Lesson 3: Practical Application of Microbiology

Section I: Microbiology of Water and Sewage

3-2

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3-2. SEWAGE MICROBIOLOGY

Microbes play an extremely important role in sewage treatment. It is largely through biological digestion that sewage is converted from a highly contaminated, infectious liquid into a relatively stable, inert sludge and a harmless effluent needing only chlorination before it may be discharged into a receiving stream, leaching bed, or other disposal area. There are two biological processes involved in sewage treatment. Aerobic digestion is exemplified by the activated sludge process, in which the wastes from primary settling tanks are thoroughly aerated until active masses of microorganisms settle out as sludge, leaving a clear effluent of relatively low organic 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.

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