Designed to be used as a post filter to remove or reduce fluoride, lead, arsenic, and MTBE. These are used in conjunction with ceramic filters, and screw onto the bottom threads. For use with gravity filter systems such as the Big Berkey and British Berkefeld systems that use white ceramic filters. Not for use with Black Berkey filters. Technical Specifications: MTBE: The PF-4 is manufactured to exceed ANSI/NSF Standard 53 which requires that MTBE be reduced by at least two thirds from an influent rate (water going in) of 15ppb to an effluent rate (water going out) of less than 5ppb (> 95% reduction*). LEAD: Tests have shown that when 2000 gallon (7,600 liters) of influent water at a concentration of lead of 200 ppb pass through 200 grams of media at a flow rate of 1 gpm (3.8 lpm), the lead level was reduced to less than 1ppb (>99% reduction). FLUORIDE: Testing for fluoride was based on 20-30ppm of the ion in the influent aqueous solution at a flow rate of no more than 3 gpm (11 lpm) per cubic foot of media. Results of 95% reduction). Under optimum conditions, effluent concentrations of less than 50 ppb were readily achieved (>99.75% reduction). REPLACEMENT: Under normal conditions it is recommended that PF-4 elements be replaced at six-month intervals (Actual capacity is dependent on the presence of other competing contaminants in the source water. High levels of total organic carbons, volatile organic compounds or certain inorganic minerals may reduce the capacity and efficiency of these elements.)