Smart COD sensor using UV–Vis spectroscopy against optical window surface contamination

• An optical window surface contamination (OWSC) compensation method is proposed. • A smart COD sensor using the combination of UV–Vis LED and PN tube is designed. • The COD sensors with compensation methods can against OWSC effectively. • The COD sensor is low cost, low power consumption and no secondary pollution. • The COD sensor can realize long-term real-time monitoring after compensation. A typical method for real-time and unpolluted measurement of chemical oxygen demand (COD) is UV–Vis absorption spectroscopy, however, contamination of the measuring system’s optical window surface affects the measurement accuracy during long-term monitoring. This study proposes an optical window surface contamination (OWSC) compensation method, and a multi-wavelength smart sensor for long-term accurate COD measurement was developed. This study uses equivalent absorption spectrum (EAS) to describe OWSC, and explores its spectral characteristics. Therefore, we can predict the EAS of OWSC between 250 and 380 nm based on the measured absorption spectrum between 380 and 440 nm, and subtract the EAS of OWSC to get the absorbance of COD to complete the spectrum compensation. Furthermore, we designed a smart COD sensor which has the advantages of real-time warning and maintenance-free. We proved that the OWSC compensation has excellent applicability through experiments using the COD sensor, and long-term accurate COD measurement can be achieved.