Biodegradation 2: 43-51, 1991. © 1991 KluwerAcademic Publishers. Printedin the Netherlands. Kinetics of aerobic biodegradation of benzene and toluene in sandy aquifer material Pedro J.J. Alvarez, Paul J. Anid & Timothy M. Vogel* Environmental and Water Resources Engineering, Department of Civil and Environmental Engineering, The University of Michigan, Ann Arbor, M1 48109-2125, USA (* requests for offprints) Received 5 March1991;accepted27 May 1991 Key words: benzene, biodegradation kinetics, indigenous microbes, Monod coefficients, toluene Abstract Monod's equation adequately described aerobic biodegradation rates of benzene and toluene by the microbial population of a sandy aquifer when these compounds were initially present at concentrations lower than 100 mg/1 each. Concentrations higher than 100mg/1 were inhibitory, and no benzene or toluene degradation was observed when these compounds were initially present at 250 mg/l each. The Monod coefficients were calculated as k = 8.3g-benzene/g-cells/day and Ks = 12.2mg/1 for benzene, and k = 9.9g-toluene/g-cells/day and Ks = 17.4mg/l for toluene. Specific first-order coefficients would be 0.681/ mg.day for benzene and 0.57 1.mg.day for toluene. Introduction Biological treatment of contaminated aquifers is receiving increasing interest. Where applicable, in situ biodegradation of the toxic petroleum hydro- carbons benzene and toluene can serve as a cost effective groundwater cleanup method (Barker et al. 1987; Hutchins et al. 1990; Lee et al. 1988; Sheehan et al. 1988; Thomas et al. 1990; Verheul et al. 1988; Werner 1985). Assessing the biodegrada- tion kinetics of these compounds is essential in predicting the extent to which contamination will spread and in estimating the duration of in situ biodegradation cleanup operations. Monod kinetics are widely used to describe bio- degradation rates of organic contaminants in aquif- er systems (Borden & Bedient 1986; MacQuarrie et al. 1990; Molz et al. 1986; Widdowson et al. 1988). Much of the versatility of Monod's equation is due to the fact that it can describe biodegradation rates following zero- to first-order kinetics with respect to the target substrate concentration. How- ever, Monod's equation cannot account for sub- strate inhibition and may be inappropriate to de- scribe biodegradation kinetics of benzene and tolu- ene when these contaminants are present at elevat- ed concentrations. Monod's equation is empirical, and the two coef- ficients that describe the equation are system spe- cific. Therefore, caution should be exercised in extrapolating coefficients measured in one envi- ronment to another. Several kinetic studies with pure cultures or with mixed cultures using sludge from biological reactors have measured Monod coefficients for benzene and toluene (Table 1). However, reported values of Monod coefficients for benzene and toluene degradation in aquifer material are virtually nonexistent. Barker et al. (1987) reported zero-order maximum degradation rates for benzene in aquifer material. However, the microbial concentration was not reported, and the maximum specific substrate utilization rate (k)