should be cone or hopper shaped to
lagoons should be about 6 feet deep
facilitate the outflow of sludge.
and large enough to provide a 6-month
Concrete lined tanks are preferred,
storage capacity, equal to about 4
although
brick
or
stone
masonry,
cubic feet per capita.
The sludge is
gunite, sheet-metal, or wood tanks may
removed from the lagoons by hand or by
be used. In firm ground such as clay,
mechanical
loading
equipment
and
an unlined pit from which the sludge
buried.
Lagoons are discussed more
and liquid are removed by pumping may
fully in paragraph 8a.
be feasible.
The scum and hard mat
that forms on the surface should be
(2) Drying beds without drains.
broken up and forced down into the
Natural sludge drying beds without
sludge.
underdrains
are
constructed
by
building earth dikes.
They should
d. Imhoff tanks. An Imhoff tank is
provide 3 to 4-1/2 square feet of
a combined sedimentation or settling
surface
per
capita,
depending
on
tank and digestion tank (fig 3).
It
climate and permeability of the soil.
consists of an upper compartment for
Liquid sludge from the digestion tank
settling
out
solids
from
slowly
is applied about 12 inches deep. When
flowing sewage and a lower compartment
dried, it contains about 65 percent
for septic digestion of the sludge.
moisture and forms a cake about 4
The upper compartment forms a channel
inches thick.
This cake is removed
with an approximately 8-inch slot in
with a fork or shovel and can be used
the bottom. Sides of the slot have a
as humus.
1 horizontal to 1-1/2 vertical slope
and are overlapped to prevent gases
(3) Drying
beds
with
drains.
formed
by
digesting
sludge
from
Underdrain sludge drying beds (fig 4)
escaping into the upper or "flowing-
consist of a surface layer of sand 6
through" compartment. With an average
to 12 inches thick, a 6- to 12-inch
flow, solids settle in the upper
layer of gravel below the sand, and
compartment in 2 to 2-1/2 hours, pass
underdrains below the gravel layer.
downward through the slot, and settle
The underdrains should be 4- to 6-inch
to the bottom of the lower compartment
open joint or perforated pipe spaced
where they are digested.
Accumulated
10 to 20 feet apart and should be laid
solids
are
removed
periodically
in a V-shaped trench and surrounded
through a sludge drawoff pipe having
with coarse gravel. The required area
its inlet about 1 foot above the tank
of a sludge bed is 1 to 1-1/2 square
bottom.
Design of the upper or
feet per capita with a maximum length
"flowing-through" compartment is based
of about 100 feet.
The bed is
on the retention period. The lower or
subdivided into sections by wood or
digestion compartment is designed to
masonry curbs spaced midway between
hold 3 cubic feet per capita below a
the drainpipes.
Characteristics and
plane 18 inches beneath the bottom of
handling of sludge are the same as for
the slot.
If sludge from secondary
natural beds.
Drying requires 2 to 4
settling
is
returned
to
this
weeks,
depending
on
humidity
and
compartment
for
digestion,
the
rainfall.
Sand removed with the
capacity of the compartment must be
sludge must be replaced when the
increased to 4-1/2 cubic feet per
thickness of the sand layer is reduced
capita.
to 4 inches.
e. Sludge drying beds.
f. Septic tanks and cesspools. For
small numbers of people, suspended
(1)
Lagoons.
Sludge
from
sewage
solids
can
be
removed
Imhoff and digestion tanks can be
and digested in a septic tank or
disposed of in lagoons or can be dried
cesspool.
Although
they
become
on natural or artificial beds. Sludge
more
inefficient
as
the
numbers
4-6
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