ESTIMATION OF THE MAXIMUM SPECIFIC GROWTH RATE CONSTANT FOR AUTOTROPHS – EXPERIENCE FROM THE LONGTERM OPERATION OF A LAB-SCALE SBR SYSTEM

The maximum specific growth rate constant for autotrophs, μA,max, has been considered the most critical parameter for design and performance of nitrifying activated sludge systems. In literature reviews, a wide range of the μA,max values have been reported (0.25–3.0 d−1). In this study, experiences gained from the long-term (almost two-year) operation of a lab-scale SBR system are evaluated in terms of its applicability for estimating this coefficient according to the methods (low F/M bioassay vs. simulation) described in the Water Environment Research Foundation (WERF) report on “Methods for Wastewater Characterization in Activated Sludge Modeling” (Melcer et al., 2003). The comparison of these two approaches revealed that the value of this parameter could be influenced by the applied approach. The estimated values of μA,max based on steady-state model calculations for four operating periods ranged from 0.83 to 0.99 d−1, which fit to the relatively narrow range (0.75–1.17 d−1) anticipated in the WERF report. On the other hand, the Activated Sludge Model No. 1 (ASM1) simulations revealed that a single value of μA,max (=1.2 d−1) could be used despite variations in the measured specific nitrification rates during these periods. Moreover, the observed discrepancies between the XA/YA ratio predicted by ASM1 and the simple steady-state model reached approximately 200% in some cases. This implies that estimating this ratio from a complex activated sludge model and using it in simple steady-state model calculations, as recommended in the WERF report, should be accepted with great caution and the issue requires further investigations.