BACKGROUNDCoupled systems of photocatalytic and biological oxidation processes for the treatment of non-biodegradable wastewater have been widely studied. Additionally, the use of membrane bioreactors (MBR) can improve the coupled system since the photocatalytic pretreatment can be shortened. Optimization of the combined process involves adjusting the intensity of the photocatalytic pretreatment to minimize the hydraulic retention time (HRT) in MBRs thus ensuring the shortest total time possible. The present study presents a new fast and easy assay based on biological oxygen demand (BOD) measurement to predict the optimum HRT in the MBR for a photo-Fenton pretreated effluent. RESULTSA non-biodegradable wastewater was mineralized by photo-Fenton to 40% and 57%. Both pretreated effluents were further treated in a MBR and the minimum HRTs were experimentally determined, being 7.7h for both 40% and 57% mineralization. The BOD based tool comprises the parameters ' and ' related to the biodegradation rate and the effluent quality, respectively. The estimated HRTs were 7.5h for both 40% and 57% mineralization, confirming the results obtained in the coupled system. CONCLUSIONThe use of the BOD based tool allowed the correct HRT values to be obtained in order to achieve the maximum treatment capacity of the system maintaining the best effluent quality. (c) 2013 Society of Chemical Industry