Although wastewater treatment plants are a line of defense in minimizing indiscriminate output of microconstituents to natural waters, we do not possess a fundamental understanding of the mechanisms involved in microconstituent removal during wastewater treatment. With this in mind, experiments were designed to investigate the factors that can influence the fate of four microconstituents, carbamazepine (CBZ), 17alpha-ethinylestradiol (EE2), iopromide (IOP), and trimethoprim (TMP), during biological suspended culture treatment. Specifically, the role that various ecological members of biological treatment systems play in biotransforming these compounds was evaluated. Sorption assays were performed with inactivated biomass samples (ammonia oxidizing bacteria (AOB), laboratory enriched heterotrophic cultures free of active nitrifiers with low (Ox⁻) or high (Ox⁺) oxygenase activity, and a nitrifying activated sludge (NAS) from a full-scale wastewater treatment plant) to determine whether partitioning dictates removal of individual microconstituents. No microconstituents sorbed to the AOB culture. Neither CBZ nor IOP sorbed to Ox⁻, Ox⁺ and NAS cultures; however, EE2 and TMP sorbed to the Ox⁻, Ox⁺ and NAS biomass. Sorption was positively influenced by the presence of exopolymeric substances (EPS) associated with the cultures. The protein content of EPS affected EE2 and TMP sorption more appreciably than the polysaccharide content of EPS. Further experiments were performed to investigate microconstituent biodegradation by AOBs, Ox⁻ and Ox⁺ cultures. The influence of growth state and oxygenase activity on biotransformation by each culture was also evaluated. Results indicate that EE2 was the only microconstituent that was amenable to biotransformation by batch cultured AOB and heterotrophic cultures. EE2 was biotransformed but not mineralized by AOB chemostat and batch cultures. TMP was not transformed by AOB batch or chemostat cultures; however both EE2 and TMP were transformed by Ox⁻ and Ox⁺ chemostat cultures. Radiolabeled studies showed that EE2 was mineralized by this culture. Kinetically, AOBs dominated EE2 transformation to monohydroxylated metabolites; however, both Ox⁻ and Ox⁺ cultures further degraded and mineralized EE2 and metabolites generated by AOBs. These results indicate that biotransformation of EE2 by NAS may be limited by heterotrophic activity whereas TMP fate may be a function of heterotrophic activity only. Oxygenase activity did not limit EE2 or TMP biotransformation in chemostat cultures. Subsequent experiments that were performed to identify the factors that influence heterotrophic degradation of EE2 and TMP indicated that the presence of readily biodegradable substrates slows EE2 and TMP biotransformation. The impact of slowly biodegradable substrates like EPS on EE2 and TMP degradation was unclear. These results suggest that EE2 and TMP are most amenable to biodegradation in bioreactors where endogenous conditions dominate. / Ph. D.