Treatment Plants for Denitrification
Example 7.4
What is the necessary C/N ratio, (BOD/N)practice, 1 , for denitrification in a recycle plant
where organic matter from wastewater functions as carbon source?
The treatment efficiency in the plant is 80 per cent for total nitrogen and 95 per cent
for BOD.
The C/N ratio in practice can be found by using Expression (7.21)
1
ErN
(BOD/TN)practice 1 = (BOD/TN)optimum · -f - · -E'
C/N
BOD
From Table 7.3 it is estimated that (C/N)optimum = 3.2 kg BOD/kg TN, and from Table
7.4 we have fc/N = 0.4-0.6.
This means that if we "substitute" the range in the fc!N interval, we find
(BOD/TN)= 3.2 · 0 ~ 6 · 0 ~: 5 - 3.2 · 0 ~ 4 · ~g 8 5 = 4.5 -6.7 kg BOD/kg TN.
The higher end of the interval is normally outside what is available in municipal wastewater, see Tables 1.7 and 1.8. Therefore we must be careful not to squander the organic matter in the treatment process.
7.3.2 Stirring/ oxygen
For processes with activated sludge, stirring is required to ensure the suspension of
the sludge (prevent settling) and to ensure contact between the wastewater and the
sludge. Stirring must take place without unnecessary air/oxygen supply as this
would inhibit the denitrification process by reducing the denitrification rate and by
reducing the efficiency factor, fc;N for the organic matter. The result of the inhibition of the denitrification process is that some of the denitrifying bacteria respire
with oxygen instead of nitrate. Respiration of 32 g oxygen(= 1 mole of Oz) corresponds to a removal of 4 electron equivalents (as oxygen is reduced from oxidation
step 0 to -2). That is, 8 g of Oz- 1 electron equivalent. Correspondingly, 14 g of
nitrate nitrogen (= 1 mole of N) represent 5 electron equivalents (reduction of
nitrogen from oxidation step +5 to 0), that is, 2.8 g N03-N - 1 electron equivalent.
That means that the nitrate respiration is reduced by
2.8 g N03-N /1 eqv
_
8 0 I 1
= 0.35 g N03-N/Oz
g 2
eqv
(7.22)
Inhibition with oxygen may also occur by the supply of oxidized wastewater to the
denitrification tank, for example in a recycle plant.
253
Example 7.4
What is the necessary C/N ratio, (BOD/N)practice, 1 , for denitrification in a recycle plant
where organic matter from wastewater functions as carbon source?
The treatment efficiency in the plant is 80 per cent for total nitrogen and 95 per cent
for BOD.
The C/N ratio in practice can be found by using Expression (7.21)
1
ErN
(BOD/TN)practice 1 = (BOD/TN)optimum · -f - · -E'
C/N
BOD
From Table 7.3 it is estimated that (C/N)optimum = 3.2 kg BOD/kg TN, and from Table
7.4 we have fc/N = 0.4-0.6.
This means that if we "substitute" the range in the fc!N interval, we find
(BOD/TN)= 3.2 · 0 ~ 6 · 0 ~: 5 - 3.2 · 0 ~ 4 · ~g 8 5 = 4.5 -6.7 kg BOD/kg TN.
The higher end of the interval is normally outside what is available in municipal wastewater, see Tables 1.7 and 1.8. Therefore we must be careful not to squander the organic matter in the treatment process.
7.3.2 Stirring/ oxygen
For processes with activated sludge, stirring is required to ensure the suspension of
the sludge (prevent settling) and to ensure contact between the wastewater and the
sludge. Stirring must take place without unnecessary air/oxygen supply as this
would inhibit the denitrification process by reducing the denitrification rate and by
reducing the efficiency factor, fc;N for the organic matter. The result of the inhibition of the denitrification process is that some of the denitrifying bacteria respire
with oxygen instead of nitrate. Respiration of 32 g oxygen(= 1 mole of Oz) corresponds to a removal of 4 electron equivalents (as oxygen is reduced from oxidation
step 0 to -2). That is, 8 g of Oz- 1 electron equivalent. Correspondingly, 14 g of
nitrate nitrogen (= 1 mole of N) represent 5 electron equivalents (reduction of
nitrogen from oxidation step +5 to 0), that is, 2.8 g N03-N - 1 electron equivalent.
That means that the nitrate respiration is reduced by
2.8 g N03-N /1 eqv
_
8 0 I 1
= 0.35 g N03-N/Oz
g 2
eqv
(7.22)
Inhibition with oxygen may also occur by the supply of oxidized wastewater to the
denitrification tank, for example in a recycle plant.
253