||Estrogens are potent endocrine disrupting compounds commonly found in municipal wastewater. While they can be removed in the activated sludge wastewater treatment process, the degree to which they are removed varies considerably among wastewater treatment plants, even of the same type. This study demonstrated that one possible reason for this variation is the differences in microbial ecology as well as solid retention time (SRT). Two laboratory-scale sequencing batch reactors (SBRs), each performing simultaneous carbon oxidation and nitrification, were fed different organic carbon sources. The SBR receiving peptone, a complex organic carbon source, resulted in a more diverse overall bacterial community and ammonia oxidizing bacteria (AOB) community than the SBR receiving glucose, a simple sugar. The peptone-fed SBR also had a greater proportion of AOB than the glucose-fed SBR. This increased proportion of AOB helps to explain the faster nitrification kinetics in the peptone-fed SBR than the glucose-fed SBR, although both reactors shared similar COD oxidation kinetics. Each SBR was also fed 17?-estradiol (E2), a natural estrogen, and 17?-ethinylestradiol (EE2), a synthetic estrogen. Although the removal of E2 and EE2 in the liquid phase was rapid and similar for both SBRs, the estrogens sorbed onto the solid phase indicated clear differences in the removal characteristics between the two SBRs. Specifically, the EE2 removal in the peptone-fed SBR was significantly more complete than the glucose-fed SBR. Therefore, the nitrifying kinetics heavily influenced the degradation characteristics of EE2, but not nearly as much for E2. However, heterotrophic activity can still be responsible for significant EE2 removal as well as E2 removal. After the SRT had been increased from 20 days to 40 days, the EE2 eventually decreased to approximately 40% of the previous amount in each SBR, and the E2 similarly decreased to 30%. These results suggest that increased SRT leads to more complete estrogen degradation.