At monthly intervals, use a probe to check the gravel bed surface
for unevenness. If ridges or sinkholes are indicated, the filter may need overhauling.
Probing a Filter During Backwash. This method uses a
metal rod long enough that the operator can reach the gravel layer while standing on
the top of the filter. The rod has a heavy grade screen attached to the end so that it
can penetrate the expanded filter media bed (the rod is stopped by the gravel layer).
By probing every few feet along the filter, mounds or holes in the gravel layer can be
discovered. A variation in the gravel level of over 2 inches (50 mm) indicates serious
Probing a Filter at Rest. The filter can be probed at rest
using a metal rod of about 1/4-inch (6-mm) diameter that penetrates the sand layer but
not the underlying gravel.
Remove media from an area of about 3 square feet (0.3 sq m)
twice a year, taking care not to disturb the gravel. Examine the gravel by hand to
determine whether it is cemented with encrustation or mud balls and whether it is
If any undesirable conditions exist to a marked degree, remove the
media and re-lay the filter gravel. If unevenness or layer mixing is caused by a faulty
underdrain system, repair it; if it is caused by faulty backwashing, correct the
Filter Underdrain System. Inspect the filter bottom as needed. Sand boils
(during backwashing), sand craters on the surface, or marked unevenness of the gravel
layers indicate trouble in the underdrain system. Inspection and treatment procedures
are as follows:
To inspect the bottom, remove the media over an area of about 10
square feet (1 sq m). Select an area where sand boils or other indications of trouble
have been noticed. Place planking over the gravel to stand on and remove gravel from
areas about 2 feet square (0.2 sq m). Check underdrains for deterioration of any
nature. If underdrains need repair, remove all sand and gravel, make repairs, and
replace gravel and sand in proper layers.
Where underdrains are of the porous-plate type and are clogged
with alum floc penetration, flood the underdrain system with a 2 percent sodium
hydroxide (caustic soda) solution for 12 to 16 hours.