A flow-type electrochemical wastewater treatment cell that uses a natural water flow without circulation or stirring device was investigated. Aluminium pipes were used as substrates; they were coated with Sb-doped SnO2 in order to fabricate Sb-SnO2/Al electrodes, and stainless steel pipes of a different diameter were used as counter electrodes. Three types of cells were prepared, in which electrodes (anode or cathode) were placed perpendicularly; these electrode pipes were arranged in concentric circles in a container. Electrolysis was conducted using a solution containing N,N-dimethyl-p-nitrosoaniline that can selectively react with OH radicals, and the changes in concentration were measured. The concentration decreased clearly when the solution passed through the electrodes only once. It was evident that the treatment efficiency differed due to the cell design, even when the same electrode (property and area), electrolyte, current density and water volume were used. This study suggests that in order to treat wastewater efficiently, it is important to consider not only the material of the electrodes but also their shape or arrangement and the water flow in a cell.