Mass balances, denitrifying treatment plants
The organic matter is limiting for the reaction rate when
Scoo,2< 5No3,2. DNo3,2· VN03,codDcoo,2
(7.6a)
where VN03,COD is the stoichiometric coefficient for the biological process between the organic matter (here measured as COD) and nitrate
(typically 5.5-6 kg COD /kg N03-N),
Dcoo,2 is the diffusion coefficient for organic matter in the biofilm,
DNo3,2
is the diffusion coefficient for nitrate in the biofilm.
Nitrate is limiting when
5N03,2< Scoo,2· Dcoo,2· VCODN03/DN03,2
(7.6b)
If the organic matter is limiting, and if the biofilm is fully penetrated(~> 1), see
Expression (5.13), the reaction rate is:
rA,coo= kov£,coo· L
The reaction rate for a partially penetrated biofilm (~< 1) is:
rA,coo= (2 · Dcoo· kov£,coo) 112 · Scoo} 12
(7.7a)
(7.7b)
If nitrate is limiting and the process is a zero order reaction in the biofilm, we have
the following expression for the reaction rate for a fully penetrated biofilm (~ > 1):
rA,No3= kov£,No3· L
and for partial penetration(~< 1) applies:
rA,N03= (2 · DNo3· kovf,N03) 112 · SN03,2 1 / 2
(7.7c)
(7.7d)
Notice that kov£ has a unit corresponding to the limiting substance; that is, in
Expression (7.7c) the unit is for example kg N03-N I (m 3 biofilm · d).
An estimate of the quantity DN03· VN03,coo/Dcoo for different organic materials
and for kt/2A,N03,D, VN03,COD and kovtN03 is found in Table 7.2.
k1/2A,N03
D
VNOJ,COD
DNo3 VNOJ,coo/Dcoo
ko!,N03
Acetic acid
1-3
0.3-0.7
5-6
5-20
10-150
Methanol
1.4-3.7
0.8-4
3.5-4.1
1.8-2.3
20-200
Glucose
1-3
0.1-0.7
5-6
5-20
10-150
Unspec.
1-3
0.3-0.6
4-5
5-20
1D-150
COD
g N1/2. m -1/2 d-1 10-4m 2 /d kg COD/kg N03-N kg COD /kg NOJ-N kg NOJ-N/(m 3 ·d)
Table7.2
Kinetic constants in a denitrifying biofilm reactor (20°C}.
The diffusion coefficient for nitrate is (0.5-1.0) · 10-4m 2 I d /2/,/3/.
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