Which membrane filtration process requires the least amount of operating pressure?

Microfiltration is the membrane filtration process that operates at the lowest pressure compared to other membrane technologies such as ultrafiltration, nanofiltration, and reverse osmosis. This process is designed to remove larger particles, including bacteria and some suspended solids, with pore sizes typically in the range of 0.1 to 10 microns.

The lower operating pressure required for microfiltration is primarily because the contaminants it targets are larger in size, allowing the process to function effectively without the need for high-pressure application. In contrast, ultrafiltration, nanofiltration, and reverse osmosis involve smaller pore sizes and are aimed at removing finer particles, dissolved substances, and ions, which necessitate higher pressures to overcome the osmotic potential and achieve adequate permeate flow.

Therefore, microfiltration's capacity to operate effectively at lower pressures is a key characteristic that distinguishes it from the other filtration processes listed, making it the correct choice for this question.