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Plant brochure designed by Charles L.
Woodruff 1999 and revised in June 2004.
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Kruger Oxidation Ditch - Phosphorus Removal |
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A three-stage anaerobic
selector is incorporated prior to the distribution chamber. The anaerobic
residence time is approximately 50 minutes based on the design flow of 2.9
MGD, which is not long enough to promote the build-up of sulphides or other
noxious products sometimes associated with anaerobic treatment processes.
The RAS enters the first stage of the anaerobic selector where chemically
bound oxygen in the form of nitrates is removed. The RAS then passes into
the second stage where it is mixed with influent wastewater. At a
microbiological level, the process functions by selectively feeding
specialized activated sludge micro organisms under anaerobic conditions. The
micro organisms have the ability to store energy in the form of
polyphosphate chemical bonds. In the anaerobic zone, where substrate (BOD)
concentration is high, the absence of oxygen causes the micro organisms to
release the stored intracellular polyphosphates by decomposition to simple
orthophosphate. |
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End View of
Anaerobic Selector |
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| The decomposition of
polyphosphate to orthophosphate results in an increase of soluble phosphorus
in the mixed liquor and also releases energy. The energy is used by the
micro organisms to transport soluble BOD through the cell wall and to store
the soluble BOD inside the cell (See Figure 2.4). Thus, the BOD
concentration in the mixed liquor is reduced without the
use of oxygen. |
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Figure 2.4: Biological
phosphorus removal reactions |
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| In the oxic phases of the
process, the organisms, in the presence of dissolved oxygen, convert the
stored BOD to CO2, water, and increased cell mass. A portion of the energy
from this reaction then goes to recreating the intracellular polyphosphate
using the orthophosphate released in the anaerobic zone. Since new cells are
grown, the amount of phosphate removed from solution is greater than that
previously solubilized in the anaerobic zone, thus providing for a net
phosphate removal. Phosphorus
is removed from the system as a fixed biological material in the waste
sludge. The amount of phosphorus in the sludge will be dependent upon the
amount of BOD and phosphate in the influent and the volume of sludge
produced. In addition, biological phosphorus removal is dependent on
maintaining an adequate sludge blanket in the secondary clarifiers. Kruger's
experience has shown that a sludge blanket of at least one foot must be
maintained to promote a “stressful” environment that allows phosphorus
removing organisms to proliferate. As a result of this requirement,
modifications to the RAS pumping controls may be required to ensure that an
adequate sludge blanket is maintained in the secondary clarifiers. |
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