2 Combined high-and low-capacity filters. Excellent results are
obtained by using a high-capacity or roughing filter followed by an intermediate-settling
tank, a low-capacity filter, and a final-settling tank (see Figure 2-16B). Continuous flow
is maintained by recirculation.
Maintenance operations and control of health hazards.
(a) Distributors and spray nozzles. Orifices should be cleaned daily.
All scum and growths should be removed from rotary distributors weekly. Wastewater
from the distribution apparatus should not be sprayed on the ground around the filter.
Shields on distribution arms and adjustments of spray velocity are appropriate
(b) Filter surface. The filter surface should be kept free of leaves;
removal of nearby trees may be necessary. In winter, the filter surface-face should be
kept sufficiently free of snow to permit rotation of distributor arms. If small particles of
rock or sand are washed out of the filter, they should be removed from the collection
channels manually to prevent their discharge to settling tanks. If this material reaches
the digester, it may plug sludge lines and reduce digester capacity . At some
installations, a small grit chamber is installed to remove these particles.
(c) Ponding. Ponding of filters may occur if prolific fungus growths fill
the voids between the rocks to the extent that water passage through the filter is
inadequate. Such growths may develop if the filter stone is too small or has broken up,
if the filter is heavily loaded or poorly ventilated, or if wastes containing large amounts of
carbohydrates are treated. Excessive grease or nofiltrable residue may plug the filter.
Ponding may be reduced by the following methods.
1 Flushing the filter surface with a fire hose.
2 Raking or forking the surface.
3 Stopping the distributor over the affected area and allowing the
continuous flow to wash down the growth.
4 Punching holes through the top layer of the filter rock with an
iron bar or pick, but not with heavy equipment which would crush the underdrains or
otherwise damage the filter.
5 Adding heavy applications of chlorine or chlorinated lime up to
5 ppm residual to filter influent for 2 to 4 hours at weekly intervals, preferably at night
when the flow and chlorine demand are low. This is after a residual up to 5 ppm applied
3 or 5 hours daily for several days starts the solids sloughing from the stone. Filter flies
can be controlled somewhat by chlorinating to a residual of 0.5 to 1.0 ppm for several
hours at 2-week intervals.