drain the bed. Remove the sand from within the
Collect several such samples from well-
scattered locations on the filter bed, mixing until
about 2 pounds remain. Dry this sample; mix,
quarter, and reduce to a usable sample size.
Determine loss of weight of a 10-gram sample
during acid treatment. Treat the sample with 10
percent hypochloric acid in a Pyrex evaporating
dish in a water bath for 24 hours. Replace acid
loss during treatment period. Wash, dry, and
weigh the sand. Determine the weight loss and
compare it to the previous analysis.
From the remainder of the sand sample, remove
100 grams and run a sieve test. Compare the
results to previous tests.
When either inspection, weight loss or sieve
analysis shows growth of sand grains to a point where
filtration efficiency is impaired, treat the sand as
Add inhibited muriatic acid at the surface and
allow it to pass downward through the bed and
out the filter drain, or rewash the line; or add it
to an empty filter through a small tap on the bed
side of the wash waterline.
Adjust the water treatment process as necessary.
If treatment is not effective, replace the filter
Gravel inspection procedures include the
1. At monthly intervals, check the gravel bed
surface for unevenness, using a garden rake or a pole as
a probe during backwashing. When ridges or sinkholes
are shown, the filter may need overhauling.
2. Every 6 months, remove sand from an area
about 3 square feet, taking care not to distribute the
gravel. Examine the gravel by hand to determine if the
gravel is cemented with incrustation or mud balls, or if it
is not layered properly.
3. When any undesirable conditions exist to a
marked degree, the sand should be removed and the
filter gravel relaid. When unevenness or layer mixing is
caused by a faulty underdrain system, repair it; when it
is caused by faulty backwashing, correct the
Annually, or as observations indicate the need, the
filter bottom should be inspected. Sand boiling during
backwashing or sand craters on the surface indicate
trouble in the underdrain system, as does marked
unevenness of the gravel layers. Inspection and
treatment procedures are as follows:
1. To inspect the bottom, remove the sand over an
area of about 10 feet square. Select an area where sand
boils or other indications of trouble have been noticed.
Place planking over the gravel to stand on, and remove
the gravel from areas about 2 feet square. Check the
underdrains for deterioration. When the underdrains
need repair, remove all sand and gravel, make repairs to
the underdrain, and replace the gravel and sand in
2. Where the underdrains are porous plate and are
clogged with alum floc penetration, flood the
underdrain system with a 2 percent sodium hydroxide
solution for 12 to 16 hours.
At quarterly intervals, the level and elevation of
troughs should be checked. Water should be drawn
below the trough lip, the wash water valve cracked,
and any low points observed where water spills over
the lip, before the lip is covered completely.
The troughs should be adjusted as necessary to
produce an even flow throughout their lengths on both
At 6 month intervals, metal troughs should be
inspected for corrosion. When corrosion exists, the
troughs should be allowed to dry, and then cleaned by
wire brush and painted with a protective paint or
Operating controls for filter valves may be
mounted on a console, panel, or table. The controls
actuate the filter valves which may be powered
either by hydraulic or pneumatic means. The
controls may be connected to the valve mechanism
either mechanically, electrically, hydraulically, or