CIVE 3610
–
Water Supply and Treatment
Fall Semester 2004
Homework #6
–
Solution
1)
A rectangular clarifier (L=2W) is to be designed for a 6 MGD treatment
facility. The overflow rate (surface loading rate) is 600 gal/day ft
2
, and the detention
time is 5 hour
s.
a. Determine the tank dimensions (L, W, and depth) in 5
–
ft increments.
b. Depth is not used for performance (removal) calculations. However, a
minimum clarifier depth (side water depth) is often specified. Why?
Solution
Given:
Q = 6*
10
6
gal/day
Vo =600 gal/day ft
2
θ = 5 hrs
V = Q * θ = [6 *10
6
gal/ day * ( 1 ft
3
/ 7.48 gal)]* [5 hrs * (1day/24hrs)] = 167,112 ft
3
Vo = Q/As
As = Q/Vo = (6*10
6
gal/day) / (600 gal/day ft
2
) =
10, 000 ft
2
a. Determine the
tank dimensions (L, W, and depth) in 5
–
ft increments.
As = 2W * W
10, 000 ft
2
= 2W
2
W = 70.7 ft →
75 ft
L = 2W =
150 ft
V= As * depth = 167, 112 ft
3
Depth = 167, 112 ft
3
/ (75*150)
= 14.8 ft
Depth =
15ft
2 W
W
b. Depth is not used for performance (removal) calculations. However, a minimum
clarifier depth (side water depth) is often specified. Why?
Minimum clarifier depth is specified to a minimum of 10 ft in orde
r to
slow down the
horizontal velocity
of the water through the tank to avoid scouring of sludge.
2) The operator from Midtown Water treatment Facility calls you because their
filter run lengths are too short. The filters at Midtown are traditional rapid
sand
filters with gravel underdrain systems. The raw water has an influent turbidity
ranging from 3 to 10, 000 ntu. They are using 15 mg/L FeCl
3
for coagulation.
a. Draw an ideal head loss curve.
b. What do you think is happening with their filters?
c. P
rovide two suggestions to the operator for improving performance of the existing
system?
d. If the midtown plant were updated in the near future, what design changes would
you suggest?
Solution
a. Draw an ideal head loss curve.
Optimum curve would be
where the
maximum head loss corresponds with the effluent
turbidity standard
.
b. What do you think is happening with their filters?
The top of the filters are being
clogged with the turbidity
, since they are sand and only
have minimal pore spaces.
c
. Provide two suggestions to the operator for improving performance of the existing
system?
1. Increase
coagulant dose
to create floc so that influent turbidity to filters would
be lowered.
2. Increase
θ
in sedimentation tank to increase turbidity removal prior to filtration.
3. Reduce
surface loading rates
.
4. Reduce
influent turbidity
to the plant by having a reservoir for influent flow.
d. If the midtown plant were updated in the near future, what
design changes would
you suggest?
1. Change to
multimedia
filters
2. Add an
extra
filter
3. Add
reservoir
to improve quality of influent flow
3) An existing chlorine contact basin receiving a flow of 1 MGD has a volume of
100,000 gallons and is poorly
baffled.
Given: 99.9% (3

log) inactivation of
Giardia lamblia
Disinfectant used is chlorine dioxide (ClO
2
)
pH = 8; Temp. =5
o
C; Required residual = 0.2 mg/L
CT requirement = 54
1
st
customer
–
water travels 10 minutes from
treatment facility
Last customer
–
water travels 60 minutes from treatment facility
a. Is the tank volume big enough to meet CT requirements? (show calculations)
b. How could you modify the existing system to meet stringent CT requirements
(give two meth
ods)?
c. If the operator decided to use chlorine (Cl
2
) gas, would the existing design meet
CT requirements? Why or why not?
Solution
Q = 1*10
6
gal/day
V=100, 000 gal
θ = V/Q = (100,000 gal) / (1*10
6
gal/ day) = 0.10 days
= 0.10 days * (24 hrs/1 day) * (60 min / 1 hr)
=
144 min
A
ctual T = (0.3)*144 min = 43.2 min (0.3 for poor baffling conditions)
To the 1
st
house
, T = 43.2 +10 =
53.2 min
C = 0.2 mg/L
a. Is the tank volume big enough to meet CT requirements? (Show calculations)
C*T = 53.2 min * 0.2 mg/L =
10.64
(mg/L)*min
10.64<<< 54; Tank not big enough.
b. How could you modify the existing system to meet stringent CT requirements
(give two methods)?
1. Higher dose of
residual
2. Better
disinfectant
3. Add
baffling
c. If the operator decided to use chlorine (Cl
2
) ga
s, would the existing design meet
CT requirements? Why or why not?
No.
ClO
2
works better than Cl
2
gas. ClO
2
with the current system isn’t enough.
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