Treatment Plants for Denitrification
When the equation is solved, the following expression for the nitrogen concentra~
tion behind the anoxic zone in the biofilm /8/ is found to be
DNo3
SN2 L = SN2 x=O + - 0 · SN03,x=O
'
'
N2
Example 7.6
How high can the nitrate concentration be to cause a noticeable supersaturation and bubble formation inside the biofilm?
(7.24)
At the surface of the biofilm, the nitrogen concentration is at least 16 g N2/m 3 (equilibrium with the atmosphere at 1s•c). Inside the biofilm, the concentration may increase
to 20 g N2/m 3 before bubble formation. This is equal to a supplement of 4 g N2/m 3 •
DNo3 = 0.7 · 10-4 m 2 /d
DN2= 1.1 ·10-4m 2 /d
SNo3,max = 4 · ~:; = 6 g NOj-N/m 3
Any higher concentration in the water outside the biofilm will cause super-saturation
and bubble formation. It is a phenomenon which must be taken into account in any denitrification filter.
7.3.4 Oxygen consumption
If we leave out of consideration the unwanted oxygen consumption, as discussed in
Section 7.3.2, the denitrification process represents in itself a negative oxygen
consumption as nitrate replaces oxygen by oxidation of organic matter. It should be
remembered, however, that a larger amount of oxygen is used for the nitrate
production than the amount which is "saved" by the denitrification.
If we look at nitrification and denitrification as an overall process:
NH! ..._. N03
N03 ---. 0.5Nz
NH! ..._. 0.5Nz
we can find the oxygen consumption for the overall process:
NH! + 0.75 Oz + mr----. o.s Nz + 2.5 HzO
(7.25)
The "oxygen saving" by denitrification, viewed separately; corresponds to 1.25 mol
Oz/mol NO)-N, or 2.86 g 02/g N03-N (as the oxidation step for nitrogen is
decreased by 5, which is equivalent to 1.25 moles of 02, which is also decreased by
(4 ·1.25 =) 5 in the overall oxidation step).
257
When the equation is solved, the following expression for the nitrogen concentra~
tion behind the anoxic zone in the biofilm /8/ is found to be
DNo3
SN2 L = SN2 x=O + - 0 · SN03,x=O
'
'
N2
Example 7.6
How high can the nitrate concentration be to cause a noticeable supersaturation and bubble formation inside the biofilm?
(7.24)
At the surface of the biofilm, the nitrogen concentration is at least 16 g N2/m 3 (equilibrium with the atmosphere at 1s•c). Inside the biofilm, the concentration may increase
to 20 g N2/m 3 before bubble formation. This is equal to a supplement of 4 g N2/m 3 •
DNo3 = 0.7 · 10-4 m 2 /d
DN2= 1.1 ·10-4m 2 /d
SNo3,max = 4 · ~:; = 6 g NOj-N/m 3
Any higher concentration in the water outside the biofilm will cause super-saturation
and bubble formation. It is a phenomenon which must be taken into account in any denitrification filter.
7.3.4 Oxygen consumption
If we leave out of consideration the unwanted oxygen consumption, as discussed in
Section 7.3.2, the denitrification process represents in itself a negative oxygen
consumption as nitrate replaces oxygen by oxidation of organic matter. It should be
remembered, however, that a larger amount of oxygen is used for the nitrate
production than the amount which is "saved" by the denitrification.
If we look at nitrification and denitrification as an overall process:
NH! ..._. N03
N03 ---. 0.5Nz
NH! ..._. 0.5Nz
we can find the oxygen consumption for the overall process:
NH! + 0.75 Oz + mr----. o.s Nz + 2.5 HzO
(7.25)
The "oxygen saving" by denitrification, viewed separately; corresponds to 1.25 mol
Oz/mol NO)-N, or 2.86 g 02/g N03-N (as the oxidation step for nitrogen is
decreased by 5, which is equivalent to 1.25 moles of 02, which is also decreased by
(4 ·1.25 =) 5 in the overall oxidation step).
257