MIL-HDBK-1164
d)
Inspect the media twice a year. If visual inspection does not reveal
the condition of the media, locate the elevation of the top of the bed to determine if the
bed has "grown" in depth. Also, remove a media sample and analyze it as follows:
(1)
Make a sampling tube 12 inches (300 mm) square by
36 inches (1 m) deep. Force a tube into the gravel and drain the bed. Remove the
sand from the tube. Collect several such samples from well-scattered locations on the
filter bed, mix thoroughly, and reduce sample size by quartering until about 2 pounds (1
kg) remain. Dry this sample and mix, quarter, and reduce it to a usable sample size.
(2)
Determine the loss of weight of a 10-gram sample during
acid treatment. Treat the sample with 10 percent hydrochloric acid in a Pyrex
evaporating dish on 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.
(3)
From the rest of the sand sample, remove 100 grams and
run a sieve test. Pass the sand through several standard sieve sizes, weighing the
sand grains retained on each sieve. Compare the results to a previous test. Retention
of greater amounts of sand on the larger sieve sizes indicates growth of the filter
media.
(4)
If visual inspection, weight loss, or sieve analysis shows
growth of sand grains to a point that filtration efficiency is impaired, treat the sand as
outlined in par. 11.5.5.1(b) above and adjust the water treatment process as necessary.
If treatment is not effective, remove and replace the filter media.
11.5.5.2
Loss of Filter Media. Media can be washed from the filter along with the
backwash water or can filter through the gravel layer along with the product water.
Losses of media in the backwash water can be kept to a minimum by controlling the
backwash flow rate, maintaining level backwash troughs at the proper elevation above
the media surface, and controlling hydraulic short circuiting as a result of clogged
media or gravel. Losses through the filter gravel can be controlled by placing a layer of
coarse garnet or ilmenite between the media and the gravel and by controlling
mounding of the filter gravel. Leakage of media can be detected by a small trap
located in the effluent line from each filter. Many new filters leak sand for a period and
then stop. Such leakage poses no real problem. However, if sand leakage increases
over a period, it is probably an indication of mounded gravel.
11.5.5.3
Gravel Inspection. Gravel inspection includes the procedures described
in (a) through (c) below.
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