The two nonsteroidal anti-inflammatory drugs diclofenac (DCF) and aceclofenac (ACF) were monitored for the first time together with their major human phase-I metabolites, namely, 4'-hydroxydiclofenac (4'-OH-DCF) and 4'-hydroxyaceclofenac (4'-OH-ACF), in untreated and treated sewage samples, collected from a municipal wastewater treatment plant which operated a continuous activated sludge (CAS) treatment in parallel with membrane bioreactor (MBR) technology. Mean concentrations of DCF and 4'-OH-DCF in the influent samples amounted to 349 and 237 ng/L, respectively, whereas levels of 4'-OH-ACF (average, 59 ng/L) exceeded those of its parent drug approximately 2-fold (31 ng/L). Removal rates of 26 and 56% were achieved for 4'-OH-DCF following CAS and MBR treatment, respectively. The most efficient elimination was observed for 4'-OH-ACF in the MBR with only 4% of the influent concentration remaining in the treated sewage. Biodegradation experiments in batch reactors loaded with mixed liquor demonstrated that ACF underwent rapid ester cleavage to liberate DCF, thus constituting a possible source of DCF release during biological sewage treatment. Studies on the microbial metabolism of DCF (295 Da) in controlled laboratory settings allowed us to identify three novel aerobic biotransformation products. Structure elucidation by means of ultraperformance liquid chromatography-electrospray ionization-hybrid quadrupole-time-of-flight-mass spectrometry in conjunction with H/D-exchange experiments unequivocally identified them as deriving from nitrosation of the hydroxyl group in the carboxylic acid moiety (324 Da) and from nitration of one of the aromatic ring systems (340 Da). A third microbial metabolite emerging in the test medium was assigned as dichlorobenzoic acid (190 Da), possibly formed by N-dealkylation of DCF and subsequent carboxylation. Taken together, this work constitutes the first report on the occurrence of ACF and the human metabolites 4'-OH-DCF and 4'-OH-ACF in wastewater, underpinning the need of incorporating metabolites excreted by humans in monitoring surveys as part of a risk evaluation for environmentally relevant pharmaceuticals.