Sludge Digestion by Sulfate Reducing
Bacteria
WESLEY O. PIPES, JR. , Assistant Professor of Civil Engineering
Northwestern Technological Institute
Evanston, Illinois
INTRODUCTION
The primary aim in the treatment of many wastes is to convert the organic matter of the waste into inorganic compounds which exert no oxygen
demand or into organic compounds which are temporarily stable because they
are part of a living system. In the vast majority of cases the most economical
method of achieving this has been found to be some form of biological treatment.
Microorganisms are able to assimilate all naturally occurring organic
compounds and many of the synthetic ones. During the process of assimilation much of the carbon and hydrogen of the organic substrate is oxidized to
carbon dioxide and water. A small amount of the original organic matter is
converted into the variety of organic substances which the microorganisms require for growth, metabolism, and reproduction.
All microorganisms derive the energy which they require to carry out their
metabolic activities from oxidation-reduction reactions. In waste treatment
processes the reducing agents for these reactions are the organic compounds
which are originally present in the waste. The oxidizing agents which may be
employed are molecular oxygen, nitrates and nitrites, sulfates, and carbonates.
In aerobic biological waste treatment processes the oxidizing agent employed is oxygen which is obtained from the air. Air is a cheap oxidizing
agent if it is needed in relatively small quantities and at a low rate. However, if oxygen is required to maintain a high rate of utilization, the technical difficulties of supplying air and dissolving oxygen in water economically become immense. For wastes containing relatively great concentrations
of oxidizable organic matter, it has proved economical to resort to anaerobic
treatment.
TWO STEPS
The conventional anaerobic waste treatment process consists of two
steps. The first of these is the actual digestion or liquification process. Insoluble organic compounds are hydrolysed by extracellular enzymes to soluble
compounds. Some of the soluble organics are fermented. Fermentation is a
series of oxidation-reduction reactions by which a portion of the substrate
molecule is oxidized to carbon dioxide while the remainder of the molecule
is reduced to an oxidation state lower than that of the original substrate. The
organic end products of fermentations are normally alcohols, organic acids,
and ketones.
The second step in the anaerobic process is commonly known as gasification. Methane bacteria oxidize the organic fermentation products to carbon
dioxide and water. The oxidizing agent which they employ is carbon dioxide
01 carbonate. The carbonate is reduced to methane during the oxidation of
the fermentation products.
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