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Natural and synthetic estrogens present in municipal wastewater can be biodegraded during treatment, particularly in activated sludge. The objective was to assess the extent of transformation of 17‐β‐estradiol (E2) and 17‐α‐ethinylestradiol (EE2) by nitrifying activated sludge and evaluate potential relationships between availability of oxygen, nitrification rate, and estrogen removal. For each batch experiment, two reactors were set up—aerobic and alternating anoxic/aerobic—which were then amended with E2 and EE2 from methanolic stock solutions. The EE2 was persistent under anoxic conditions; under aerobic conditions, the observed level of its removal was 22%. The E2 was readily converted to estrone (E1)—faster under aerobic (nitrifying) than anoxic (denitrifying) conditions. During the initial anoxic conditions, a metabolite consistent with 17‐α‐estradiol transiently accumulated and was subsequently removed when the reactor was aerated. Higher removal rates of estrogens were associated with higher nitrification rates, which supports the contention that the nitrifying biomass was responsible for their removal.

Sequencing batch reactors were acclimated under aerobic and alternating anoxic/aerobic conditions. Greater nitrification rates in the alternating reactor were investigated by comparing environmental conditions. In the alternating reactor, pH, alkalinity, oxygen, and nitrite were higher at the onset of aerobic nitrification. Kinetic studies and batch tests, with biomass developed under aerobic and alternating conditions, revealed that these factors were insufficient to explain the divergent nitrification rates. Nitrifying genera vary in nitrification kinetics and sensitivity to environmental conditions. Nitrosospira and Nitrospira spp. could dominate in aerobic reactors, as they are adapted to low nitrite and oxygen conditions. Nitrosomonas and Nitrobacter spp. are better competitors with abundant substrates and have higher nitrite tolerance, so they could excel under alternating conditions. This theoretical explanation is consistent with the kinetics and environmental conditions in these reactors and argues for using alternating treatment, because the harsh conditions select for populations with inherently faster nitrification rates. Water Environ. Res., 80, 388 (2008).

The purpose of this study was to evaluate the influence of partial ozonation of return activated sludge on settling properties and dewaterability of sludge. Sequencing batch reactors with two sets of aerobic and alternating anoxic/aerobic conditions were used. In each set, one reactor served as a control and the other was subject to the ozone treatment (doses in the range of 0.016-0.080 mg O3/mg TSS of initial excess sludge). The level of total suspended solids (TSS) in each reactor was controlled at 1,800 mg/l. To evaluate settleability and dewaterability, settling kinetic studies, sludge volume index (SVI) and capillary suction time test (CST) were used. For extraction and quantifying sludge biopolymers, thermal-ethanolic extraction was employed. The ratio of bound-to-total extracellular polymer substances (EPS) was higher for the strictly aerobic reactor than for the alternating anoxic/aerobic one, indicating the stronger structure of the aerobic flocs. After ozone treatment, the fraction of bound EPS was released and solubilized, increasing soluble EPS. Increased apparent food-to-microorganism (F/M) ratio favoured production of EPS in ozonated reactors, enhancing flocculation, which had potential to improve settling. Dewaterability, measured by CST test, was better in alternating anoxic/aerobic reactors than in aerobic ones, indicating that incorporation of an anoxic zone for biological nutrient removal leads to improvement in sludge dewatering. The negative impact of ozonation on dewaterability was minimal in terms of the long-term operation.