.
CT
Calculation
“Required by Law”
Math for Water Technology
MTH 082
(pg. 468)
C.T calculation
• “C” is the concentration of chlorine,
• “T” is the amount of time the water has
been in contact with chlorine before the
first user.
• Calculated daily and compared against
the EPA CT “Required” Table
EPA rules states - “Measurements must be taken on the same
day of the week, every week, for one year (52 measurements),
during peak hourly flow for that day. Data can be measured
manually or with on-line instrumentation.”
.
CT
Calculation Problems
• To calculate the contact time T, the results
of a recent tracer study must be used!!
• The peak flow must be measured from the
effluent side of the clearwell or contact
chamber, NOT the flow through the plant.
• The chlorine residual, pH, and temperature
values used to calculate CT must be measured
at the effluent side of the clearwell or contact
chamber, or before the first user.
CT Calculation Problems
• When using the EPA CT tables, make sure you
round down for temperature, and
round up for pH.
• log-reduction must equal 3.0 for Giardia
For surface water systems without
filtration, the Surface Water
Treatment Rule requires public water
systems to?
.
li.
.
ai
nt
ai
n
M
ou
sl
tin
u
on
pu
b
s.
..
y
C
a
C
ai
nt
ai
n
M
id
e
co
a
gu
l..
.
...
25% 25% 25% 25%
Pr
ov
1. Provide coagulation and
filtration
2. Maintain a C X T value
above the minimum value
3. Continuously sample for
total coliforms
4. Maintain public
notification
In the application of chlorine for
disinfection, which of the following is
not normally an operational
consideration?
...
th
es
e
pH
of
e
on
is
s
D
N
on
ol
ve
d
ta
c
ox
tt
yg
...
im
e
ng
ix
i
C
Mixing
Contact time
Dissolved oxygen
pH
None of these answers
are correct
M
1.
2.
3.
4.
5.
20% 20% 20% 20% 20%
The chlorine residual, pH, and
temperature values used to
calculate CT must be measured
at?
o.
..
..
of
ny
A
be
fo
re
th
e
th
e
ab
fir
.
...
e
si
d
nt
flu
e
ef
flu
e
nt
si
d
e
...
25% 25% 25% 25%
ef
1. effluent side of the
clearwell
2. effluent side of the
contact chamber
3. before the first user
4. Any of the above would
be ok
The C.T calculation must be
calculated
y
da
ee
k
a
w
O
nc
e
a
O
nc
e
a
e
nc
O
nc
e
a
m
on
ye
th
ar
Once a year
Once a month
Once a week
Once a day
O
1.
2.
3.
4.
25% 25% 25% 25%
The peak flow must be
measured as total flow through
the plant?
ls
e
50%
Fa
50%
Tr
ue
1. True
2. False
The final C.T calculation has what
units?
tim
e
gm
in
/
L
25% 25%
m
s/
da
y
25%
Lb
g/
L
Mg/L
Lbs/day
mg-min/L
time
M
1.
2.
3.
4.
25%
What is the acceptable log reduction
for Giardia?
25%
25%
4
25%
3
25%
2
1
2
3
4
1
1.
2.
3.
4.
What is the acceptable log reduction
for viruses?
25%
25%
4
25%
3
25%
2
1
2
3
4
1
1.
2.
3.
4.
Four log removal is?
99
.9
9
%
25%
%
.9
0
99
.0
0
99
.0
0
25%
%
25%
%
90.00 %
99.00 %
99.90 %
99.99 %
90
1.
2.
3.
4.
25%
If your temperature is 8 degrees with
a pH of 7.2, you should use the ____
degree page and the _____ pH table
(please look at your tables).
,7
25%
10
7.
5
25%
5,
,8
.5
25%
10
19
5, 19
10, 8.5
5, 7.5
10, 7
5,
1.
2.
3.
4.
25%
Disinfection CTs have been
established for?
1. Free chlorine
2. Chloramine
3. Chlorine dioxide
4. Ozone
5. All of the above
ov
e
ne
ll
o
ft
he
ab
zo
O
A
xi
de
di
o
e
C
hl
or
in
e
hl
or
am
in
C
Fr
ee
ch
lo
r in
e
20% 20% 20% 20% 20%
Disinfection C.T are
impacted by which of the
following?
ov
e
H
ab
rp
ft
he
at
e
A
ll
o
W
em
pe
ra
tu
re
(m
W
at
e
rT
tT
ta
c
on
C
hl
or
in
e
re
s
id
u
al
im
e
co
...
in
)
20% 20% 20% 20% 20%
C
1. Chlorine residual
concentrations (mg/L)
2. Contact Time (min)
3. Water Temperature
4. Water pH
5. All of the above
C.T Calculation
T =contact time= Detention Time
“the length of time water is retained in a
vessel or basin ”
The units of T in the final CT
calculation are ?
