Mass balances, denitrifying treatment plants
Similar to Expression (6.1), Expression (7.8) can be converted into:
ex,A = 1 I llobs,A,net
(7.9)
Remember that only tank V3 is included in the calculation of the aerobic sludge age.
Denitrification may occur in biofilm even if there are aerobic conditions in the water
flowing through the filter. This is the case where nitrate enters deeper into the
biofilm than oxygen, for example due to a high concentration of nitrate and a low
concentration of oxygen. Denitrification may occur in the innermost part of the
biofilm if organic matter is present. The denitrification rate is, however, reduced due
to the longer diffusion path.
Fig 7.5 shows concentration profiles in the biofilm. For the three zones, it applies:
aerobic zone rl SNo 3
- - = 0
O ai
rlsoz = kovf,oz
ai
Doz
(7.10)
anoxic zone rl SNo3 kovf,N03
xo < X < XN
ax2
DNo3
anaerobic zone
XN d 2 5Me kovf Me
- - = - -' -=constant
ai
DMe
(7.11)
(7.12)
In the aerobic zone the concentration profile for nitrate is straight. In the anoxic
zone, the concentration profile is a parabola with top point in XN.
aerobic denitriSo2
fication
I
I
anI
aerobic
1.0
I
oxygen
I
r....,N03
I
st.4e
k112A-SNoj 12
0.5
excess
methanol
0
10" 2
10' 1
nitrate
ko.No3 D02
S"o2
limiting 8
--·--· - -
ko.o2 DN03
S* N
NOJ
,.,
Criterium
2 2 ·DN03 ·SN03 2. 002· So2
•
L
for partial
L>
+
penetration
ko.N03
kaf.
Fig 7.5
The figure slwws Jww denitrification may occur in spite of aerobic conditions in the
free water. Nitrate diffuses through the aerobic part of the biofilm and is denitrified in
the anoxic zone. To the right the reduced denitrification rate is shown compared with
denitrification witlwut oxygen.
238