Physical filtration acts much like the sieve you use to separate your pasta from the water in which you cooked it. As water passes through a screen or in some cases layers of sand, the largest particles can be caught by the screen and thus filtered out. Membrane-like materials can remove smaller particles based on the size of the pores of the membrane. For example, nanofiltration can remove particle sizes down to 0.0001 to 0.005 microns like viruses, pesticides, and herbicides. Adding coagulants like lime to water can cause particles to clump together so that they can be more easily filtered.
Another form of physical filtration is the process of reverse osmosis where water is pushed through a membrane at pressure. That pressure results in a blockage by the membrane of particles dissolved within the water while the water passes through. Reverse osmosis can remove metal ions, and is also the technique used in most desalination plants to remove aqueous salts.