Denitrification
The substrate removal rate can be written as follows:
rvs = j.lmax .
SNo3
. __ S_. Xs
'
Y max SNo3 + Ks,N03 S + Ks
(3.32)
SNo3
where
describes the effect of the nitrate concentration.
SNo3 + Ks,Nm
Ks,N03 is the saturation constant for nitrate,
S
is the concentration of energy source (for example organic matter
in wastewater),
Ks
is the saturation constant for the energy source.
In many cases, Expression (3.32) can be simplified to a zero order expression in
respect of nitrate and energy source:
j.lmax
rv,s= - - . Xs
Ymax
as
SN03»Ks,N03
S»Ks
Compared with the aerobic processes, the essential difference is that by denitrification it is normally nitrate (the electron acceptor) we want to control, whereas by the
aerobic heterotrophic processes it is normally the organic matter (the electron
donor), we want to control.
90
Example 3.9
Calculate the removal rate for substrate, COD, and nitrate, N03-N, for a process where the following constants apply:
j.lmax = 3 d- 1
Y max,COD = 0.50 kg COD(B)/kg COD(S)
Ks,N03 = 0.1 g N03-N/m 3
Ks = 20 g COD/m 3
Xs = 2 kg COD(B)/m 3
The process is assumed to correspond to the one discussed in Example 3.7.
The process occurs in an ideally mixed tank, with influent and effluent concentrations
of:
in
SN03
20
scoo
200
Expression (3.32) is used:
out
1 g NQ3-N/m 3
10 g COD/m 3