It is shown that the most important parameter in batch cultivation of mixed cultures is the ratio of the initial substrate concentration to the initial biomass concentration (So/Xo as COD/biomass). When the ratio is sufficiently low (below 2-4 depending on the mixed culture history) no cell multiplication takes place during the exogenous substrate removal. Under these conditions, a biomass increase is mostly due to the synthesis of storage polymers. It is also shown that the observed yield, Yobs, decreases with increasing So/Xo ratio. Under the high So/Xo conditions, more energy is spent for cell multiplication, which results in greater part of substrate being oxidized. Batch cultivation at high So/Xo ratios results also in higher concentrations of microbial polymers produced as waste products of mixed culture microorganisms. It is concluded that for the biodegradation studies with the aim to obtain kinetic constants it is necessary to work at low So/Xo ratios to prevent mixed culture microorganisms from substantial multiplication. This is necessary because cell multiplication during batch cultivation of mixed culture changes the proportion among slow-growers and fast-growers. This is the only way to obtain the kinetic constants which are representative of the original mixed culture.

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