How to Get the Best Data - Secrets of COD measurement with Optical Sensors

Efficient operation of Water Resource Recovery Facilities (WRRF) requires monitoring of the composition of wastewater. The parameters Biochemical oxygen demand (BOD) and Chemical Oxygen Demand (COD) are critical measures of wastewater organic (carbonaceous) load. Laboratory methods require a chemical or bio-chemical reaction to proceed to a desired endpoint. In the case of BOD this requires 5 days. A COD measurement usually requires over 2 hours with the addition of heat and chemicals. An alternative to direct measurement in the lab is simulated measurement through direct measurement of optical absorbance. The direct optical technique is simpler than traditional analytical measurements but the technology is not well understood. A spectral type optical sensor was deployed in untreated wastewater for five months with routine COD and cBOD5 reference measurements for verification. After careful data filtering to remove outlier data points a user calibration was created in Microsoft Excel using the linear regression tool. The user calibration was able to achieve greater accuracy than the COD factory calibration and to create a calibration for cBOD5 which is correlated with COD.