25% 25%
25%
s
ay
D
rs
ou
H
te
s
in
u
M
co
nd
s
Seconds
Minutes
Hours
Days
Se
1.
2.
3.
4.
25%
Determining T in CT?
Detention time (DT) = volume of tank =
flow rate
MG
(1440min)
MGD
(1day)
Want T value to be in minutes
T = volume X Baffle Factor (table)
flow rate
Baffle Factor?
Poor circulation
Baffling efficiency = 5-10%
in
out
Poor circulation
Baffling efficiency = 5-10%
in
out
No circulation
Baffling efficiency = 0%
in
out
Baffle Factor?
No circulation
Baffling efficiency = 0%
in
Good circulation
Baffling efficiency = 30-50%
in
in
out
out
Superior circulation
Baffling efficiency = 70%
Baffle Factor?
in
out
Perfect circulation
Baffling efficiency = 100%
Poor circulation/hydropneumatic tank
Baffling efficiency = 10%
in
out
No circulation/Bladder type pressure tank
Baffling efficiency = 0%
Baffle Factor?
Unbaffled Tank
0.1
Poor Baffling
0.3
Average Baffling
0.5
Superior baffling
0.7
Pipe Flow
1.0
“How well the chlorine is in contact with the water”
What is the average detention
time in a basin that is 1240 ft3
when the flow rate is 4.1 MGD?
3.3 min
.03 min
303 min
435,512 min
51
2
5,
43
30
3
m
in
m
in
m
in
3
.0
3
m
in
25% 25% 25% 25%
3.
1.
2.
3.
4.
What is the average detention
time in a basin given the
following: diameter = 80' depth =
12.2' flow = 5 MGD
10
5
m
in
25%
m
in
4
1
10
2
25%
m
in
25%
m
in
25%
14
132 min
101 min
144 min
105 min
13
1.
2.
3.
4.
Determine T at the effluent side
of a superior baffled contact
chamber basin that has the
following: diameter = 30' depth =
15' flow = 700 gpm
25%
m
in
.
25%
86
.1
m
in
.
.1
m
in
25%
79
.8
m
in
.
25%
75
65.8 min.
79.1 min
75 min.
86.1 min.
65
1.
2.
3.
4.
Determine T at the effluent side of a superior baffled
contact chamber basin that has the following: Length
of = 30ft, width of 20 ft, depth = 15' flow = 700 gpm?
• Given: tank= L X W X D , Flow rate 700 gpm, BF=0.7
T= DT X BF
• Formula:T= volume of tank/flow rate X BF
Vt= L X W X D
• Solve: Vt=30 ft X 20 ft X 15 ft= 9000 ft3 20 in
30 ft
700 gpm
out
9000 ft3 (7.48 gal/1ft3)= 67320 gal ft
DT=VT/FR X BF
15 ft
Time = 67,320 gal/700gpm X 0.7
Time= 96.17 min X 0.7
Time= 67.32 min
5
.8
12
m
in
9
in
m
m
in
67
.3
2
m
in
7
.1
96.17 min
67.32 min
12.85 min
9 min
25% 25% 25% 25%
96
1.
2.
3.
4.
DRAW:
Disinfection C•T Requirements
Chlorine residual concentrations
(mg/L)
Contact Time (min)
Water Temperature
Water pH
Disinfection power= [residual chlorine] • time o
its contact
Disinfection C•T Calculation
C•T= [disinfection concentration mg/L] • contact
time (min)
C•T units= mg/L• min or mg •min/L
Required C•T
CTrequired is # established by EPA to provide
log inactivation. Based on Giardia cysts. Look
up in charts
Disinfection C•T Calculation
C•T actual =[Residual chlorine]•hydraulic
detention time•baffle factor (table)
Hydraulic detention time = volume/flow rate
C•Tact/ CTreq= ratio must be > or = to 1.
C•Tact/ CTreq > 1.0
“You want to make sure that you are actually
meeting the necessary requirements”
Disinfection C•T Rules
1.Chlorine residuals used for C•T
calculations are measured after
contact, but before first customer.
2. Contact times are determined by
calculating the hydraulic detention
time (HDT) as water flows through
pipes and tanks. Based on highest
flow of day.
Disinfection C•T Rules
3. Water flowing through pipes
provides contact times that are
equal to the calculated HDT.
Circular or rectangular tanks used
for contact time are given only
partial credit due to short
circuiting.