**1. Introduction**

The growth of population, industrialization, and rapid urbanization increases the pollution of nitrogen (N). Nitrate and nitrite are often used as pollutants; they present a water-quality problem [1]. Nitrate is primarily responsible for water eutrophication and infectious diseases in the environment [2]. In comparison to nitrate, the nitrite has a higher level of toxicity to human health. In the body, methemoglobin can be formed when it combines with hemoglobin, reducing oxygen transport. Additionally, this compound can be converted into carcinogenic nitrosamine, which is linked to hypertension, leukemia, brain tumors, stomach cancers, and bowel cancers [3]. The World Health Organization (WHO) suggests a maximum nitrate content of 50 mg/L

in irrigation water and 0.5 mg/L in drinking water to protect the public health from the harmful effects of high nitrate and nitrite concentrations [4].

It was possible to remove nitrate and nitrite chemically using chlorination and physicochemically using coagulation-flocculation [5]. In contrast to traditional biological approaches [6], bioadsorption [7], and photocatalytic denitrification [8], most methods require oxidation in order to remove nitrite. Removing nitrate and nitrite simultaneously avoids this oxidation step, which requires chemical products to turn nitrite into nitrate.

Donnan dialysis (DD) was chosen because it is mostly economical. Because it only needs, a small amount of chemicals, pumping energy, and is simple to operate; this process is continuous and inexpensive. In the DD process, an ion-exchange membrane separates a compartment containing the solution to be treated (feed) from a compartment containing the solution that receives the target ions (receiver). The concentration gradients of the ions transported through the membrane (counter-ions) control the ion-exchange kinetics [9, 10]. The DD process involves the stoichiometric exchange of counter-ions, or ions with the same charge, over an ion-exchange membrane, and it is ended when Donnan equilibrium is attained [11].

As is common knowledge, the DD process is used to purify, concentrate, and remove various ions from wastewater and industrial effluents, including boron [12, 13], fluoride [14, 15], chromium [16, 17] and nitrates, nitrites [18, 19] using different type of anion-exchange membranes. Although Donnan dialysis has certain advantages in terms of cost and energy efficiency, industry does not adopt it primarily due to its slow kinetics. The almost of applications have been studied at laboratory scale.

The typical "one factor at a time" method of optimizing multivariate systems is not only time-consuming, but also often does not take into account the effects of crossinteractions between experimental factors. Furthermore, this approach implies that the best levels must be determined by multiplying experiments, which is not always true. By integrating Doehlert's experimental design [20] with response surface methods to simultaneously optimize all influential parameters, these drawbacks of the single-factor optimization procedure can be avoided.

The Box and Wilson-developed Response Surface Methodology (RSM) is a set of mathematical and statistical methods for studying situations like the one that is being posed using an empirical model. The RSM is a useful tool for process optimization. The benefit of this method over the traditional one is that it takes less time and costs less. Doehlert designs have a number of advantages over other designs, such as central composite or Box-Behnken designs. Because the number of levels can vary from one variable to another, there is greater flexibility in assigning a high or low number of levels to the selected variables, saving time on studies. Additionally, adjacent hexagons may successfully occupy a space because they do not overlap, making Doehlert designs more effective at mapping space [20].

The present investigation presents the application of the RSM applying Doehlert experimental design studies to look into the simultaneous removal of nitrates and nitrites by Donnan dialysis. In order to set up the experimental field, one component (nitrate or nitrite) was first eliminated from the feed compartment using different parameters, such as the concentration of counter-ions and the concentration of nitrate and nitrite separately. In order to improve the procedure and comprehend the simultaneous transport of nitrites and nitrites, the removal of two components (nitrate and nitrite) in the feed compartment was then explored by the Response Surface Methodology (RSM) by Doehlert design.
