MUNICIPAL WASTEWATER 729
Sewage contains the waste of a city and its inhabitants.
It is possible to find almost any microorganism in sewage.
In fact, in the sewer system can be found quite unexpected
creatures. About 1960, sewer workers in New York City
found a number of live and exceedingly unfriendly alligators
in that city’s sewers. These animals, gifts to city residents,
had been disposed of through household toilets. The warm,
wet environment of the sewers, rich in food, was excellent
for rapid growth.
Traditionally, public health practice applied to water san-
itation has placed major emphasis on prevention of typhoid
fever, the causative organism of which is S. typhosa. This
disease is spread by the pathway of anus to mouth. S. typhosa
is a fairly delicate organism and is usually not found in high
concentrations. It is felt that if pollution arising from human
intestinal discharges is removed to a high degree, then S.
typhosa will also be removed. As BOD is used as an indi-
cator of pollutional loading on oxygen resources, so is the
indicator organism E. coli used as an indication of pollu-
tional loading due to human intestinal waste. E. coli is a
normal inhabitant of the human intestinal tract. It is excreted
in huge numbers and the presence of this organism in receiv-
ing waters may be evaluated quantitatively. The lactose
broth-brilliant green bile test is commonly used to determine
the presence of E. coli. A common soil organism, A. aero-
genes, gives a false positive test and further confirmatory
tests are necessary. Serial dilutions of the water being tested
are prepared and, on the assumption that one organism is
responsible for a positive test, the Most Probable Number
(MPN) of organisms is determined. The MPN is based on
statistical reasoning. Work by Kupchick, using the enzyme
urease to determine the presence of uric acid, has shown a
high degree of correlation between the concentration of this
acid in wastewater and the MPN.
Most pathogenic organisms are not hardy and do not com-
pete well for food. The use of E. coli as an indicator organism
is in the way of a margin of safety. This is consistent with
Phelps’ concept of multiple barriers.
Microorganisms form the basis of secondary, or biologi-
cal, wastewater treatment. Stated briefly, microorganisms
establish themselves on trickling filter slime or in activated
sludge liquor and feed on waste material in the sewage.
Large particles are removed in the primary settling por-
tion of treatment. The larger particles, grease, etc. are in some
ways not as objectionable as the colloidal or truly dissolved
materials. The larger particles are, of course, objectionable
from an aesthetic point of view, but the smaller particles
place more of an immediate load upon the receiving waters.
In almost all areas of environmental engineering surface
area is one of the controlling parameters. Microorganisms,
in carrying out their vital processes, utilize the waste as food
and the smaller particles offer greater surface area per unit
volume. Microbial activity is correspondingly greater and
the oxygen required is also greater.
Microorganisms require a readily available source
of oxygen. Aerobic conditions are said to exist when the
oxygen is in the form of dissolved molecular oxygen or
nitrates. At the point of exhaustion of nitrates and nitritesthe system is in the anaerobic state and the oxygen sources
are then sulfates, phosphates, borates, etc. Reduction of
sulfates to give mercaptans (HS − ) carriers with it charac-
teristic vile odors. Secondary treatment is an essentially
aerobic process while conventional sludge digestion is an
anaerobic process.
In the trickling filter an activated sludge processes
microorganisms extract their food from the flowing waste-
water. By means of extracellular enzymes large molecules
are broken down so that they may pass through the microbial
cell wall by diffusion. The food is further broken down for
cell synthesis and energy by means of intracellular enzymes.
End products are largely carbon dioxide and water. The
waste material, now part of the cell mass, is removed in the
final stage of treatment.
The primary reason for treating any waste is the need for
avoiding nuisance and dangerous conditions in the environ-
ment. It is necessary to remove some of the waste so that the
remaining can be discharged to the surroundings. This is, in
effect, disposal by dilution. Discharge standards are usually
based on concentration and total quantity.
Sewage purification works were formerly constructed
for reasons based primarily on public health. More modern
thinking has expanded the original rationale to include pro-
tection of oxygen resources of the receiving waters. If the
second criterion is satisfied, the first will almost always be
also satisfied. Demand for clean waters is increasing even
more rapidly than population. Water will be used many
times in its passage to the sea. Lakes are essentially a closed
system. Leisure time is increasing and the question of water
pollution has reached the point where aesthetics is now a
significant parameter in planning. It is no longer possible to
treat a watercourse as a separate entity. The approach now
finding wider and wider application is that of basin manage-
ment. This is the systems approach. An excellent example
of this is the Ohio River Sanitation Commission. Here it has
been possible to rise above local jealousies and self inter-
est. The results are most gratifying and should serve as an
example to other areas. It is unfortunate, but true, that one
heavy pollution source can undo the efforts of many groups
with social responsibility and foresight.
Because objectives in waste management can change
drastically at political boundaries, it has occasionally been
necessary for intervention by representatives of larger politi-
cal entities when pollution problems effect, for example,
several states. Federal agencies in the Unites States and
other countries have come to play an increasingly larger role
in waste management, particularly when problems do not
respect political boundaries. The discussion presented here
involves primarily technology of wastewater treatment and
the underlying philosophy will not be extensively covered.
It has been mentioned that larger particles in wastewater
can be removed by physical means. Removal of colloidal
and dissolved components requires other methods and this
must be accomplished economically. Treatment is classified
as primary if it is without biological basis. Secondary treat-
ment is generally accepted as biological treatment. A few
regulatory authorities have endeavored to classify treatmentC013_007_r03.indd 729C013_007_r03.indd 729 11/18/2005 10:43:40 AM11/18/2005 10:43:40 AM