Estrogen concentration affects its biodegradation rate in activated sludge

The effect of concentration on the biodegradation rate of the steroid estrogens, estrone (E1) and 17-α-ethinylestradiol (EE2), was studied in batch and continuous-flow reactor systems using fresh activated sludge from two sewage treatment plants. Between the concentrations of 0.03 to 10 µg/L in the batch system no consistent difference was found in the biodegradation rates for either estrogen. The biodegradation half-life was 0.3 to 0.7 h for E1, and 1.5 to 4.4 h for EE2 at 15 to 20 °C. However, at 100 µg/L, biodegradation rates for both estrogens decreased, with the half-life prolonged to around 2.5 h for E1 and 12 to 18 h for EE2. In continuous-flow experiments, over a 2 h residence time, 95% of E1 and 48% of EE2 were removed on average at 0.1 µg/L, whilst 52% of E1 and 20% of EE2 were removed at 100 µg/L. In general, spiking concentration of estrogens did not appear to affect biodegradation rates between the ng/L to low µg/L levels in activated sludge; however, the rates greatly slowed down when the concentration increased up to 100 µg/L. The results suggest activated sludge biodegradation studies with estrogens in the high µg/L levels could give misleading results and should be avoided.