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Plant brochure designed by Charles L.
Woodruff 1999 and revised in June 2004.
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Kruger Oxidation Ditch - BioDenipho
Process Theory of Operation |
| The BioDenipho process is
best understood by following the nitrogen and phosphorus removal stages
through one complete 4-hour cycle of operation. One complete cycle consists
of four separate phases. The phases are labelled B, E, G, and J. Please note
that G and J are simply “mirror images” of Phases B and E.
The BioDenipho process follows the
sequence of operation that will be described below. In all of the phases,
the influent wastewater is mixed with the Return Activated Sludge (RAS) in
the anaerobic selector prior to being directed to either ditch. The
reactions that take place in the anaerobic selector were described in the
previous section.
Phase B
The cycle begins with phase B, which in this example has a duration of 90
minutes. The duration of the phases can be varied to provide different
operational strategies. This is discussed further in the section entitled
Flexible Operation. In Phase B, the effluent weir in Ditch 1 is raised and
the effluent weir in Ditch 2 is lowered. The hydraulic gradient is now
shifted so that the flow direction is from Ditch 1 to Ditch 2, with Ditch 2
discharging effluent (See Figure 2.6). |
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Figure 2.6: Ditch 1 -
denitrification (anoxic), Ditch 2 - nitrification (oxic) |
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| Ditch 1 is in the
denitrification mode of operation. The rotors in Ditch 1 are turned off and
will remain off throughout Phase B to produce anoxic conditions. The
activated sludge in Ditch 1 is kept in suspension by submerged mixers. The
ammonia concentration in Ditch 1 will increase due to the ammonia and
organic material in the influent. The ammonia concentration in Ditch 2 will
decrease due to the oxidation of ammonia and organic materiel in the
wastewater. The concentration of nitrate, which was produced during an oxic
phase in Ditch 1 in the previous cycle (Phase J), will decrease as a result
of denitrification. As stated
previously, denitrification requires a carbon source to fuel the process.
This carbon source is provided by the influent wastewater. Because the
influent wastewater is directed to the anoxic ditch (Ditch 1 in phase B),
the organic carbon is readily available for use during denitrification. If
the influent had been initially sent to the oxic ditch (Ditch 2 in phase B),
the organic carbon would have been degraded aerobically and an external
carbon source, such as methanol, would have to be added to the system to
promote denitrification.
Focusing on Ditch 2 in phase B, the
ammonia concentration will decrease while the nitrate concentration
increases, because the oxic environment promotes the oxidation of ammonia to
nitrate (nitrification). The motorized effluent weir is lowered in Ditch 2
to allow the activated sludge to exit the ditch and proceed to the
clarifier. As the influent enters Ditch 1, a hydraulic gradient is produced
that forces the mixed liquor from Ditch 1 to Ditch 2, over the effluent weir
and on to further treatment.
Effluent will always be discharged from
an oxic ditch. This ensures that influent ammonia will be exposed to a
nitrification period prior to exiting the ditches, thereby guaranteeing that
the concentration of ammonia is minimized.
The treated and clarified effluent is either discharged from the facility or
continues on for further treatment, such as filtration or disinfection. The
underflow from the clarifier is either wasted or directed back to the
distribution chamber, where it is mixed with the influent wastewater.
Phase E
Phase E has a duration of 30 minutes, during which oxic conditions are
maintained in both ditches, and is initiated by changing the flow direction
from the distribution chamber. The automated influent weirs in the
distribution chamber switch position and direct the influent to the inlet
for Ditch 2 instead of Ditch 1 (Figure 2.7). The distribution chamber is
operated automatically via the PLC; however, the unit can also be operated
manually in the event of an emergency.
In Phase E, Ditch 1 is isolated from
the influent. The rotors in Ditch 1 are turned on to produce oxic
conditions. The increased ammonia concentrations in Ditch 1 from Phase B
will now decrease as a result of nitrification. |
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Figure 2.7: Ditch 1 -
nitrification (oxic), Ditch 2 - nitrification (oxic) |
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| Ditch 2 is oxic from the
previous phase and will remain oxic throughout phase E. In addition, the
effluent will continue to be discharged from Ditch 2 throughout phase E.
Phase G
Phase G is a mirror image of Phase B
and has a duration of 90 minutes. Phase G is initiated by raising the
effluent weir in Ditch 2, and lowering the effluent weir in Ditch 1. The
resulting hydraulic gradient alters the direction of flow from Ditch 2 to
Ditch 1, and on to the clarifiers. At the same time, the rotors are turned
off in Ditch 2 to promote anoxic conditions so that the nitrate produced
during the previous three oxic phases for Ditch 2 can be denitrified (Figure
2.8). |
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Figure 2.8: Ditch 1 -
nitrification (oxic), Ditch 2 - denitrification (anoxic) |
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Phase J
Following Phase G, the process will
operate in Phase J. Phase J is a mirror image of Phase E and has a duration
of 30 minutes. During this phase, Ditch 1 remains in the nitrification mode
of operation. Influent flow is again redirected into Ditch 1. The rotors in
Ditch 2 are turned on and will remain on throughout Phase H to product oxic
conditions. The ammonia concentration in Ditch 2 will decrease due to the
oxidation of ammonia (nitrification). The concentration of nitrate will
increase as a result of nitrification (Figure 2.9). |
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Figure 2.9: Ditch 1 -
nitrification (oxic), Ditch 2 - nitrification (oxic) |
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| Clarified effluent
continues for further treatment such as filtration and disinfection. The
underflow from the secondary clarifier is discharged to a wet well by means
of a telescoping valve. The sludge is then either wasted or returned to the
anaerobic selector. At the
end of phase J, the cycle will be completed. The weir in Ditch 1 will be
raised, the weir in Ditch 2 will be lowered and another 4 hour cycle of
operation will begin.
As discussed in Section 1, due to aeration limitations the system can only
operate in the BioDenipho mode up to a flow rate of 2.3 MGD. Based on a
typical Hydraulic Retention Time (HRT) of 16 hours at a flow rate of 2.3
MGD, one complete 4 hour cycle accounts for only one-fourth of the HRT. |
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