Aerobic Digestion of Extracellular
Microbial Polysaccharides
ALAN W. OBAYASHI, Graduate Student
ANTHONY F. GAUDY, JR., Professor and Director
Bioenvironmental Engineering Laboratories
Oklahoma State University
Stillwater, Oklahoma
INTRODUCTION
The extended aeration, or total oxidation, process differs from conventional activated
sludge processes because it is operated with total sludge recycle, and a longer detention time
is provided to allow for autoxidation of biological solids. The theoretical premise upon
which the operational concept is based is that the increase in biological solids resulting from
metabolism of the incoming waste is balanced by the decrease in biological solids due to
their aerobic digestion. The theory requires that all organic constituents of the cell, i.e.,
those in the cytoplasm, the walls and membrane, and the capsular slime layer, can be
metabolized and thus converted to C02. It is a concept which has been the subject of much
controversy in the water pollution control field during the past two decades.
Porges, et al (I), on the basis of laboratory studies on the biological treatment of skim
milk waste, theorized that total oxidation was possible under the proper conditions. The
theory was further supported by Forney and Kountz (2), who also concluded that total
oxidation was possible. Later, however, Kountz and Forney (3) as well as other researchers
(4,5,6) concluded that the concept of total oxidation was theoretically unsound. Symons
and McKinney (4) studied batch-fed systems in which nitrogen was limiting and theorized
that total oxidation was not possible because of a buildup of extracellular polysaccharides
which they had noted by microscopic observation of sludge stained with Alcian blue. They
pointed out that extracellular polysaccharide often accumulated as a waste product of
internal metabolism and that, being a waste product, it would not be subject to further
biological action, i.e., it would form an inert portion of the sludge. Washington and Symons
(6) also reasoned that the so-called "inert" material which would accumulate in an extended
aeration unit would be. "the remains of cell capsular and external slime, for it is this material
which is least degradable by the organisms themselves." Busch and Myrick (5) also
concluded from their continuous flow pilot plant studies that total oxidation was
impossible, and they noted that at times appreciable amounts of biological solids were
present in the effluent.
On the other hand, Washington, et al (7) reported a long-term adaptation of
microorganisms to an accumulated sludge mass. In their studies, batch reactors were
operated for one year with no sludge wasting. Under these conditions, the biological solids
did not build up continually, as predicted by other researchers (3,4,5,6). The biological
solids level showed a cyclic trend, i.e., a period of increase in solids followed by a decrease in
solids. Although positive retention of all biological solids was not provided for in the
experimental apparatus, the decrease in solids cannot be attributed to loss of solids in the
effluent, since it was possible for them to make the observation that'"there was essentially
no loss of volatile solids in the effluent."
Recently, a long-term study in which return of all biological solids was positively
controlled has been reported by Gaudy, et al (8,9). The experimental apparatus consisted of
a continuously fed, completely mixed laboratory extended aeration plant in which the
biological solids which did not settle out in the clarifier chamber were returnedjdaily to the
unit through use of a Sharpies centrifuge. The three-year study demonstrated conclusively
that the total oxidation theory was not unsound, and that biological solids would not
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