Contact Stabilization Activated Sludge Process
355
Viable cell growth rate in the contact tank is defined by a steady-state materials
balance around this basin:
Rx s - (F + R)x c + V c ? £ = o
d x c
ance w
= Mnc x
L C
X C
At low stabilization basin substrate concentrations, a net decrease of viable cells and
sludge mass occurs.
A steady-state cell materials balance around the stabilization basin yields:
net
=
x
r ~
x
s = J_ f X _ ,1
»8*8
h L*S
J
I net
S
and
k d , - " ^ ^
* '*
M
( 3 )
where t s = stabilization basin hydraulic residence time = V s /R.
The overall contact stabilization process can be described by the following cell and
substrate materials balances:
- ( F - w) Xl - w x c + Mn c V c x c - k d s V s x s - 0
Defining a & β as the fractions of the total sludge in contact basin and stabilization
basin respectively:
»■w Λ ΐ , . and p -
V s X
^
c
x c
+
s
x s
c
x c
+ "s
x s
then -(F - w) Xl - w x c + (V c x c + ν ^ Μ α μ ^ - ßk d ) = 0
and since
( F _ w ) X i + W X c
Mn =
V c x c + V s x s
i
4 >
Mn = a^n c - /3k d
(5)
s
F ( s 0 - SO
q
= V x + V x
= a 9 c
(6)
c c
s
A s
Equation (5) embodies the idea that cell growth is the major activity in the contact
basin and cell decay is the major process during stabilization and provides a quantitative
description of the ability of contact stabilization to unlock the cell continuity expression
defined by Pearson (1966). Thus, in a conventional "single aeration" activated sludge