The paper presents the results of a study focused on the occurrence and removal of dioxins and furans (PCDD/F) and poly-chlorinated biphenyls (PCB) in both conventional and membrane wastewater treatment processes. It was found that the conventionally activated sludge process could perform a good removal of PCDDs/Fs and PCBs, but the relatively low solid retention time applied and the presence of suspended solids in the effluent limited the removal capability of the system. On the other hand, the membrane bioreactor was capable of perfectly removing PCDDs/Fs and PCBs giving an effluent characterised by concentrations under the limit of detection for most of the tested compounds. This efficiency was the result of both the solids removal from the effluent (permeate) and the application of prolonged solid retention times which enabled the bioconversion of those compounds as demonstrated by the mass balances. A mathematical model was developed to predict the final fate of a given molecule according to the operational conditions applied in the wastewater treatment process. (C) 2010 Elsevier Ltd. All rights reserved.