## 1 Introduction  
Many researchers have recently been interested in the fabrication of ultrafine and uniform fibers using various materials such as organics, inorganics, and composites [1]. Electrospinning is one of the versatile methods for producing the nanofibers, which have one-dimensional nanostructures [2]. The interconnected nanostructuring of nanofibers in distinction from the conventional nonwoven media can show unique properties. Thus, electrospun nanofibers have been applied in the field of electronics, catalysis, drug delivery, tissue engineering, and filtration [3; 4; 5; 6]. Electrospun nanofiber membranes (ENMs) were widely applied for the particle separation from water. Thus, they can be used for treatment of wastewater prior to the treatment by UF (Ultrafiltration), NF (Nanofiltration) and RO (Reverse osmosis) [7; 8; 9; 10]. Fiber diameter is a very important factor in determining the performance of ENMs and is dependent on the electrospinning parameters. The electrospinning parameters are comprised of solution properties, operating conditions, and ambient conditions. Parametrizationis a mathematical process consisting of expressing the state of a system, process, or model as a function of some independent quantities called parameters. The state of the system is generally determined by a finite set of coordinates, and the parametrization thus consists of one function of several real variables for each coordinate. The parametrization study is important to fabricate the desired fiber morphology. However, there are few reports related to the salt effect on the diameter of PES nanofibers, and the methods of predicting the fiber diameter from information of one parameter alone does not currently exist because the parameters interact simultaneously in the electrospinning process. Therefore, the experimental investigation of individual parameters and interactions between parameters acting simultaneously was carried out, introducing the response surface methodology (RSM) [1]. The fiber diameter influences various properties of the electrospun nanofibrous membranes (ENMs) (i.e., morphology, specific surface, water treatment performance, and so on). To fabricate electrospun nanofibers which have the desired fiber diameter and morphology, it is necessary to adjust various processing parameters (i.e., flow rate, tip-to-collector distance, and so on) and solution parameters (i.e., salt concentration, polymer concentration, and so on). Response surface methodology (RSM) is a collection of mathematical and statistical techniques used to analyze the experimental data acquired in the experimental design and to develop the suitable model. In this study, the PES solution, including lithium chloride (LiCl), was used to produce electrospun nanofibrous membranes (ENMs). The electrospinning parameters considered were solution properties (i.e., salt concentration, polymer concentration) and operating conditions (i.e., flow rate, tip-to-collector distance), and the response in the experiment was the average fiber diameter. A mathematical model of the fiber diameter was obtained during the RSM process. Since the fiber morphology can have a direct effect on the membrane structure, a thorough understanding of the electrospinning parameters is needed for the wide application of nanofibers [11]. When applied to filters for removing the particulate contaminants in air or water, ENMs provide many advantages, such as the high porosity for higher permeability, the interconnected pores for lower fouling, and the high surface area for larger particle retention [12]. The permeability tests and the filtration tests with micro-particles presented in water were conducted to demonstrate the effect of electrospinning parameters on filtration properties. The membranes for the filtration experiments were manufactured utilizing the information obtained in response surface models.