The purpose of filtration is to remove suspended particles from water by passing the water through a medium such as sand. As the water passes through the filter, floc and impurities get stuck in the sand and the clean water goes through. Filtration is now required for most water treatment systems, but filters must reduce turbidity to less than 0.5 NTU in 95% of each month's measurements and the finished water turbidity must never exceed 5 NTU in any sample. Regulations require that at least 99.9% of Giardia cysts and 99.99% of viruses be removed from drinking water, since it is difficult to test directly for these microorganisms, turbidity in water can be used as an indicator for their presence. The most common method of filtration is conventional filtration, where filtration follows coagulation/flocculation and sedimentation, this type of filtration results in flexible and reliable performance, especially when treating variable or very turbid source water.