A model describing ibuprofen and ketoprofen biodegradation by activated sludge from three different wastewater treatment plants (WWTP) was developed in this study. This model successfully described the biodegradation profiles observed at two different initial concentrations of each compound, where a lag-phase was observed prior to the biodegradation of each compound. Twelve ibuprofen and ketoprofen degrading isolates were identified in this study from the WWTP sludge showing the best removal performance. One of these isolates was characterised via another model, where biodegradation was dependent on biomass growth rate as well as the ibuprofen concentration. The fact that different models were needed to describe the biodegradation by activated sludge and a pure culture suggests that the biodegradation of non-steroidal anti-inflammatory drugs (NSAIDs) depends on the microbial community, thus pharmaceutical biodegradation models may require adaptation depending upon the system. This study provides an advance towards modelling pharmaceutical biodegradation in WWTPs.