Accuracy analysis of a respirometer for activated sludge dynamic modelling.

The aim of the paper is to assess the experimental errors arising from the operation of a closed respirometer using autotrophic biomass. A closed, intermittent-flow device has been set-up for the measurement of oxygen uptake rate (OUR) and parameter calibration. After describing the device structure and operation, the factors affecting accuracy have been assessed. Inaccuracies may be caused by two groups of parameters: design parameters, including flow rate, volume, sampling time, numerical algorithm, sample injection and environmental parameters, concerning the physicochemical conditions of the experiment, such as unwanted oxygen transfer, pH, and the influence of sludge condition on "start-up" behaviour. It is shown to what extent each of them affects the final accuracy of the OUR measurement. In the second part of the paper, the respirometric data are used to calibrate a two-step nitrification model and their impact on the accuracy of the estimation of model parameters is assessed. Confidence limits are derived for the identifiable parameter combinations and the practical identifiability assessed with the aid of trajectory sensitivity analysis.