Filter
Cleaning
When to Backwash
Rapid sand
filters, pressure filters, and diatomaceous earth filters can all be
backwashed. During backwashing,
the flow of water through the filter is reversed, cleaning out trapped
particles.
Three factors can be used to assess when a filter needs
backwashing.
Some plants use the length of the filter
run, arbitrarily scheduling
backwashing after 72 hours or some other length of filter
operation.
Other plants monitor turbidity of the effluent water and head loss
within the filter to determine when the filter is clogged enough to
need cleaning.
Head loss is a loss of
pressure
(also known as head) by water flowing through the filter. When
water
flows through a clogged filter, friction causes the water to lose
energy, so that the water leaving the filter is under less pressure
than the water entering the filter. Head loss is displayed on a
head
loss
gauge. Once the head loss within the filter has reached between
six
and ten hours, a filter should be backwashed.
The Process of Backwashing
In
order to backwash a filter, the influent valve is closed and a waste
line is opened. A backwash pump or tower forces treated water
from
the
system back up through the filter bed. The
dirty backwash water is collected by the wash troughs and can be
recycled to the beginning of the plant or can be allowed to settle in a
tank, pond, or basin.
Backwashing should begin slowly. If begun too quickly, backwash
water
can damage the underdrain system, gravel bed, and media due to the
speed of the water. Beginning backwashing too quickly will also
force
air bound in the filter out, further damaging the filter.
After a slow start, the backwash rate should be accelerated to reach
around 10 to 25 gpm/ft.2
The backwash water must have enough velocity and volume to agitate the
sand and carry away the foreign matter which has collected there.
Backwashing normally takes about 10 minutes, though the time
varies depending on the length of the filter run and the quantity of
material to be removed. Filters should be backwashed until the
backwash water is clean.
Surface Washing
At the same time as backwashing is occurring, the surface of the filter
should be additionally
scoured using surface washers.
Surface washers spray water
over the sand at the top of the filter breaking down mudballs.
