Environmental Engineering Reference
In-Depth Information
Example 8.9
Problem:
Given the following data, what is the MCRT?
Influent flow = 4.2 MGD
Aeration volume = 1.20 MG
Influent BOD = 135 mg/L
Settling volume = 0.60 MG
Influent TSS = 150 mg/L
MLSS = 3350 mg/L
Effluent flow = 4.2 MGD
Waste rate = 0.080 MGD
Effluent BOD = 22 mg/L
Waste concentration = 6100 mg/L
Effluent TSS = 10 mg/L
Desired MCRT = 8.5 days
Solution:
3350 mg/L
×
(1.2 MG
+
0.6MG)
×
8.34
MRCT
=
(6100mg
/L
×
0.08 MGD8.34)
×
+
(10mg/L4.2 MGD8.34)
×
×
= 1.4days
1
8.14.6.1 Mean Cell Residence Time Control
Because it provides an accurate evaluation of the process condition
and considers all aspects of the solids inventory, the MCRT is an excellent
process control tool. Increases in the waste rate will decrease the MCRT,
as will large losses of solids over the effluent weir. Reductions in waste
rate will result in increased MCRT values. You should remember these
important process control parameters and their impact on the MCRT:
•
To increase F/M
, decrease MCRT.
To increase MCRT, decrease
•
waste rate.
•
If the MCRT is increased, the
MLTSS and 30-minute settling volume
increase.
Return sludge rate has no impact on MCRT.
•
•
MCRT has no impact on F/M when the number of aeration tanks in
service is reduced.
8.14.6.2 Typical MCRT Values
The following chart lists the various aeration process modifications
and associated MCRT values:
Process
MCrT (days)
Conventional
5-15
Step aeration
5-15
Contact stabilization
5-15
Extended aeration
20-30
Oxidation ditch
20-30
Pure oxygen
8-20
