The more stringent requirements imposed on wastewater treatment systems result in an increasing demand for on-line monitoring equipment for these processes. The paper presents the modification of a fully operational respirographic biosensor to allow simultaneous and on-line biokinetic characterization of both BOD-removal and nitrification capacities of activated sludge.

For this characterization, advantage is taken of the calibration measurements that are performed regularly (every 2-3 h) during normal operation of this biosensor. The injection of a well chosen calibration mixture of readily biodegradable BOD and ammonia to a small activated sludge filled bioreactor results in oxygen uptake rate profiles that are interpreted in the framework of a mathematical model that describes the combined oxygen consumption for BOD removal and nitrification. Model identification gives rise to biokinetic parameters (max. conversion rates and affinity constants) that are indicative of the different metabolic capacities of the activated sludge. It is shown that this information can be used to assess the effect of injected wastewater samples on the sludge condition, e.g. adaptation or inhibition, and allows one to follow the change in relative abundance of heterotrophic and nitrifying populations.

Special attention is paid to the proper composition of the calibration solution. Especially the ratio between NOD and BOD is found to be critical. With the aid of simulation, sub-optimal experimental designs are proposed that allow reliable estimation of the biokinetic parameters. Finally, validation of the proposed design is given with experimental data.

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