Indian Journal of Chemical Technology Vol. 15, May 2008, pp. 209-215 Effect of substrate concentration on the transient dynamics of specific cell growth during bioconversion of Cr +6 to Cr +3 using polyculture consortia K Samanta, R Chowdhury & P Bhattacharya* Chemical Engineering Department, Jadavpur University, Kolkata 700 032, India Email: pinaki_che@yahoo.com Received 15 June 2007; revised 4 January 2008 The transient dynamics of specific cell growth rate during bioconversion of hexavalent chromium (Cr +6 ) to trivalent chromium (Cr +3 ) has been studied covering both Cr +6 uninhibited and Cr +6 inhibited conditions in a batch contacting system. Suitable polyculture consortia necessary for this bioconversion have been isolated and purified from tannery waste. It is observed that above the Cr +6 concentration of 33 mg/dm 3 , effect of Cr +6 inhibition on the specific cell growth rate is appreciably high. Detailed simulation and modeling work indicate that while Monod model equation is capable of describing the cell growth dynamics under Cr +6 uninhibited condition, the same can well be represented by Briggs-Haldane uncompetitive type model equation under Cr +6 inhibited condition. Since for the present system the difference in magnitudes of two intrinsic kinetic parameters, namely, Monod constant, K S and the inhibition constant, K i is not appreciably large, a unified transient specific cell growth dynamic equation has been presented. Keywords: Transient growth dynamics, Cr +6 inhibition, Briggs-Haldane equation, Biological switch, Unified rate equation Hexavalent chromium (Cr +6 ) is regarded as one of the major hazardous chemicals due to its toxic 1,2 , carcinogenic 3,4 and mutagenic 5,6 effects on living body. Conversion of Cr +6 to its trivalent form by chemical route is economically feasible when the former is present in an appreciable concentration. However, if the initial concentration of hexavalent chromium is low or very low, as is the case in related industrial effluent but still sufficiently hazardous to cause health hazard 7-10 , use of bioconversion of hexavalent form of chromium to its harmless trivalent form may be an important route to abate environmental pollution threat from hexavalent chromium compounds. A considerable quantum of investigation has been reported on isolation and purification of Cr +6 reducing microorganisms from native source as well as on their identification with the help of classical microbiological and biochemical tests or even 16S rRNA identification method 11-13 . Notably, Xu et al. 14 and Cheung and Gu 15 have recently successfully applied the bioreduction technique for converting toxic hexavalent chromium compound to its non-toxic trivalent form. These classical investigations, however, are more related to physicochemical studies of the system and a detailed bioprocess study with respect to specific cell growth response under the forcing function of change in initial Cr +6 concentration is still awaited. As such the understanding of such cell growth response is probably the first important step in bioprocess study of any bioremediation system. In the present investigation the transient dynamics of specific cell growth, perturbed by the change in initial Cr +6 concentration during bioconversion of hexavalent chromium to its trivalent form using mixed culture consortia has been studied. The main interest of the present investigation was to identify the inhibitory effect of Cr +6 concentration on the specific cell growth rate and subsequently to correlate the response of cell growth as a function of initial inhibitor, Cr +6 , concentration encompassing both inhibition free and inhibited regimes through judicious simulation and modeling work. The unified response equation so obtained is expected to be useful in predicting the cell growth rate in both uninhibited and inhibited regimes of bioconversion of Cr +6 . Such prediction is considered as an apriori requirement for understanding of any biochemical system. Microorganisms used for this study were isolated from tannery waste. These were purified and characterized by standard microbiological tests 16-18 as well as scanning electron microscope (SEM). The mixed culture consortia, so obtained, consisted of Pseudomonas sp., Bacillus sp. and Micrococcus sp. The chromium resistant nature of the above