Figure 7.
Dosing tank.
a. Dosing
tank.
Most
of
the
flushing head is made by changing the
methods of secondary treatment use
length of the nipple on the air vent
aerobic bacteria.
Because of this,
or overflow pipe.
some
of
the
secondary
treatment
processes must have the liquid applied
b. Trickling
filter.
Trickling
in doses or intermittently. This will
filters,
often
called
sprinkling
allow oxygen to get to the bacteria
filters, are beds of crushed rock,
between doses.
A dosing tank with an
crushed slag, or gravel.
Primarily
automatic siphon is usually built as a
treated sewage is applied through
part of the primary facility to do
rotary
distributors
or
stationary
this. It discharges the effluent from
sprays to the surface (fig 8).
The
the primary treatment facility at
sewage then trickles in a thin film
intervals and provides the hydraulic
over the stones to a system of
head. The size of the dosing tank is
underdrains.
These beds are not
80 percent of the treatment facility
actually filters in the usual sense,
it is to serve.
The action of the
but accomplish their function in the
dosing siphon is as follows (fig 7):
treatment of sewage mainly through the
As liquid rises in the dosing tank,
action of oxidizing bacteria of the
air trapped in the dome covering the
aerobic type.
There are different
outlet siphon is compressed and forced
theories regarding the chemistry of
downward through the shorter leg.
this
action,
but
whatever
the
Further rise in liquid level forces
chemistry may be, the effect is that
the air around the bend, allowing it
dissolved and suspended organic matter
to escape through the longer leg.
in the sewage is absorbed by bacterial
Water rushing into the dome to replace
growth on the stones and converted to
this air starts siphon operation,
more stable compounds.
Some of these
which continues until the liquid level
compounds coagulate and settle in the
drops below the bottom rim of the
final settling tank.
Others are
dome, breaking the siphon action and
soluble and pass on with the final
stopping the flow.
Adjustment in
effluent
sewage.
The
two
main
4-9
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