A comparison of biodegradation kinetic models applied to estrogen removal with nitrifying activated sludge

The biological degradation of estrone (E1), estradiol (E2) and ethinylestradiol (EE2) was studied in batch experiments at typical concentration levels using nitrifying activated sludge from a membrane bioreactor (MBR). Since first-order, pseudo first-order and Monod-type kinetics were observed. Pseudo first order kinetic was reformulated using only the soluble concentrations S and assuming adsorption coefficient K_D of the estrogens. For the adsorption coefficients K_D determination, activated sludge from MBR was spiked with the respective target compounds and stirred. Finally, the water was analyzed. The K_D values of estrogens ranged from 0.323 to 0.474 L/g. Greater than 98% of E1, E2 and EE2 were found to be removed in batch reactors. The measured data were linearly regressed giving R² values ranging from 0.748 to 0.990. According to these results, the biodegradation kinetics were adjusted to pseudo first-order assuming adsorption coefficient K_D and Monod-type kinetic. The biodegradation rate constant k of the estrogens were: E1 and E2 > 78.52 L/g_VSS d and 12.41 L/g_VSS d for EE2. Monod-type kinetic indicates that these compounds are biodegradated by co-metabolism. E2 was oxidized into E1.