Evaluation of different methods for the determination of maximum heterotrophic growth rates

This work evaluated the most commonly used methods for determining maximum heterotrophic specific growth rates (mum) in batch reactors. Parallel batch experiments were conducted under various initial substrate-to-biomass (S0/X0) ratios and values of the solids retention time (thetac). The maximum specific growth rate, mum, was determined simultaneously according to measurements of oxygen consumption (i.e., oxygen uptake rate, OUR) and volatile suspended solids (VSS) increase. The S0X0 ratio was found to significantly influence mum values. Under high S0/X0 ratios (= 20), fast-growing bacteria seemed to gain a competitive advantage resulting in higher mum values than those obtained under low S0/X0 ratios (= 1.5). The OUR-based estimate of mum (mum(OUR)), under certain circumstances, is differentiated from mum that is based on exponential bacterial growth (mum(VSS)), and seems to be more a measure of substrate oxidation than a measure of bacterial growth. At high S0/X0 ratio and low thetac, mum(OUR) was significantly higher than the mum(VSS), indicating that considerable uncoupling between anabolism and catabolism was occurring under these conditions. Batch experiments conducted at high S0/X0 ratios seemed to be a more sensitive method for determining mum values in the presence of an inhibitor than tests conducted at low S0/X0 ratios.