Estimation of wastewater biodegradable COD fractions by combining respirometric experiments in various So/Xo ratios

In this work, a new respirometric method is proposed, in order to estimate readily biodegradable COD, slowly biodegradable COD, and the active heterotrophic biomass present in a wastewater, necessary for modelling and upgrading the biological treatment processes. As the initial conditions of the batch procedures proposed need to be adapted to each sewage under study, the presented method is based on the combination of two experiments, at high and low So/Xo ratio (initial ratio between wastewater and biomass in gCOD/gVSS). The first ones are obtained by direct aeration of wastewater without sludge addition, and the second by mixing wastewater sample with an excess aerated sludge volume, at low So/Xo, in the range 0.01–0.2 gCOD/gVSS. The use of a biological model and an identification procedure allows simultaneous estimations to be made of the kinetic parameters and the initial concentrations in COD biodegradable fractions, and of the active heterotrophic biomass. Firstly, assuming that the yield coefficient is known (YH), the structural identifiability of the COD fractions is proved. Then it is shown that the combination of two experiments with the same sample allows the readily biodegradable COD concentration to be clearly identified, and greatly improves the estimation of the hydrolysable fraction of COD, dividing by three the standard deviation of the results. The determination of heterotrophic biomass concentration is obtained from the same respirograms, but need a simplified interpretation for some experiments. Concerning the French sewage studied (12 samples), the concentration of heterotrophic biomass measured in (around 23% of total COD) is high compared with classical data. The same wastewater contains a readily biodegradable COD concentration varying from 6 to 14% of total COD, and a hydrolysable fraction in the range 41–66%. These results are compared with those obtained by the classical flow-through method and good agreement is shown between the two procedures.