MIL-HDBK-1164
(4)
Open the filter each year and remove the sand from an area
large enough to allow inspection of the gravel. If the sand or gravel distribution
indicates non-uniform distribution of backwash water, the filter media and gravel may
need to be removed and the underdrain system checked.
(5)
Clean and paint the exterior of the shell each year.
(6)
Every 3 years (or more often if necessary), remove the filter
medium and gravel and check the underdrain system for wash water distribution.
Repair if necessary. Clean the underdrain system, and paint it or apply a protective
coating to all parts subject to corrosion, including the inside of the shell. Replace the
gravel and the filter media.
11.5.7
Precoat Filters. In general, the maintenance procedures for cleaning the
filter element are the same for both pressure- or vacuum-filter types. The following
procedures apply:
a)
Each month, or as often as operating conditions require, check the
filter elements. The need for cleaning is evident when the precoat shows bare spots on
the elements. Iron oxide deposits, manganese dioxide deposits, and algae growths
cause element clogging.
(1)
For iron oxide removal, treat the elements with a 0.5 percent
solution of oxalic acid. Information is available from the manufacturer on the amount of
oxalic acid to use for units of different sizes. The following procedures are used:
a.
Start with an empty filter after a regular washing.
b.
Close the drain valve and the main outlet valve; open
the recirculation valve.
c.
Fill the tank to a level covering the top of the
elements.
d.
Add the proper quantity of oxalic acid and recirculate
for 1 hour.
e.
Drain and hose down the elements and the tank
interior.
f.
Close the drain valve; refill, circulate a few minutes,
and then drain again. If the cleaning is not completely effective, repeat the procedure.
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