Design of denitrifying plants
266
Example 7.9
A plug flow filter for the denitrification of 1,800 m 3 /d municipal wastewater has to be disigned. The water is treated in a nitrifying activated sludge plant after which it is denitrified with acetic acid, HAc,
as the carbon source.
The influent concentration to the filter is SNo3,1 = 23 g N03-N/m 3 .
The effluent concentration, SNo3.3 , must be 3 g N03-N/m 3 .
An acetic acid dose is chosen. From Table 7.2, the stoichiometrically needed dosage is
found (inclusive of assimilation) to be 5.5 g COD/g N03-N.
We choose to add more acetic acid than this ratio, say 6.5 g COD/g N03-N. That is,
g N03-N
g COD
ScoD,1,min = (5N03,1 - SNQ3,3)V = (23 - 3)
3
· 6.5
_
m
g N03-N
which corresponds to
3
gCOD
~
9HAc
130gCOD/m ·VCQD,HAc=130 m 3 ·0.937 gCOD=122 m 3
By means of Expression (7.6b) it is considered whether nitrate is limiting in the influent
and effluent.
DN03
From Table 5.1 we find -D-- = 1.6.
COD
Scoo,1 _ 130 9 COD /m 3 5 6 9 COD
DN03
-
·
<--·VN03COD
SN03,1 23 g N03-N/m3
g N03-N Dcoo
.
= 1.6 · 5.5 = 8.8 9 COD
g N03-N
That is, acetic acid is limiting and nitrate is not limiting.
In the effluent, the acetic acid content (and the nitrate content) is reduced.
From the ratio vNo3,coo = 5.5 g COD/g N03-N from Table 7.2 and a mass balance for
COD, the effluent concentration is determined:
in + removed = out
Scoo.1 - (SNo3,3 - SNo3,1) vNo3.coo = Scoo,3
9 COD
g N03-N
COD
Scoo,3 = 130
- (23 -3)
· 5.5 = 20 g - -
m3
m3
m3
For the effluent it is considered whether nitrate is limiting