Design of denitrifying plants
Example 7.5
In a denitrification tank with paddle stirring, a KLa-value of 3 d- 1 at sac is measured in
the wastewater. The oxygen concentration is 0 g/m 3 and the sludge concentration 4 kg
VSS/m 3 . Calculate how much the denitrification rate is reduced at sac.
The oxygen flux from the atmosphere to the tank is~- N = KLa (Cm- C)
accord1ng to the theory for aeration.
The saturation concentration of oxygen, Cm. at sac is found to be 11.S g/m 3 . Hence
the oxygen flux is:
= 3 d- 1 (11.S- 0) g 0 2/m 3 = 35.4 g 0 2/(m 3 · d)
If the bacteria which utilized the supplied oxygen had denitrified instead, they would
have denitrified:
35.4 g 0 2/(m 3 · d) · 0.35 g NO:J-N/g 0 2 = 12.4 g N03-N/(m 3 · d)
If it is assumed that the denitrification rate without the influence of oxygen, rx.s. corresponds to the rate stated in Fig 3.13 for raw wastewater, approx. 12 g NO:J-N/(kg VSS ·
d), a specific volumetric denitrification rate is found:
rv.s = rx.s ·X= 12 · 4 = 4S g N03-N/(m 3 ·d)
Hence the current denitrification rate is:
rv.s (current)= rv.s -12 = 4S -12 = 36 g N03-N/(m 3 ·d)
That is, the denitrification rate is in this case reduced by 12/4S = 25 per cent. At a higher temperature, the reduction is lower.
7.3.3 Nitrogen gas in settling tanks and biofilters
If denitrification occurs in the settling tank, it may cause problems with rising
sludge as a result of nitrogen gas bubbles. It is difficult exactly to say when it
becomes a problem as the water in a settling tank can be super-saturated with
nitrogen without nitrogen bubbles being formed.
The increased pressure at the bottom of the settling tank will increase the saturation
concentration for nitrogen and will subsequently counteract the tendency of supersaturation.
Oxygen in the sludge in the settling tank will reduce the denitrification (see Example 7.5).
With a saturation concentration of nitrogen at an atmospheric pressure of about 20
g N2/m 3 , an increased hydraulic head of 2m means that the saturation concentration increases to approx. 24 g N2/m 3 . A denitrification of 4 g N03-N /m 3 , while the
water and the sludge are in the settling tank, will therefore not worsen the situation.
For a normally well-functioning denitrification process and a well-functioning
settling tank, the amount of nitrate, which can be denitrified, will not cause prob254
Example 7.5
In a denitrification tank with paddle stirring, a KLa-value of 3 d- 1 at sac is measured in
the wastewater. The oxygen concentration is 0 g/m 3 and the sludge concentration 4 kg
VSS/m 3 . Calculate how much the denitrification rate is reduced at sac.
The oxygen flux from the atmosphere to the tank is~- N = KLa (Cm- C)
accord1ng to the theory for aeration.
The saturation concentration of oxygen, Cm. at sac is found to be 11.S g/m 3 . Hence
the oxygen flux is:
= 3 d- 1 (11.S- 0) g 0 2/m 3 = 35.4 g 0 2/(m 3 · d)
If the bacteria which utilized the supplied oxygen had denitrified instead, they would
have denitrified:
35.4 g 0 2/(m 3 · d) · 0.35 g NO:J-N/g 0 2 = 12.4 g N03-N/(m 3 · d)
If it is assumed that the denitrification rate without the influence of oxygen, rx.s. corresponds to the rate stated in Fig 3.13 for raw wastewater, approx. 12 g NO:J-N/(kg VSS ·
d), a specific volumetric denitrification rate is found:
rv.s = rx.s ·X= 12 · 4 = 4S g N03-N/(m 3 ·d)
Hence the current denitrification rate is:
rv.s (current)= rv.s -12 = 4S -12 = 36 g N03-N/(m 3 ·d)
That is, the denitrification rate is in this case reduced by 12/4S = 25 per cent. At a higher temperature, the reduction is lower.
7.3.3 Nitrogen gas in settling tanks and biofilters
If denitrification occurs in the settling tank, it may cause problems with rising
sludge as a result of nitrogen gas bubbles. It is difficult exactly to say when it
becomes a problem as the water in a settling tank can be super-saturated with
nitrogen without nitrogen bubbles being formed.
The increased pressure at the bottom of the settling tank will increase the saturation
concentration for nitrogen and will subsequently counteract the tendency of supersaturation.
Oxygen in the sludge in the settling tank will reduce the denitrification (see Example 7.5).
With a saturation concentration of nitrogen at an atmospheric pressure of about 20
g N2/m 3 , an increased hydraulic head of 2m means that the saturation concentration increases to approx. 24 g N2/m 3 . A denitrification of 4 g N03-N /m 3 , while the
water and the sludge are in the settling tank, will therefore not worsen the situation.
For a normally well-functioning denitrification process and a well-functioning
settling tank, the amount of nitrate, which can be denitrified, will not cause prob254