Environmental Engineering Reference
In-Depth Information
The main idea of all biological methods of wastewater treatment is to
provide contact with bacteria (cells), which feed on the organic materials in
the wastewater, and thereby reduce its biological oxygen demand (BOD).
In other words, the purpose of biological treatment is BOD reduction. The
natural process of microbiological metabolism in aquatic environment
is capitalized on in the biological treatment of wastewater. Under proper
environmental conditions, the soluble organic substances of the wastewa-
ter are completely destroyed by biological oxidation. A part of it is oxidized
while the rest is converted into biological mass in the biological reactors.
The biological treatment system usually consists of biological reactors and
a settling tank to remove the produced biomass or sludge [159].
1.3.3.1
Aerobic and Anaerobic Degradation
Aerobic means in the presence of air (oxygen); while anaerobic means in
the absence of air (oxygen). These two terms are directly related to the
type of bacteria or microorganisms that are involved in the degradation
of organic impurities in a given wastewater and the operating conditions
of the bioreactor. Therefore, aerobic treatment processes take place in the
presence of air and utilize those microorganisms (also called aerobes),
which use molecular/free oxygen for the assimilation of organic impu-
rities, which are converted into carbon dioxide, water and biomass. On
the other hand, the anaerobic treatment processes occur in the absence
of air (i.e., molecular/free oxygen) by those microorganisms (also called
anaerobes) which do not require air for the assimilation of organic impuri-
ties. The final products of organic assimilation in anaerobic treatment are
methane and carbon dioxide and biomass. The simplified principles of the
two processes are illustrated in Figure 1.1 [160].
Anaerobic and aerobic treatments have been used together or sepa-
rately for the treatment of textile effluents. Hence aerobic treatment is not
Water
Carbon
Dioxide
Organic
Contaminants
Carbon
Dioxide
Bacteria
(Aerobes)
Methane
Bacteria
(Anaerobes)
Organic
Contaminants
Oxygen
Excess Cell
Mass
Nutrients
Excess Cell
Mass
Nutrients
Figure 1.1
A simple illustration showing the principles of aerobic and anaerobic
degradation [160].
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