module 3 effective contact time for verification of chemical disinfection mjoyce 23032012
TRANSCRIPT
-
8/13/2019 Module 3 Effective Contact Time for Verification of Chemical Disinfection MJoyce 23032012
1/13
MMODULEODULE 33
THE CONCEPT OFTHE CONCEPT OFEFFECTIVE CONTACT TIME (Ct)EFFECTIVE CONTACT TIME (Ct)
FOR THE VERIFICATION OFFOR THE VERIFICATION OFCHEMICAL DISINFECTIONCHEMICAL DISINFECTION
-
8/13/2019 Module 3 Effective Contact Time for Verification of Chemical Disinfection MJoyce 23032012
2/13
Chlorine dose
(mg/l)
Free chlorine residualconcentration (C)
Nominal contact t ime ()= tank volume/flowrate
Ct = mg/l x minutes = mg.min/l
where t is the Effective Contact Time
How is t (effective contact time) defined?
WHO recommend Ct of 15WHO recommend Ct of 15 mg.min/l at pH
-
8/13/2019 Module 3 Effective Contact Time for Verification of Chemical Disinfection MJoyce 23032012
3/13
tt1010
= t= t
1010
xx == DDff
DfDf (t(t1010//)) is the measure ofis the measure ofeffective mixing within aeffective mixing within a
contact tankcontact tank configurationconfiguration
Time
Tracerconcentration
t10
t0
tp
t50
t90
t0= time to initial detection of tracer
t10 = time for 10% of tracer to pass
tp= time to m aximum tracer concentration
t50 = time for 50% or tracer to pass
= hydraulic retention time
t90 = time for 90% of tracer to pass
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.5 1 1.5 2 2.5 3
t/
C/C0
t10/= 0.436
Allow for contact tank hydraulics
Identify tx value time for fastestx% of flow to pass through tank usually with tracer tests
Manual uses USEPA value t10 i.e. Time for which 90% of the flow has longer residence
DEFININGDEFININGEFFECTIVE CONTACT TIME (t)EFFECTIVE CONTACT TIME (t)
-
8/13/2019 Module 3 Effective Contact Time for Verification of Chemical Disinfection MJoyce 23032012
4/13
ConditionCondition DescriptionDescription DDff
Unbaffled None, agitated basin, very low
length to width ratio, high inlet and
outlet flow velocities.
0.1
Poor Single or multiple unbaffled inlets
and outlets, no intra-basin baffles.
0.3
Average Baffled inlet or outlet with some
intra-basin baffles.
0.5
Superior Perforated inlet baffle, serpentine
or perforated intra-basin baffles,
outlet weir or perforated launders.
0.7
Effective contact time (minutes)= Tank volume (m3) x 60 x Df / flow (m3/h)
For pipe, assume Df = 1
EFFECTIVE CONTACT TIMEEFFECTIVE CONTACT TIME
-
8/13/2019 Module 3 Effective Contact Time for Verification of Chemical Disinfection MJoyce 23032012
5/13
POORPOOR AVERAGE BAFFLINGAVERAGE BAFFLING
BAFFLING ARRANGEMENTSBAFFLING ARRANGEMENTS
Df = 0.3 Df = 0.5
-
8/13/2019 Module 3 Effective Contact Time for Verification of Chemical Disinfection MJoyce 23032012
6/13
SUPERIOR BAFFLINGSUPERIOR BAFFLING
Df = 0.7
BAFFLING ARRANGEMENTSBAFFLING ARRANGEMENTS
-
8/13/2019 Module 3 Effective Contact Time for Verification of Chemical Disinfection MJoyce 23032012
7/13
Tuam RWSS
Luimnagh Water Treatment Works
Baffling within Clear Water Tank
Df = 0.5 - 0.6
-
8/13/2019 Module 3 Effective Contact Time for Verification of Chemical Disinfection MJoyce 23032012
8/13
Chlorine demand - due to reaction of Cl2 withcontaminants Instantaneous reaction with ammonia
Gradual reaction with natural organic matter in water
Irrespective of type of chlorine used, chlorination
always produces mix of free chlorine constituents:
HOCl (Hypochlorous Acid) Strong Disinfectant
OCl - (Hypochlorite ion) - Weaker Disinfectant
CHLORINE CONCENTRATION IN WATERCHLORINE CONCENTRATION IN WATER
-
8/13/2019 Module 3 Effective Contact Time for Verification of Chemical Disinfection MJoyce 23032012
9/13
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
5 6 7 8 9 10
pH
Fraction(m
olar)
5oC
15oC35oC
5oC
15oC
35oC
HOCl OCl-
DEPENDENCY OFDEPENDENCY OF HOClHOCl toto OClOCl--EQUILIBRIUM ON pH AND TEMPEQUILIBRIUM ON pH AND TEMP
-
8/13/2019 Module 3 Effective Contact Time for Verification of Chemical Disinfection MJoyce 23032012
10/13
New Disinfection Manual recommends that Ct for chlorination be
adjusted as follows for variations in pH and temperatureAdjusted Ct = 15mg.min/litre x pHdx Tempd
pH range pHd
factor
8.26 8.50 1.4
8.01 8.25 1.3
7.76 8.00 1.2
7.50 7.75 1.1
-
8/13/2019 Module 3 Effective Contact Time for Verification of Chemical Disinfection MJoyce 23032012
11/13
RECOMMENDED Ct VALUES FOR99% (2-log) INACTIVATION (Chlorine)
Temperature
(C)pH Ct (mg.min L-1)
Bacteria 1-2 7 0.08
1-2 8.5 3.3
Viruses
-
8/13/2019 Module 3 Effective Contact Time for Verification of Chemical Disinfection MJoyce 23032012
12/13
Ct FOR 2 LOG INACTIVATION(USEPA/WHO) USING ClO2
Temperature 1C 5C 10C 15C 20CCryptosporidium 1220 858 553 357 232
Giardia 17 15 13
Viruses 5.6 4.2 2.8
Bacterial inactivation needs lower Ct than viruses (e.g. < 1 mg.min/l)
Generally at least as good as chlorine, probably better at higher pH
-
8/13/2019 Module 3 Effective Contact Time for Verification of Chemical Disinfection MJoyce 23032012
13/13
DISINFECTION PERFORMANCEOF OZONE CRYPTOSPORIDIUM
Ct values (Ct values (mg.min/lmg.min/l) for inactivation of) for inactivation of CryptosporidiumCryptosporidium
Log
Inactivation
Temperature, oC
1 5 10 15 20
0.5 12 7.9 4.9 3.1 2.0
1.0 (90%) 24 16 9.9 6.2 3.9
2.0 (99%) 48 32 20 12 7.8
3.0 (99.9%) 72 47 30 19 12