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/ft² or
higher are common for sand, and rates for anthracite may range from 8 to 12
gpm/ft².
· 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.
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