sprinkler nozzle outlet.
Crushed
depth of 3 to 5 inches.
As it
stone is the best filter material, but
trickles
through
the
sand,
fine
gravel, coke, clinker, broken brick,
suspended solids are filtered out and
or slag can be used.
To permit
dissolved organic matter is removed
maximum voids for passage of sewage
through the action of bacteria living
and
air
for
ventilation,
filter
in a gelatinous growth on the surface
material should be reasonably uniform
of the sand.
This type of filter
in size.
1-1/2- to 2-1/2-inch stone
usually produces as high a degree of
is best. The filter layer should be 5
treatment
as
can
be
obtained
by
to 8 feet deep.
Underdrains may be
biological processes, but is often
either whole or half tile laid with
uneconomical for large installations
open joints, or a grillage of 2- by 4-
because of the maintenance required.
inch timber laid on edge.
The
underdrain system must be constructed
(1) Design.
The design can be
so all parts of the filter bed are
based on 60,000 gallons of effluent
ventilated.
per acre of sand surface per day. The
sand is uniformly graded .01- to .05-
(2) Operation.
Settled sewage
inch sand.
The sand beds are usually
must be distributed evenly over the
2 to 4 feet deep.
The siphons and
filter surface.
Rotary distributors
distribution system should be designed
are generally used.
The force of the
for a dosing rate of 0.1 gallon per
spray leaving the distributor causes
minute
per
square
foot
of
sand
it to rotate, spreading the flow
surface.
Splash plates are required
evenly over the surface of the bed.
at the discharge to the sand to
Recovery periods should about equal
prevent
erosion.
Joint
tile
or
discharge periods.
perforated pipe drains surrounded by
gravel are spaced on 12-foot centers.
c. Final
settling
tank.
The
effluent from trickling filters should
(2) Operation.
As the settled
be passed through final settling tanks
sewage seeps through the sand, most of
to remove the bacterial gel which
the fine suspended matter stays on the
forms on the filter stone and peels
surface or adheres to the grains near
off
into
the
effluent.
Sludge
the surface. For this reason, the top
obtained from final settling tanks is
layer of sand must be scraped off
about one-half the volume obtained
occasionally.
The dissolved organic
from primary settling tanks.
It can
matter in the liquid is oxidized by
be run directly to drying beds or
the action of the aerobic bacteria in
preferably returned to the digestion
the bed. At least 16 hours are needed
tank.
Final settling tanks should be
between dosings to allow the sewage to
large enough to provide a 2- to 2-1/2-
percolate through the bed and to
hour detention period at the average
permit air to fill the voids in the
rate of flow.
The sludge should be
sand.
For continued operations, the
removed
daily
to
prevent
septic
filter should be divided into three or
action.
The slope of the hopper
more sections.
bottom is 1 to 1 or steeper.
Water
depth
of
final
settling
tanks
e. Oxidation pond.
An oxidation
(measured at the side) should not be
pond (fig 9) is a relatively large,
more than 10 feet.
Other details of
shallow
pond,
either
natural
or
construction are the same as for plain
artificial,
into
which
settled
settling tanks.
sewage
is
discharged
for
natural
purification under the influence of
d. Sand filter.
Sand filters are
sunlight and air.
These ponds should
more truly filters in the usual sense
be located one-half mile or more from
than trickling filters. They are beds
the
nearest
housing
to
prevent
of specially selected sand about 3
objectionable
odors
from
reaching
feet thick.
The sewage is applied
it.
The ponds are not practical
uniformly over the entire surface to a
where
the
temperature
is
below
4-11
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