# Electrospinning of Chitosan for Antibacterial Applications--Current Trends
## 7 Application Fields of Electrospun Chitosan Nanofibers and Chitosan Derivatives
### Water Filtration
#### 7.1.1 Filtration Types and Membrane Criteria  
Usually, the most often used characteristics to assess membrane performance are high filtration capacity, high water flux and hydrophilicity, low transmembrane pressure, high porosity, small pore size, mechanical strength, antifouling and self-cleaning ability, and space-saving.  
Water filtration can be performed in different ways and a distinction between the 4 types of water filtration systems is possible [155]:  
1. Microfiltration (MF) is extensively used in particle removal and degreasing processes;
2. Ultrafiltration (UF) is a technique used for separating ions from oil, water, and emulsions without the use of chemicals. With UF, paints can be additionally recovered, and fats, oils, and greases from the food sector can be separated, which MF cannot remove. It can remove high-molecular-weight compounds, particles, colloids, and inorganic and organic polymers, but not low-molecular-weight ions such as magnesium chloride, calcium, or sodium [155];
3. Reverse osmosis (RO) membranes are used as a popular filtering technique that removes large amounts of pollutants from water. A semipermeable membrane is used to separate dissolved solutes in this system. The distinction between RO membranes and other filtration membranes is that they do not need actual pores. These membranes are hydrophilic, which means that water may readily pass through them. They are easy to set up and provide for a high production capacity [156], but their limiting factors are biofouling and high transmembrane pressure requirements. The biological fouling increases as the number of eliminated microorganisms and other particles in water increases [157], which is one of the drawbacks of using RO membranes. Several studies have shown that at greater working pressures bacterial cells shrink in size, resulting in increased penetration through the filter. Therefore, to maintain antibacterial action during long-lasting application of the membrane, lower transmembrane pressure (TMP) is generally sought [8];
4. Electrospun nanofibers provide the possibility of producing membranes of nanocomposites. The pore size and porosity as well as the antibacterial properties are adjustable by use of antibacterial components. Therefore, the easiest method to eliminate microorganisms, metal ions, and dyes while maintaining minimal pressure drops and high water flux is by nanofiber adsorption.  
In particular, nanofiltration membranes are capable of separating particles with sizes ranging from 0.001 to 0.01 \(\upmu\)m because of their high surface-area-to-volume ratio, their pores being in the nano-meter range, the high porosity, and their high air and water permeability. The large surface area of the electrospun nanofiber membranes offer an abundance of adsorption sites for metal ions and dyes, and the higher porosity results in lower driving forces to drive water through the membrane, making the process less energy-demanding and simpler [158].  
With less fouling and lower transmembrane pressure, electrospun nylon nanofibers were able to eliminate bacteria via size exclusion as well as conventional membranes. Thisshowed that nanofibers can enhance membrane function while keeping bacteria filtration levels high [159].  
The right choice of water filtration system depends on the application and can save costs and resources. Table 1 gives an overview of the advantages and disadvantages of the different water filtration systems.