**Abstract**

The removal of nitrate and nitrite simultaneously was investigated by Donnan dialysis (DD) using a Response Surface Methodology (RSM) approach. DD is a membrane process that consists of cross-ion exchange having the same electric charge through an ion-exchange membrane separating two solutions. In addition to being easy to handle, DD process is continuous, economical, requiring only few chemicals and low pumping energy. Statistical tools were applied to investigate the simultaneous removal of nitrates and nitrites by DD. The RSM is an efficient statistical strategy to design experiments, build models, determine the optimum conditions, and evaluate the significance of factors, even the interaction between them. A preliminary study was performed with three commercial membranes (AFN, AMX, ACS) in order to determine the experimental field based on different parameters. Then, a full-factor design was developed to determine the influence of these parameters and their interactions on the removal of nitrates and nitrites by DD. The RSM was applied according to the Doehlert model to determine the optimum conditions. The use of the RSM can be considered a good solution to determine the optimum condition compared to the traditional "one-at-a-time" method.

**Keywords:** nitrate, nitrite, Donnan dialysis, anion-exchange membrane, Doehlert
