengths according to COD value to expand the measurement range and the turbidity compensation strategy based on full-spectrum data analysis to improve accuracy in wide measurement range are proposed and realized by an automatic wavelength selection calibration algorithm. Experiments were conducted to verify our idea with the self-developed micro UV-vis spectrophotometer (with a spectral range of 200-750 nm and a resolution of 5 nm) with a 10mm-path-length sample cell. By comparing various algorithms, the Savitzky-Golay (SG) convolution smoothing algorithm and the orthogonal signal correction (OSC) algorithm were chosen to compensate the additional absorption owing to turbidity. The experimental results show that the algorithm can automatically select the optimal characteristic wavelengths according to the spectral data and the measurement range of COD is enormously expanded from 10-150 mg/L to 1-1000 mg/L. The linear correlation coefficient (R-2) of model is above 0.9995 and the relative error of measurement is less than 5%. This method can be used for in-situ and online COD measurement.