There are four different types of membranes depending on pore size and the molecular weight of the solute they can reject, as illustrated in Figure 2.1.
• Microfiltration (MF)—The membranes with a nominal pore size of 0.1 μm to 1 μm are typically called microfiltration membranes, although there are no universally accepted criteria. A vast majority of membranes have pore size distributions except for track-etched and patterned membranes. Nominal pore sizes are determined based on the smallest particles that a membrane can reject at 90% to 99% efficiency (depending on the manufacturer). It can be also determined based on pore size distribution curve, where the pore size that falls in the top 1% to 10% is called the nominal pore size. Pore size distribution is typically measured by a porometer, which measures gas flow rate through the membrane of which pores are prefilled (or wetted) with the liquids of known surface tension (Kim et al. 1994). While gas pressure is rising gradually, the largest pores are opened up first and the smaller pores follow sequentially in the order of pore sizes because large pores have a lower capillary pressure than small pores. The pore size distribution is calculated mathematically by analyzing the relation between the gas permeation rate and the gas pressure. In a practical membrane filtration occurring with a cake layer formed on the membrane surface, the cake layer acts as a dynamic membrane. As a result, membranes can reject much smaller particles than their pore size suggests. The quality of MF and UF permeates may be indistinguishable from each other in most applications in raw water and wastewater treatment once the cake layer is stabilized. Because cake layers dominate the membrane permeability in a vast majority of MF and UF applications, there are no substantial differences in operating pressures and other design parameters between MF and UF with few exceptions. MF and UF typically run at low pressure (e.g., 6 bar or below) whereas NF and RO run at relatively high pressure (e.g., 6 bar or higher).