**5.1. Filtration**

In the air filtration field, electrospun non-woven fabrics are taking the place of traditional filter media like activated carbon and fiberglass, because of their excellent performance in filter efficiency and pressure loss. It has been demonstrated by various authors [71] that electrospun nanofibers can remove the volatile organic compounds (VOC) in the air, with some samples filtering faster than conventional activated carbon. Scientists have found that the slip flow mechanism becomes dominant due to the ability of the smaller fiber to disturb the air flow instead of non-slip flow in traditional filters [72]. Surface loading of dust particles takes place on non-woven fabrics coating conventional filters. In one work, Heikkila et al. [73] optimized the coating thickness of polyamide nanofibers required to improve filtration efficiency and obtain an efficiency of over 95% (0.16 µm Particles) for a 0.5 g/m2 coating; in another work, Hung et al. [74] studied the effect of fiber diameter on capture efficiency and pressure drop and they observed that when fiber diameter was reduced from 185 to 94 nm, the filtration of 50–500 nm nanoaerosol can be achieved only with a significant increase in the pressure drop.

In the water filtration field, typically used membranes for ultra-filtration (UF) or nano-filtration (NF) filters are made using the phase immersion method. The torturous porosity in these membranes usually has a low flux rate. Therefore, some researchers turned to electrospun nonwoven fabrics to make use of their high flux rates. The electrospun non-woven fabrics usually appear as a functional layer in a composite membrane. Yoon et al. [75] studied a composite membrane containing an electrospun PAN scaffold with an average diameter from 124 to 720 nm and a porosity of about 70%, together with a chitosan top layer, and found this membrane exhibited a flux rate that was an order magnitude higher than commercial NF membranes over 24 hours of operation, while maintaining the same rejection efficiency (>99.9%) for oily wastewater filtration. Many other materials having composite membranes with electrospun nonwoven layers also have proved this characteristic [76-78]. Some other research has also demonstrated electrospun non-woven fabrics' notable performance in selective filtration of cells or bacteria and adsorption for viruses from fluid [79-80].
