Several manufacturers have developed gravity filters that are backwashed automatically at a preset head loss.
WATER TREATMENT : AUTOMATIC GRAVITY AND CONTINUOUS CLEANING FILTERS
AUTOMATIC GRAVITY FILTERS
Several manufacturers have developed gravity filters that are backwashed automatically at a preset head loss. Head loss (water level above the media) actuates a backwash siphon and draws wash water from storage up through the bed and out through the siphon pipe to waste. A low level in the backwash storage section breaks the siphon, and the filter returns to service.
Automatic gravity filters are available in diameters of up to 15 ft. When equipped with a high-rate, multilayer media, a single large-diameter unit can filter as much as 1,000 gpm.
CONTINUOUS CLEANING FILTERS
Continuous cleaning filter systems eliminate off-line backwash periods by backwashing sections of the filter or portions of the filter media continuously, on-line. Various designs have been introduced.
Upflow units contain a single filter medium–usually graded sand. The finest sand is at the top of the bed with the coarsest sand below. Gravel is retained by grids in a fixed position at the bottom of the unit. The function of the gravel is to ensure proper water distribution during the service cycle. Another grid above the graded sand prevents fluidization of the media. Air injection during cleaning (not considered backwash because the direction of flow is the same as when in-service) assists in the removal of solids and the reclassification of the filter media. During operation, the larger, coarse solids are removed at the bottom of the bed, while smaller solids particles are allowed to penetrate further into the media. Typical service flow rates are 5-10 gpm/ft².