maintenance procedures given here apply. For a more
detailed discussion of these procedures, consult the
The following maintenance operations are
required on a monthly basis:
1. Purge the diaphragm cases of air, and check the
cable to be sure that it leaves the segment at a tangent to
the lower end when a zero reading exists on the unit.
2. Remove dirt from the knife edges; when
necessary, tighten the cam hubs on their shafts.
3. Drain mud from the mud leg. In doing this, flush
the mud out of the water pipeline running from above
the sand to the loss-of-head gauge. Drain the mud leg
until the water runs free of sediment.
Annually, inspect the diaphragms for leakage, and
replace when necessary.
Diaphragms in stock should be stored under-
Also, disassemble the unit to clean and lubricate it
when necessary. Check the working parts and the
cables. (They should be free of knots, splices, or
fraying.) Repack the stuffing box when it is leaking.
Make certain that the knife edges rest solely on their
edges where the pendulum is hung vertically, and be
sure that all cable ends are knotted tightly.
Except where the filter medium is housed in an
enclosed pressure shell, pressure filters (fig. 8-6) are
constructed like gravity filters with respect to the
underdrain system, gravel, and the filter medium (sand
or anthrafilt). Pressure filters need the same care and
attention as gravity filters. Since their backwashing
operations cannot be observed, the filter must be
opened regularly and inspected carefully. The
recommended maintenance procedures are as follows:
Weekly, inspect piping and valves for leaks.
Lubricate and repack valves if necessary.
Quarterly, open the pressure shell and inspect
the filter bed surface. The inspection procedures
are as follows:
1. Use a garden rake during backwashing while the
manhole is open to test for mud balls in the lower part of
the filter bed and for evenness of the gravel layer
2. Determine when the sand bed level has changed
since the last inspection by comparing the bed surface
elevation with some reference point.
3. When the filter does not have a surface wash
system and shows evidence of mud balls, backwash it at
the highest rate possible while jetting the surface with a
stream of water from a high-pressure hose nozzle.
Annual maintenance requirements are as
1. Open the filter, remove the sand from an area
large enough to permit the inspection of the gravel.
When the sand or gravel distribution indicates
nonuniform distribution of backwash water, the filter
media and gravel may need to be removed, and the
underdrain system checked.
2. Clean and paint the exterior of the shell.
Every 3 years (or more often ifnecessary), the
filter medium and gravel should be removed
and the underdrain system checked for the
distribution of wash water, and repaired if
necessary. Clean the under-drain system, and
paint it or apply a protective coating to all
parts subject to corrosion, including the
inside shell. Replace the gravel and the filter
Most diatomite filter installations in potable water
supply plants are of the pressure type, although there
are vacuum-type filters that can be used in certain
installations. In general, the maintenance procedures
for cleaning the filter element are the same for both
types. The following procedures apply:
At monthly intervals, or as often as operating
conditions show the need, check the filter elements.
Cleaning is needed if the precoat has apparent bare
spots on the elements. Causes of element clogging are
iron oxide, manganese dioxide deposits, and algae