Application of Activated Sludge Model No. 1 requires an influent characterisation of COD over the different model COD components. Total biodegradable COD (CODBD) is the most important quantity in this characterisation. The use and interpretation of BOD tests for the determination of biodegradable COD were investigated. Of four grab samples, the BOD evolution in time was measured up to fifteen days, with addition of nitrification inhibitor. From the BOD curves, total BOD (BOD, or BOD infinite) was estimated with first and second-order models. Parameters were fitted with one (or two, for the second-order models) kinetic parameter per individual sample as well as one (two) for all samples together. The BOD curves were adequately described by first-order kinetics. The estimated kinetic parameters showed considerable variation over the samples from the same wastewater. As a consequence, the ratio BOD/BOD5 was not constant for a given wastewater. The ratio BOD/CODtot on the contrary proved much more constant. Therefore, the total COD instead of BOD5 is suggested as a basis for determination of biodegradable COD. The BOD estimated has to be converted to CODBD. The conversion factor was computed by simulating the BOD tests using ASM1, reduced for aerobic conditions. This approach provides a quantitative basis for this conversion factor. The sensitivity of this conversion factor to model parameter values and COD fractionation was determined, which showed that the conversion has to be adjusted for non-default values of the heterotrophic yield.

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