The principle of water filtration is that the filter material is able to remove contaminants from the water by trapping them within its pores or on its surface. The size of the pores in the filter material determines which contaminants can be removed. For example, a filter with smaller pores will be able to remove smaller contaminants, such as bacteria and viruses, while a filter with larger pores may only be able to remove larger particles, such as sediment and dirt.