content. A portion of the sludge is returned and mixed with the incoming raw sewage,
while the remainder is pumped to digester tanks. Anaerobic digestion is a slower
process, which is typified by large digestion tanks, septic tanks, and cesspools.
a. Aerobic Digestion. In aerobic digestion, as found in the activated sludge
process, the key to successful operation is twofold: providing a continuous supply of
oxygen; and preventing excessive variations in raw sewage input, which may upset the
balance in the biological population. Aerobic digestion of sewage is a cyclic process.
Activated sludge contains bacteria, fungi, protozoa, rotifers, and sometimes nematodes.
As raw sewage, high in organic matter, enters the system, bacteria begin to grow in
logarithmic proportions. As the bacteria grow, the protozoa, their predators, also grow.
When the point is reached at which food is the limiting factor, both bacteria and
protozoa begin to die off, forming a floc. Rotifers are able to eat small particles of floc.
This floc is subsequently removed in a sedimentation tank. If the process were allowed
to continue unaltered, all biological forms would eventually die off. In practice, however,
fresh sewage is added only daily and the cycle repeats.
b. Anaerobic Digestion. Anaerobic digestion is simpler than the activated
sludge process, but it is more sensitive to an imbalance in the biological population. As
the sludge in a digestion tank or septic tank settles to the bottom, where there is no free
oxygen, the aerobic organisms (aerobic bacteria, fungi, and protozoa) die off or form
spores or cysts. Then two distinct biological processes, or phases, occur. Acid-forming
bacteria, predominantly facultative anaerobes, begin to metabolize the organic matter,
converting it into organic acids, aldehydes, and alcohols. During this first phase (acid
phase) the pH is lowered, retarding further bacterial action. The second phase
(methane phase) begins when a group of obligate anaerobes begins to increase.
These bacteria have the ability to metabolize the organic acids, producing carbon
dioxide and methane. The metabolism of amino acids results in the liberation of
ammonia, which in turn raises the pH to a level favorable for bacterial growth. As long
as a balance in the bacterial flora exists, maintaining the pH at about 6.8-7.4, the
digestion process continues with the production of both acids and methanes. Sudden
adding of large amounts of solids to the tank may cause an excess of acid production
and thereby upset the process by inhibiting the methane-forming bacteria.
Section II. DISINFECTION AND STERILIZATION
Disinfection and sterilization are indispensable procedures in modern medicine
and public health. Without the benefit of these procedures, it would be virtually
impossible to carry out many of the routine surgical procedures that are often taken for
granted. It would also be impossible to isolate specific microorganisms for their study.