Environmental Engineering Reference
In-Depth Information
planned. Wastewater treatment plant processes can also cause organic
overloads returning high-strength wastes from the sludge treatment
processes. Regardless of the source, an organic overloading of the plant
results in increased demand for oxygen. This demand may exceed the
air supply available from the blowers. When this occurs, the activated
sludge process may become septic. Excessive wasting can also result
in a type of organic overload. The food available exceeds the number of
activated sludge organisms, resulting in increased oxygen demand and
very rapid growth.
Organic underloading may occur when a new treatment plant is ini-
tially put into service. The facility may not receive enough waste to allow
the plant to operate at its design level. Underloading can also occur when
excessive amounts of activated sludge are allowed to remain in the sys-
tem. When this occurs, the plant will have difficulty in developing and
maintaining a good activated sludge.
8.8 aCTivaTed sludge ModifiCaTions
First developed in 1913, the original activated sludge process has
been modified over the years to provide better performance for specific
operating conditions or with different influent waste characteristics.
Conventional activated sludge
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Employing the conventional activated sludge modification requires
primary treatment.
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Conventional activated sludge provides excellent treatment; how-
ever, a large aeration tank capacity is required, and construction
costs are high. In operation, initial oxygen demand is high.
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The process is also very sensitive to operational problems (e.g.,
bulking).
Step aeration
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Step aeration requires primary treatment.
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It provides excellent treatment.
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Operation characteristics are similar to conventional.
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It distributes
organic loading by splitting influent flow.
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It reduces oxygen demand at the head of the system.
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It reduces solids loading on the settling tank.
Complete mix
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Complete mix may or may not include primary treatment.
It distributes the waste, return, and oxygen evenly throughout the
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tank.
Aeration may be more efficient.•
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