minute (8 gallons per square foot per minute for anthrifilt media filters). The
backwashing should be done about 5 minutes or until the backwash water appears
clear for 2 minutes.
(a) Air binding. Short-circuiting of the filter inflow can be caused by air
trapped in the top of the filter. An air release valve located in the top of the shell will
release this air. Automatic or manual valves are available. When painting the filter unit,
avoid painting any automatic valves.
(b) Mechanical loss of filtering mat. Shutting down for extended
periods of time, such as during the night, prevents good operation based on filter design
and results in a partial loss of the uniform filter mat.
(c) Inability to read pressure differential. High range gauges should be
replaced with those that read in the range of the filter's operation. In general, gauges
reading up to only 30 pounds per square inch are desirable. Another operational
difficulty may be avoided by mounting gauges on the tops of filter lines. This prevents
clogging with sediment.
(d) Rate of flow change through filters. With the pump running at the
same speed, the flow of water through the filter will slowly decrease throughout the filter
run. A rate of flow controller can be installed to deliver a constant 3 (or 2) gallons per
square foot per minute through the filter.
(3) Filter media difficulties. Filters that are failing to produce a clear effluent
should be inspected by removing the manhole cover and looking at the sand surface
(a) Clean sand surface. This indicates satisfactory operation.
(b) Dirty sand surface. This indicates unsatisfactory conditions. The
filter should be backwashed slowly (test backwash) and the rising water observed.
(c) Water emerges evenly during test backwash. More water is
needed for backwashing in this case. While an inadequate backwash pump may cause
the trouble, temporary relief may be obtained by removing the top layer of sand and
washing it in a lye (caustic) solution. Another technique is to wash the whole sand bed
with caustic soda. One pound of lye per square foot of filter surface should be applied
after the water has drained to within 2 inches of the sand bed surface. After 4 to 6
hours of soaking, the filter may be drained and then thoroughly backwashed.
(d) Water emerges unevenly during test backwash. This indicates
obstructions in the filter bed or underdrains.