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 GOD, slowly biodegradable GOD, and the active heterot rophic biomass present in a wastewater; necessary for modelling and upgradi ng the biological treatment processes. As the initial conditions of the batch procedures proposed need to be adapt ed to each sewage under study, the presented method is based on the combina tion 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 direc t aeration of wastewater without sludge addition, and the second by mixing wastewater sample with an excess aerated sludge volume. at low So/Xo, in th e range 0.01-0.2 gCOD/gVSS. The use of a biological model and an identifica tion procedure allows simultaneous estimations to be made of the kinetic pa rameters and the initial concentrations in COD biodegradable fractions, and of the active heterotrophic biomass. Firstly, assuming that the yield coefficient is known (Y-H), 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 biod egradable COD concentration to be clearly identified, and greatly improves the estimation of the hydrolysable fraction of COD, dividing by three the s tandard deviation of the results. The determination of heterotrophic biomas s concentration is obtained from the same respirograms, but need a simplifi ed interpretation for some experiments. Concerning the French sewage studie d (12 samples), the concentration of heterotrophic biomass measured in (aro und ?3% of total GOD) is high compared with classical data. The same wastew ater contains a readily biodegradable COD concentration varying from 6 to 1 4% of total GOD, and a hydrolysable fraction in the range 41-66%. These res ults are compared with those obtained by the classical flow-through method and good agreement is shown between the two procedures. (C) 2000 Elsevier S cience Ltd. All rights reserved.