WATER TRETMENT: FILTER WASHING-GRAVITY FILTERS
Periodic washing of filters is necessary for the removal of accumulated solids. Inadequate cleaning permits the formation of permanent clumps, gradually decreasing filter capacity. If fouling is severe, the media must be cleaned chemically or replaced.
For cleaning of rapid downflow filters, clean water is forced back up and through the media. In conventional gravity units, the backwash water lifts solids from the bed into wash troughs and carries them to waste. Either of two backwash techniques can be used, depending on the design of the media support structure and the accessory equipment available:
High-rate backwash, which expands the media by at least 10%. Backwash rates of 12-15 gpm/ft≤ or higher are common for sand, and rates for anthracite may range from 8 to 12 gpm/ft≤.
∑ Low-rate backwash, with no visible bed expansion, combined with air scouring.
Where only water is used for backwash, the backwash may be preceded by surface washing. In surface washing, strong jets of high-pressure water from fixed or revolving nozzles assist in breaking the filter surface crust. After the surface wash (when there is provision for surface washing), the unit is backwashed for approximately 5-10 min. Following backwash, a small amount of rinse water is filtered to waste, and the filter is returned to service.
High-rate backwash can cause the formation of mud balls inside the filter bed. A high backwash rate and resulting bed expansion can produce random currents in which certain zones of the expanded bed move upward or downward. Encrusted solids from the surface can be carried down to form mud balls. Efficient surface washing helps prevent this condition.
Air scouring with low-rate backwashing can break up the surface crust without producing random currents, if the underdrain system is de-signed to distribute air uniformly. Solids removed from the media collect in the layer of water between the media surface and wash channels. After the air is stopped, this dirty water is nor-mally flushed out by increased backwash water flow rate or by surface draining. Wash water consumption is approximately the same whether water-only or air/water backwashing is employed.