The Use of COD, TOC, Fluorescence, and Absorbance Spectroscopy to Estimate Biochemical Oxygen Demand in Wastewater

All wastewater treatment facilities must obtain a National Pollution Discharge Elimination System (NPDES) permit, which regulates the quality of water that is discharged. Common to all NPDES permits is a limit on organic matter, as determined by the five-day biochemical oxygen demand (BOD5) test. More rapid methods, such as chemical oxygen demand (COD), total organic carbon (TOC), fluorescence, and absorbance spectroscopy are also capable of quantifying organic matter. Previous studies indicate it is possible to develop correlations between these parameters. This study explored the correlations using influent, primary clarifier effluent, and finished effluent samples from an operational wastewater treatment plant located in Nevada. It was concluded that COD could be used to estimate BOD5 for influent, primary clarifier effluent, and finished effluent. TOC could be used as a surrogate for finished effluent, but it was not suitable for influent or primary clarifier effluent. The relationship developed for fluorescence and BOD5 was nonlinear, presumably due to inner filter effect (IFE) interference. Power functions were developed for region I of the excitation emission matrix (EEM) and peak T (excitation=275 nm, emission=340 nm) that could be used to estimate BOD5 for finished effluent and primary clarifier effluent, but they were poor estimators for plant influent. Comparison of removal efficiencies indicated that TOC and fulvic-like fluorescence peaks increased in the primary clarifier, presumably due to the return of centrate that is sent back to the primary clarifier. The fluorescence removals indicated that over 80% of protein-like (typically associated with BOD5) fluorescence was removed during treatment and approximately half of humic- and fulvic-like fluorescence was removed during treatment. Quality control experiments indicated that holding the samples overnight biased the test results low, which was more pronounced when samples were held with headspace. Degradation likely occurs due to the biological consumption of organic matter that is occurring within the sample during the holding time