Design of denitrifying plants
the amount of nitrogen which has been removed. The three 3 elements mentioned
above contribute to the C/N ratio in the following way as organic matter is here
calculated in COD which gives an exact calculation of the C/N ratio in the form of
the COD /N ratio.
1.
N03 + 1.25 "COD"_. 1/2 N2 + C02
COD-consumption= 1.25 moles/mole N03-N
= 2.86 kg COD /kg N03-N
The 1.25 "COD" represents an amount of organic matter which would consume
1.25 moles of oxygen= 40 g oxygen by a COD analysis, whereas N03 represents
1 mole of nitrogen= 14 g nitrogen.
If the denitrified amount of nitrate is i'lN03-N, the COD-consumption is 2.86
· i'lN03-N
2.
Sludge production= Fsp. If it is measured in COD-units we have the COD-consumption directly.
COD-consumption = FsP,COD
3.
Oxygen respiration. If the oxygen consumption is Fo2, the
COD-consumption = F02.
The total COD-consumption is then:
Total COD-consumption per volume of wastewater= 2.86 · ~N03-N + FsP,coo+ F02
The total nitrogen removal is denitrified nitrate (~N03-N) plus assimilated nitrogen (in the sludge production):
Total N removed = ~N03-N + Fsr,coo· fB,N
Hence the C/N ratio with the unit kg COD /kg N, realized by the current process, is
C/N = (2.86 · ~03-N + Fsr + F02)/(i'lN03-N + Fsr · fs,N)
(7.15)
The C/N ratio can also be determined on the basis of an equation of reaction.
Note that what is included in the C/N ratio, as defined in Expression (7.15), is not
the total COD and total N in the influent, hence it is not (COD/Nh; for both COD
and nitrogen it applies that the amounts in the effluent should be deducted.
For a low-loaded nitrification-denitrification plant, the observed yield constant will
for example be= 0.4 kg COD(B)/kg COD(S). A corresponding equation of reaction
with organic matter, C 1sl-h909N, and biomass, CsH7NO:z., will look as follows:
C1sH1909N + 8.4 N03- + 8 H+ + 0.4 NH4 + -t
1.4 CsH7N02 + 11 C02 + 4.2 N2 + 9.4 H20
(7.16)
Here the COD-consumption (C 1sH 1~9N has a COD-content of 560 g 02/ mol):
250
(560/(8.4 · 14)) = 4.8 kg COD/kg N03-N or
560/(8.4 ·14 + 0.4 ·14) = 4.5 kg COD/kg TN
the amount of nitrogen which has been removed. The three 3 elements mentioned
above contribute to the C/N ratio in the following way as organic matter is here
calculated in COD which gives an exact calculation of the C/N ratio in the form of
the COD /N ratio.
1.
N03 + 1.25 "COD"_. 1/2 N2 + C02
COD-consumption= 1.25 moles/mole N03-N
= 2.86 kg COD /kg N03-N
The 1.25 "COD" represents an amount of organic matter which would consume
1.25 moles of oxygen= 40 g oxygen by a COD analysis, whereas N03 represents
1 mole of nitrogen= 14 g nitrogen.
If the denitrified amount of nitrate is i'lN03-N, the COD-consumption is 2.86
· i'lN03-N
2.
Sludge production= Fsp. If it is measured in COD-units we have the COD-consumption directly.
COD-consumption = FsP,COD
3.
Oxygen respiration. If the oxygen consumption is Fo2, the
COD-consumption = F02.
The total COD-consumption is then:
Total COD-consumption per volume of wastewater= 2.86 · ~N03-N + FsP,coo+ F02
The total nitrogen removal is denitrified nitrate (~N03-N) plus assimilated nitrogen (in the sludge production):
Total N removed = ~N03-N + Fsr,coo· fB,N
Hence the C/N ratio with the unit kg COD /kg N, realized by the current process, is
C/N = (2.86 · ~03-N + Fsr + F02)/(i'lN03-N + Fsr · fs,N)
(7.15)
The C/N ratio can also be determined on the basis of an equation of reaction.
Note that what is included in the C/N ratio, as defined in Expression (7.15), is not
the total COD and total N in the influent, hence it is not (COD/Nh; for both COD
and nitrogen it applies that the amounts in the effluent should be deducted.
For a low-loaded nitrification-denitrification plant, the observed yield constant will
for example be= 0.4 kg COD(B)/kg COD(S). A corresponding equation of reaction
with organic matter, C 1sl-h909N, and biomass, CsH7NO:z., will look as follows:
C1sH1909N + 8.4 N03- + 8 H+ + 0.4 NH4 + -t
1.4 CsH7N02 + 11 C02 + 4.2 N2 + 9.4 H20
(7.16)
Here the COD-consumption (C 1sH 1~9N has a COD-content of 560 g 02/ mol):
250
(560/(8.4 · 14)) = 4.8 kg COD/kg N03-N or
560/(8.4 ·14 + 0.4 ·14) = 4.5 kg COD/kg TN