During the filling of sterile beer kegs and bottles, the beer is typically filtered to 0.45µm either with one pleated cartridge or a two / three stage system (typically 5µm > 1µm > 0.45µm). This is for clarification and to prevent microorganisms spoiling the final packaged product.
Beer is stored in sterile tanks, which have 0.1µm or 0.2µm air vent filtration to prevent ingress of bacterial contamination.