The objective of this paper is to demonstrate how occurrence, transport and fate of trace chemicals can be assessed when modelling wastewater treatment plants (WWTP). A modified version of the International Water Association (IWA) Benchmark Simulation Model No 1 (BSM1) used to evaluate control strategies in activated sludge systems is expanded with the ASM-X framework, to describe the behaviour of xenobiotic trace chemicals. In this paper, the capabilities of the BSM – ASM-X combined approach are illustrated with two cases studies. First, the occurrence of the antibiotic sulfamethaxole (SMX) is modelled using an influent generator. Administration patterns, bioavailability and body residence time are the basis to generate the user-defined profiles that will describe SMX daily variation patterns in the raw wastewater. Additional simulations also show that transport conditions such as sewer length, oxygen concentration and total suspended solids (TSS) loading might have a strong effect on the concentration and the dynamic behaviour of SMX and its metabolites. The second case study presents the fate of tetracycline (TCY), ciprofloxacin (CIP), diclofenac (DCF) and carbamazepine (CMZ) in the benchmark activated sludge system. The results of these investigations demonstrate that different operating conditions can have opposite effects on the studied compounds, especially when they present co-metabolic/inhibitory behaviour with other substances present in the influent wastewater. Finally, the paper is complemented with: i) a critical discussion of the presented results; ii) a thorough analysis of the limitations of the proposed approach; and, iii) future pathways to improve the overall modelling of trace chemicals.