A fixed bed electrochemical cell was tested as reactor for treatment of waters by direct electrolysis. The aim was the development of a reactor configuration for effective and controllable primary and secondary disinfection. Stacks of titanium grids coated with Ru/Ir oxides or platinum were used as anodes and cathodes, respectively. The electrode packings were arranged in series, the system worked in continuous mode. The reactor was characterised for mass transfer (by the limiting current densities technique) and flow behaviour (by pulse-response curves with an inert tracer). Electrolyses were carried out with 100 mg dm(-3) of chloride ions, and current density ranging from 2.5 to 7.5 mA cm(-2). Active chlorine concentrations from 0.3 to 1.2 mmol dm(-3) were obtained in the reactor outlet, depending on the conditions. The higher value was measured with i = 7.5 mA cm(-2) and flow conditions corresponding to a Reynolds number of 5: under these conditions, about 1 mg dm(-3) of chlorates were detected, otherwise the selectivity of the process towards active chlorine was about unit. A mathematical model at steady state was implemented, and pseudo first order kinetics was used to describe chemical and electrochemical reactions. The model allowed obtaining the concentration profiles of chloride and chloride oxidation by-products which were compared with experimental data, with good agreement in a wide range of flow rates. The configuration allowed a well-defined concentration profile: the active chlorine in the outlet was always 50% less than the value inside the reactor, allowing to exploit the process for primary and secondary disinfection. (C) 2012 Elsevier B.V. All rights reserved.