Natural and synthetic estrogens are present in municipal wastewater treatment plants effluents. Biodegradation of estrogens, including the natural estrogens estrone (E1) and 17-beta-estradiol (E2), as well as the synthetic 17-alpha-ethinylestradiol (EE2), can occur during sewage treatment, particularly in activated sludge. Investigations of removal of estrogens in wastewater treatment plants, as well as batch experiments with activated sludge, have demonstrated the potential for conversion of

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... to E1 and subsequent removal of E1 and to a lesser extent EE2. The objective of this study was to assess the extent of transformation of E2 and EE2 by nitrifying activated sludge under aerobic, anoxic and alternating conditions, and evaluate potential relationships between availability of oxygen, nitrification rate and estrogen removal. The nitrifying biomass originated from laboratory sequencing batch reactors. For each batch experiment, two reactors were set up: aerobic and alternating aerobic/anoxic, which were then amended with E2 and EE2 from methanolic stock solutions at the same time as the addition of synthetic wastewater feed. E2 was readily converted to E1 in the activated sludge. The conversion was faster under aerobic (nitrifying) than anoxic (denitrifying) conditions. EE2 was persistent under anoxic conditions; whereas under aerobic nitrifying conditions the maximum observed level of its removal was 22%. During anoxic denitrifying conditions, E2 was converted to E1, and a metabolite consistent with 17-alpha-estradiol transiently accumulated, and was subsequently removed when the reactor was aerated. The total removal of estrogens was similar in aerobic and alternating reactors. Higher rates of E1 and EE2 removal were associated with higher nitrification rates, which supports the contention that the nitrifying biomass was responsible for their removal.