PF-2 filters are optional filters, placed underneath the Black Berkey filters in the lower tank and are specifically used to filter out arsenic, fluorides and heavy metals from the water. Although the Black Berkey filters themselves perform well on this type of pollution, a high concentration of these pollutants could saturate them before their normal life span. To determine if these filters are necessary for your system, refer to the analysis of the water you are filtering. More details on these filters can be found in the product sheet.