This work focuses on the scale-up of electrochemical and photoelectrochemical oxidation processes with diamond anodes for the removal of organic pollutants and disinfection of treated urban wastewater, two of the most important parameters for the reclaiming of wastewater. The removal of organics was studied with actual biologically treated urban wastewater intensified with 100 mg dm(-3) of caffeine, added as a trace organic pollutant. The disinfection was also studied with biologically treated urban wastewater, and Escherichia coli was used to monitor the efficiency of the process. Results obtained with a single DiaCell® 101 were compared with those obtained with a single-stack DiaCell® 1001 and with a pilot plant made up of five of these stacks. Results obtained demonstrate that scale-up is not a simple but a very complex process, in which not only the electrode and the irradiation dose are important but also mass transfer conditions. Enhanced mass transport conditions have a determining and very positive effect on the removal of organics and a negative effect on the disinfection. Likewise, ultraviolet (UV) irradiation affects in a different way in the different setups used, having a great influence on the removal of complex organics and on the speciation of oxidants produced during disinfection. This works helps to understand the key differences observed in the scale-up, and it is a first approach for future works focused on the real application of conductive diamond electrochemical oxidation.