Mass balances, denitrifying treabnent plants
7.1.1 Separate denitrifying plant
The process can be carried out in activated sludge plants or in biofilters - see the
schematic representation in Fig 7.1.
A mass balance for a denitrifying plant can be written when the processes and their
kinetics are known. In Table 7.1, a process matrix is shown for a separate denitrifying plant. llg is the fraction of heterotrophic bacteria which can denitrify.
Activated sludge plant
A nitrate balance for the plant in Fig 7.la looks as follows:
Ql · 5N03,1- rv,s · V 2 = Q3 · SNo3,3 + Qs · SNo3,5
(7.1)
from which the nitrate concentration in the effluent, SN03,3 is found:
Assuming that
Qs · SNo3,s= o
the following simplified expression is found:
SNo3,3= 5No3,I- rv;s · 82
(7.2)
where the hydraulic retention time e2 = ~~
The reaction rate expression, rv,s, can be found from:
rv,s = vx,s· ry, ...
where rv, ... is found in the last column at the right hand side of Table 7.1, and the
stoichiometric coefficient for the biological process, VXB,N03 is found in the SN03
column:
(1- Y max,H) llmax,H
Ss
Ks 02 H
SN03
rv,s=
. - .
' '
·11 ·XBH
2.86 · Y max,H
Ks + 5s Ks,02,H + S02 Ks,N03 + SNo3 g '
(7.3)
By combining Expressions (7.2) and (7.3), the nitrate concentration in the effluent,
5No3.3' can be found for an ideally mixed tank.
Often the denitrification process can be approximated by a zero order process with
respect to organic matter, oxygen and nitrate, that is,
232
7.1.1 Separate denitrifying plant
The process can be carried out in activated sludge plants or in biofilters - see the
schematic representation in Fig 7.1.
A mass balance for a denitrifying plant can be written when the processes and their
kinetics are known. In Table 7.1, a process matrix is shown for a separate denitrifying plant. llg is the fraction of heterotrophic bacteria which can denitrify.
Activated sludge plant
A nitrate balance for the plant in Fig 7.la looks as follows:
Ql · 5N03,1- rv,s · V 2 = Q3 · SNo3,3 + Qs · SNo3,5
(7.1)
from which the nitrate concentration in the effluent, SN03,3 is found:
Assuming that
Qs · SNo3,s= o
the following simplified expression is found:
SNo3,3= 5No3,I- rv;s · 82
(7.2)
where the hydraulic retention time e2 = ~~
The reaction rate expression, rv,s, can be found from:
rv,s = vx,s· ry, ...
where rv, ... is found in the last column at the right hand side of Table 7.1, and the
stoichiometric coefficient for the biological process, VXB,N03 is found in the SN03
column:
(1- Y max,H) llmax,H
Ss
Ks 02 H
SN03
rv,s=
. - .
' '
·11 ·XBH
2.86 · Y max,H
Ks + 5s Ks,02,H + S02 Ks,N03 + SNo3 g '
(7.3)
By combining Expressions (7.2) and (7.3), the nitrate concentration in the effluent,
5No3.3' can be found for an ideally mixed tank.
Often the denitrification process can be approximated by a zero order process with
respect to organic matter, oxygen and nitrate, that is,
232