Describe the specific physical straining mechanism that distinguishes microfiltration from ultrafiltration in terms of removed particle size range.

Physical straining is a separation mechanism where a porous membrane acts as a physical barrier, blocking particles larger than its pore openings while allowing smaller particles and the fluid to pass through. The specific physical straining mechanism distinguishes microfiltration from ultrafiltration primarily based on the relative size of the membrane pores to the particles being removed. Microfiltration (MF) is a pressure-driven membrane process that removes suspended solids, bacteria, and larger colloids. Microfiltration membranes typically have pore sizes ranging from approximately 0.1 to 10 micrometers (µm). The specific straining mechanism for microfiltration is absolute sieving or direct physical exclusion. This means that any particle larger than the membrane's defined pore size is completely prevented from passing through. The membrane functions like a direct sieve, with particles being rejected primarily at the membrane surface or within the pore mouth because they are substantially larger than the membrane's pores. For instance, a 0.2 µm microfiltration membrane will entirely block bacteria, which are generally larger than 0.2 µm. Ultrafiltration (UF) is also a pressure-driven membrane process, but it removes much finer particles than microfiltration, including viruses, smaller colloids, and macromolecules like proteins. Ultrafiltration membranes typically have pore sizes ranging from approximately 0.005 to 0.1 micrometers (µm), or 5 to 100 nanometers (nm). The specific straining mechanism for ultrafiltration is also physical sieving, but it operates at a significantly finer scale, often quantified by a molecular weight cut-off (MWCO), which represents the molecular weight above which 90% of a solute is rejected. This mechanism involves the direct physical exclusion of individual macromolecules and submicron particles based on their size and shape relative to the extremely small membrane pores. In ultrafiltration, the target particles, such as proteins or viruses, are often closer in size to the membrane pores than in microfiltration. This leads to a more precise, size-selective sieving process where even slight differences in particle dimension relative to the constricted pore opening critically determine passage or rejection. For example, a UF membrane with a 10 kDa MWCO will primarily retain proteins with molecular weights of 10 kilodaltons or higher, due to their physical inability to pass through the very narrow pores. Therefore, the distinguishing physical straining mechanism is the *scale of the physical exclusion relative to the particle size*: microfiltration employs absolute sieving to block particles that are typically significantly larger than the membrane pores (like a coarse filter), whereas ultrafiltration utilizes a much finer physical sieving to directly exclude particles and macromolecules that are on the order of tens to hundreds of nanometers, where the particles are often only marginally larger than, or sterically hindered by, the membrane pores, enabling the removal of much smaller contaminants.