Water Treatment: PRESSURE FILTERS
Pressure filters are typically used with hot process softeners to permit high-temperature operation and to prevent heat loss. The use of pressure filters eliminates the need for repumping of filtered water. Pressure filters are similar to gravity filters in that they include filter media, supporting bed, underdrain system, and control device; however, the filter shell has no wash water troughs.
Pressure filters, designed vertically or horizon-tally, have cylindrical steel shells and dished heads. Vertical pressure filters (see Figure 6-2) range in diameter from 1 to 10 ft with capacities as great as 300 gpm at filtration rates of 3 gpm/ft≤. Horizontal pressure filters, usually 8 ft in diameter, are 10-25 ft long with capacities from 200 to 600 gpm. These filters are separated into compartments to allow individual backwashing. Backwash water may be returned to the clarifier or softener for recovery.
Pressure filters are usually operated at a service flow rate of 3 gpm/ft≤. Dual or multimedia filters are designed for 6-8 gpm/ft≤. At ambient temperature, the recommended filter backwash rate is 6-8 gpm/ft≤ for anthracite and 13-15 gpm/ft≤ for sand. Anthracite filters associated with hot process softeners require a backwash rate of 12-15 gpm/ft≤ because the water is less dense at elevated operating temperatures. Cold water should not be used to backwash a hot process filter. This would cause expansion and contraction of the system metallurgy, which would lead to metal fatigue. Also, the oxygen-laden cold water would accelerate corrosion.