Pharmaceutical agents accumulate in wastewater after consumption, but the conventional sewage treatment process is unable to remove them completely. The occurrence of certain compounds in the environment brings forth serious problems even at low concentrations. In this study, the microbiological elimination ability of four non-steroidal anti-inflammatory drugs (NSAIDs)—ibuprofen, naproxen, ketoprofen and diclofenac—were investigated under laboratory, pilot and plant conditions. Mixed cultures from environmental matrices presumably have the ability to reduce the concentration of target agents effectively. According to our analytical measurements, certain mixed cultures gained from natural habitats were even capable of reducing the amount of diclofenac efficiently, after being enriched to a 10⁹ colony-forming unit—CFU/mL scale, and inoculated in adequate quantity. Target NSAIDs were detected at µg/L levels in both influents and effluents from wastewater treatment plants (WWTPs). The results showed that inoculated conventional activated sludge sewage treatment technologies have high efficiency for removing ibuprofen, ketoprofen and naproxen. The diclofenac-specific bacteria mix exhibited mild but positive removal efficiency compared to the control plant. This removal ability is also influenced by the hydraulic retention time (HRT).