A membrane is a thin film that treats water by acting as a selective barrier. It separates materials when a driving force, typically pressure, is applied across the membrane. The use of membranes for water treatment has risen significantly in recent decades. The most common water treatment membrane processes are microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). MF and UF are porous membranes, while UF and NF are nonporous.