Critical Analysis of Computing Equations for Determination of BOD by Dilution Methods

The dilution method has been accepted widely as the standard procedure for determining biochemical oxygen demand (BOD). A mass-balance approach was used to analyze four equations for computing BOD: two involving external seed correction methods (Standard Methods) and two involving internal correction methods (Sawyer and McCarty in 1978 and Kline and Gibbs in 1979). Five testing conditions were identified as being pertinent to the underlying assumptions: seed addition method, uniform BOD bottle volume, uniform initial dissolved oxygen (DO), seed uptake linearity, and data handling method. The results of this study showed that internal methods may provide more reproducible results, primarily due to the inherent correction for dilution water demand and better data handling (regression) technique to cope with the variation in BOD bottle volume and initial DO. However, internal methods may violate the requirements imposed by Standard Methods for minimum final DO (1 mg/L) for the high spike runs and minimum 5 day DO depletion (2 mg/L) for the low spike runs. Recommendations are also made to improve the reliability of these methods.