Final answer:
For filtration of particulate matter such as bacteria, membrane filters with a pore size of about 0.2 µm are commonly used. However, to remove viruses, smaller pore sizes are necessary.
Step-by-step explanation:
The size of the filter used for filtration of particulate matter depends on the specific requirements of the process and the size of the particulates being removed. For removing bacteria from liquids, membrane filters with an effective pore size of about 0.2 µm are typically used. This size can even retain bacterial cells smaller than the average bacterium (1 µm), but not small enough to remove viruses, as they are smaller than this filter's pore size. In certain cases where virus removal is required, smaller pore sizes would be necessary. Moreover, for different types of filtration processes like reverse osmosis, the filters used have very small pore sizes that can remove ions and molecules from water.