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Plant brochure designed by Charles L.
Woodruff 1999 and revised in June 2004.
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Kruger Oxidation Ditch - Nitrogen Removal |
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| The predominant forms of
nitrogen in raw wastewater are organic nitrogen and ammonia (NH3). Organic
nitrogen is converted to ammonia biologically in the first step of the
nitrogen cycle. In order to remove nitrogen from wastewater, the ammonia
must first be oxidized to nitrate (NO3). This process is commonly referred
to as nitrification. The nitrification process is carried out by two species
of bacteria referred to as nitrifiers (Nitrosomonas and Nitrobacter). These
bacteria have slow growth rates compared to the bacteria responsible for
removing BOD. Therefore, in order for nitrification to proceed, an oxic
environment must be maintained as well as an adequately long solids
retention time (SRT) due to the slow growth rate of the nitrifiers.
In the phased isolation ditches, oxic
conditions are generally maintained by a number of rotor (brush-type)
surface aerators. In the presence of dissolved oxygen and a sufficient SRT,
micro organisms will simultaneously oxidize the BOD and ammonia.
Nitrification involves the conversion of ammonia by the nitrifiers to
nitrate, as well as new cell mass. The biological reactions for
nitrification are illustrated in Figure 2.2. |
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Figure 2.2: Biological
nitrification reactions |
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Following nitrification,
nitrogen can be removed from the wastewater by reducing the nitrate to
nitrogen gas (N2), which is released to the atmosphere (See Figure 2.3).
This process is commonly referred to as denitrification. Denitrification
requires anoxic conditions, as well as a carbon source to proceed. In the
illustration below, BOD is the carbon source fuelling the process.
During anoxic conditions, dissolved oxygen is not available to the micro
organisms for respiration. Consequently, the oxygen molecules are stripped
from nitrates resulting in the production of nitrogen gas. Carbon dioxide
and water are also produced in the process, resulting from the degradation
of BOD (Figure 2.3). In addition, a portion of the alkalinity consumed
during the nitrification process is restored through the denitrification
process. |
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Figure 2.3: Biological
denitrification reactions |
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