This P5-20 is one of Everpure’s prefilters for removing sediments and dirt particles from your water, and it is NSF approved. It is designed for use in ice making machines and is 20 inches in diameter. The sediment prefilter is rated at 5 microns and because it is a prefilter, it requires at least one primary cartridge fore safe and healthy water. The water flows through the filter and the dirt and sediments are removed. If the filter is not removed at the appropriate time, the filter may clog and no water will flow through. Sometimes, the water forms a channel and will flow through the filter but will not be filtered. Also, filters can sometimes dump carbon sludge back into the water. It any of these things happens, you know that it is time to change your water filter.