Precise filter(also named protecting filter) is always used after the multi-media fiter but before the RO main machine or ultra filter system. it can remove the tiny substance escaped through the mechanic filters, such as the green sand, the carbon granule, it can protect the RO membrane not to be damaged by the big substance. The filter cartrdge inside the precise filter can be devided into 0.5us,1us,5us,10us etc, it's depends on the precise request to the water.