The detailed composition of wastewater has significant impact on the biological wastewater treatment processes. The carbon sources present govern the oxygen uptake rate, the rate of denitrification and the biological phosphorus uptake rate.

In this paper, the respiration rate determinations used for the bi- or tri-substrate determinations are coupled to removal capacity in order to develop rate-capacity diagrams, that gives a fuller picture of the wastewater and its influence on the biological processes. The directly degradable carbon source gives high reaction rates, but the concentration and thus the capacity can be very limited. In such a case the overall obtainable removal rate is of more interest than the peak rate.

The rate-capacity diagrams are shown for raw, primary settled and primary precipitated wastewater, characterized by detailed investigations of the respiration rates obtainable. It shows the effect of pretreatment not only on the total concentrations in the pre-treated wastewater, but also in the rate-capacity curves for biological denitrification. The rate-capacity curves are in practice coupled to the design and operation of the treatment plant. These two factors determine whether the carbon sources in the influent wastewater are used for denitrification (or biological phosphorus removal), for oxidation with oxygen or for sludge production.

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