Sludge/water partition and biochemical transformation of estrone and 17β-estradiol in a pilot-scale step-feed anoxic/oxic wastewater treatment system

•Estrogen biodegradation and transformation in a pilot-scale wastewater treatment system.•Total amounts of estrogens in water were lower than in the sludge phase.•Estrogens accumulated in the return sludge caused increase in estrogen levels in A1.•Estrogen removal via excess sludge wastage was 40–50% of total estrogens removed.•Concentrations of E1 and E2 in the water phase were linearly correlated.

This study investigated biodegradation, transformation and sludge/water partition of two natural estrogens – estrone (E1) and 17β-estradiol (E2) - in a pilot-scale step-feed anoxic/oxic (A/O) wastewater treatment system. The results show that the total amounts of E1 and E2 (E1 + E2) in the water phase were approximately 9–23% of those in the sludge phase in the reactor tanks. The estrogens accumulated in the return sludge led to the increase in the estrogen concentration in the first-stage anoxic zone. The biodegradation of E1 and E2 in the first-stage aerobic zone was highly efficient. The sludge/water partition coefficients (Log Kd) of E1 and E2 were gradually increased from 2.3 and 2.6 in the first stage to 3.2 and 3.3 in the third stage, respectively, and Log Kd of E2 was higher than that of E1 in each A/O stage. In addition, the Log Kd values of E1 and E2 were larger in the aerobic zones than in the anoxic zones. Under six operation conditions tested, there was a linear relationship between the concentrations of E1 and E2 in the water phase, indicating the transformation between E1 and E2. The calculated mutual conversion equilibrium constant (K) for the transformation between E1 and E2 was significantly higher in the anoxic zones (0.38–0.81) than in the aerobic zones (0.08–0.24).

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