A titrimetric method to monitor nitrification was applied on a pilot activated sludge plant for biological N removal. Mixed liquor was sampled from the aerobic compartment of the treatment plant and a titration in-sensor experiment was performed. Interpretation of the cumulative base addition curves resulting from each titration in-sensor experiment was done using both a simple slope extrapolation method and a model-based non-linear parameter estimation method. The NH4+-N concentrations obtained with both methods correlated well with the NH4+-N concentrations measured on the effluent of the pilot plant using an on-line NH4+-N analyser. Contrary to most physical/chemical NH4+-N analysers, no sample pretreatment of the mixed liquor is needed for the measurements. It is shown in detail that interpretation of the titration curves yields information about the nitrification kinetics too, which can be an important advantage for process control purposes.

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