Solution to Homework 7, Problem 1
Parts a.-k. - Original design
Design parameters
Design flow rate
Reactor volume
Influent COD concentration
Part l. - Redesign
Q
V
Sin
0.0088
25
300
m3/s
m3
mg COD/L
0.0088
50
300
m3/s
m3
mg COD/L
Solids concentration of recycled sludge
XR
12000
mg VSS/L
12000
mg VSS/L
Clarified effluent from secondary clarifier
Safety factor
Xe
SF
15
20
mg VSS/L
15
80
mg VSS/L
µmax
9.6
day-1
Kinetic constants:
Maximum specific growth rate
Half-saturation constant
Cell yield
Endogenous respiration rate
KS
Y
ke
75
0.4
0.096
mg/L as COD
g VSS/g COD
day-1
Computed characteristics
a.
Hydraulic residence time
tR
0.8
hours
1.6
hours
b.
Minimum solids retention time
θc,min
2.5
hours
2.5
hours
Washout solids retention time
θc,w
3.16
hours
3.16
hours
Design solids retention time (sludge age)
θc
51
hours
202
hours
Design solids retention time (sludge age)
Reactor biomass concentration
Effluent substrate concentration
Efficiency
Substrate utilization rate
Food:microorganism ratio
Recycle ratio
Recycle flow rate
θc
X
S
E
U
F/M
r
QR
c.
d.
e.
f.
g.
h.
2.1
6288
4.7
98.4
1.43
1.45
1.08
0.010
days
mg VSS/L
mg COD/L
percent
g COD/g VSS/day
g COD/g VSS/day
m3/s
3
m /s
i.
Sludge wasting rate
QW
j.
Sludge production rate
QW/Q
P
k.
This plant has a high F/M ratio and short SRT. As such, it would be classified as a
high-rate aeration plant. Such a plant can work, but is not compatible with the
intended low level of maintenance planned for this facility. More bluntly, this plant
is a disaster waiting to happen!
l.
The plant needs to be redesigned as a low-rate AST plant. A design safety factor of
SF = 100 is more appropriate and would raise the SRT to 8.4 days, which is more
consistent with a low-maintenance, low-rate treatment plant. However, raising the SRT
has the perverse effect of raising the biomass concentration (X) which in turn raises the
F/M ratio (see Lecture 19, Equation 41). To lower the F/M ratio, we can increase the
tank volume, which increases tR, which in turn lowers F/M according to Equation 30.
Homework 7.xls Sheet1 11/22/2005
0.00025
2.8%
3.1
kg VSS/hr
8.4
8447
1.7
99.4
0.54
0.54
2.36
0.021
0.00016
1.9%
2.1
days
mg VSS/L
mg COD/L
percent
g COD/g VSS/day
g COD/g VSS/day
m3/s
m3/s
kg VSS/hr