A Novel Hybrid Strategy for Detecting COD in Surface Water

Abstract

The prediction of chemical oxygen demand (COD) by ultraviolet–visible absorption spectrum is a common method. Many researchers use the absorbance at the characteristic wavelength to establish COD prediction models. However, selecting the characteristic wavelength is a problem. In this paper, the extreme values of absorption spectrum change rate, was proposed as a new characteristic parameter to determine the characteristic wavelengths. On this basis, a novel hybrid strategy for detecting COD in surface water was proposed. We first proposed to combine the first derivative method with the permutation entropy method (FDPE) to determine the characteristic wavelengths. Then we used partial least square (PLS) to establish a COD prediction model. Experimental results demonstrated the linear correlation coefficient (R2) of the FDPE_PLS was above 0.99 without turbidity interference. Secondly, a dual-wavelength method (DWM) was proposed to determine the turbidity values. The DWM used slopes of absorbance values at 400 nm and 600 nm to predict the turbidity values. Compared with the single-wavelength method, the DWM improves the measurement accuracy of turbidity. Finally, a new turbidity compensation method was proposed to compensate for the interference in the first derivative spectrum. After compensation, FDPE_PLS can predict COD concentrations accurately, whose R2 was 0.99.

Comparative Analysis of Different Methods used in Evaluation of Physio-chemical Parameters of Surface Water: A Review

“…Titration method and colorimetric methods are widely used for the assessment of COD level and several studies have been conducted by various researchers for the COD analysis in surface water bodies using these methods [2,19,30,43,44].…”

“…The method is simple but it has less stability. It is good for the measurement of complex samples but every time during analysis, it may lead to change in wavelength range and cause instability in obtained results [43,44].…”

Research on COD measurement method based on UV-Vis absorption spectra of transmissive and reflective detection systems

“…With the development of optical technology, there are various methods for determining COD, including single-wavelength (Shi et al, 2020), double-wavelength (Minchao et al, 2013;Zhang et al, 2020), multi-wavelength (Lv et al, 2011), and broad-spectrum spectrometry (Ye et al, 2022). Broad-spectrum spectrometry is particularly advantageous due to its wider range of absorption spectral bands compared to the other methods.…”

Deep learning–based turbidity compensation for ultraviolet-visible spectrum correction in monitoring water parameters

“…Although use of the multi-wavelength algorithm has resulted in a marked improvement in accuracy, turbidityrelated interference still limits the accuracy of measurements, especially with regard to actual environmental water samples. Various turbidity compensation methods have been developed to decrease or nearly eliminate turbidity interference resulting from suspended particles in water components (Zhang et al, 2020). In methods using the single-wavelength inversion algorithm, the numerical value of the minimum point, such as 350 nm, is subtracted from the full absorption spectrum (Li et al, 2019).…”

“…In methods using the single-wavelength inversion algorithm, the numerical value of the minimum point, such as 350 nm, is subtracted from the full absorption spectrum (Li et al, 2019). Dual-wavelength method have also been proposed to predict turbidity values (Zhang et al, 2020). Smoothing methods using the orthogonal signal correction (OSC) algorithm can remove some variation from data matrix X that is orthogonal to the response matrix Y.…”