A filter-press FM01-LC reactor electro-generating active chlorine on a synthesized Ti/RuO2-ZrO2-Sb2O3 anode is evaluated to account for whether the optimization of the active chlorine concentration, or the oxidation-reduction potential (ORP) in solution is the most important variable during the elimination of 20mgL-1 sulfamethoxazole (SMX). The three most important factors affecting the reactor performance (chloride concentration, current density and volumetric flow) are used in wide ranges via a Box-Behnken design (BBD) of experiments in the absence of SMX (4h): 0.01 to 0.2mol L-1 NaCl, 10 to 250 A m-2 and 1 to 5L min-1. An initial pH of 5.8 is utilized due to the HOCl predominance, presenting the most oxidizing potential among chlorine species. Higher chlorine concentrations are produced at higher NaCl contents (i.e. 0.2mol L-1 NaCl). Volumetric flows around 3.87L min-1 should be enough to furnish chloride ions to the anode surface by convection. The current density strongly depending on pH is the decisive parameter to either maximize the chlorine concentration or the ORP. A higher active chlorine production at 250 A m-2 eliminates the pollutant before 5min, and a lower amount of intermediates remain after 90min of electrolysis; since this condition effectively acts against the reducing character of SMX. The ORP optimization cannot overcome this reducing environment due to a lower amount of chlorine species produced at 10 A m-2, whereby traces of this contaminant and more intermediates are detected after the treatment.