Biological wastewater treatment processes exhibit variable extents of organic micropollutant (MP) biotransformation. Conventional activated sludge (CAS) systems have been frequently studied, but up-flow anaerobic sludge blankets (UASBs) have received less attention. We hypothesize that the unique wastewater microbial communities in a CAS and UASB will perform different types of MP biotransformations at different rates. To test this hypothesis, we collected influent and effluent samples from a full-scale CAS and a pilot-scale UASB receiving the same influent. We measured each sample by means of high-resolution mass spectrometry to quantify up to 175 MPs in each sample. We measured 39 MPs at concentrations greater than the limit of quantification in both systems. We used a variety of metrics to identify 17 MPs that exhibited better removal in the CAS, 16 MPs that exhibited better removal in the UASB, and 7 MPs that exhibited similar removal in both systems. To validate these findings and to investigate the types of biotransformations performed by each microbial community, we conducted batch experiments in bioreactors seeded with wastewater microbial communities harvested from the CAS and UASB into which we spiked 24 MPs of particular interest (12 MPs that exhibited better removal in the full-scale CAS and 12 MPs that exhibited better removal in the pilot-scale UASB). Temporal sampling revealed MP biotransformations in both experiments, some of which had rates that agreed with our observations made at full- and pilot-scale, and some of which had rates that did not. Nevertheless, analysis of biotransformation products revealed that those formed exclusively in the batch experiments seeded with wastewater microbial communities harvested from the CAS were primarily the result of oxidations (monohydroxylation, ether dealkylation, amine dealkylation). Interestingly, those formed in both types of experiments seeded with wastewater microbial communities harvested from either the CAS or UASB were primarily the result of redox-independent hydrolyses (ester hydrolysis, amide hydrolysis). Finally, different biotransformation products of androsterone were found in experiments seeded with wastewater microbial communities harvested from the CAS or UASB; the biotransformation product found in the CAS experiment was the result of a monohydroxylation (oxidation) and the biotransformation product found in the UASB experiment was the result of a hydrogenation (reduction). Together, our data provide novel insights on the relative functioning of two wastewater microbial communities operated under different redox conditions. Our study demonstrates the potential of the UASB reactor to biotransform MPs and the relative biotransformation potential of CAS and UASB processes. We also present structures of seven biotransformation products, five of which (transformation products of gabapenitn, diphenhydramine, amphetamine, ethyl-butylacetylamino-propionate, adrenosterone) have not been previously reported in the literature.