730 MUNICIPAL WASTEWATER
degree on the basis of degree of removal. This relegates some
biological treatment to primary status. Many experts do not
agree with this change in definition. A widely accepted defi-
nition of tertiary treatment is the use of any process, in addi-
tion to conventional secondary treatment, for the purpose of
further removals. From the foregoing it should be noted that
chemical treatment, popular in the past, is a primary treat-
ment process.
Water carriage of waste was practised in the Minoan
civilization of Crete. It may be said that sanitation practices
reflect the level of a civilization. Certainly, sanitary practices
of the Middle Ages were of a rather low order. Methodical
control of waterborne diseases was not attained until the
19th century. It is of historical interest to note that knowl-
edge of the use of creosote for odor control at the Carlisle,
England sewage purification works gave inspiration to
Joseph Lister for the birth of asepsis in surgery.
The process by which wastewater is purified can best be
understood by following the waste from its source through
the collection system and treatment plant. Organic material
discharged to a watercourse will eventually undergo stabi-
lization. This is accomplished by natural processes (unit
operations) and a wastewater treatment plant basically sets
up, under controlled conditions, the processes which act in
the river. Indeed, it has often been said that a sewage treat-
ment plant is a river in miniature. In the river heavy particles
settle out and lighter particles float to the surface. Biological
decomposition takes place. Oxygen present is used by organ-
isms that accomplish decomposition. Some of the settled
material will be resuspended, increasing the organic load-
ing. While oxygen is being withdrawn by BOD this resource
is being replenished at a rate proportional to the deficit. The
oxygen deficit is the difference between the amount that can
be held at saturation (about 10 mg/l) and the amount actually
present.
Stated mathematically, the oxygen concentration in a
river as a function of time isd
dD
tky kD 12 .This expression considers only the effects of BOD and atmo-
spheric reaeration. A more complete equation can be writ-
ten but effects of oxygen production by algae and oxygen
reduction by benthal (bottom) deposits are not of major
significance here. Solving the above givesDkL
kk
t^1 kt kt D kt
1(^100)
11 2
− {}
10 10.
This expression is commonly known as the oxygen sag
equation and is displayed in Figure 3. It is of interest here
because it illustrates the rationale underlying waste treat-
ment requirements. Waste is treated so that undesirable
conditions do not develop in the receiving waters. In effect,
a limit has been placed on the allowable oxygen deficit.
Regulatory authorities usually require that a minimum dis-
solved oxygen level be maintained. Normally, this will be
stated as a percentage of dissolved oxygen saturation. This
is the largest permissible critical deficit. Game fish may
require a minimum of 5 mg/l D.O. while scavengers can
survive in a much lower quality water.
The critical deficit is given by
D
kL
c k
ktc
1
2
10 1
.
Once the maximum deficit is specified, the BOD loading
on the stream can be immediately estimated. The allowable
BOD loading will be that impressed on the watercourse by
the wastewater treatment plant effluent. It is of interest to
observe the effect of the condition of the river at the point
of discharge. A river in poor condition will have a large ini-
tial deficit, D 0. This can raise the treatment requirements. It
can be seen that it is necessary to integrate the efforts on a
basin wide basis. Parameters other than just dissolved oxygen
must be controlled by the treatment processes. This is accom-
plished in some, or all, of the following steps.
Decomposition of the waste begins in the collection system.
Ease, or difficulty, of treatment depends to a large degree on
the condition of the sewage when it reaches the plant. Some
substances are not permitted in the sewage system. Gasoline
and other flammable substances, oil, hexavalent chromium are
examples of prohibited substances. These can damage either
the collection system or the treatment plant and processes. The
legal vehicle by which such materials are excluded is known
as a sewer ordinance.
It is most economical to collect sewage by gravity flow.
If topography does not permit, pumping will be necessary
in order to cross high points and to avoid excessively low
flow velocities and deposition of waste material in the pipes.
(1)
(2)
Dc
reoxygenation
D.O
D
critical deficit
deoxygenation
time
(1)+(2)
D.O.s at.’
DO
D=O
FIGURE 3
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