Module 3 Effective Contact Time for Verification of Chemical Disinfection MJoyce 23032012

8/13/2019 Module 3 Effective Contact Time for Verification of Chemical Disinfection MJoyce 23032012

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MMODULEODULE 33

THE CONCEPT OFTHE CONCEPT OFEFFECTIVE CONTACT TIME (Ct)EFFECTIVE CONTACT TIME (Ct)

FOR THE VERIFICATION OFFOR THE VERIFICATION OFCHEMICAL DISINFECTIONCHEMICAL DISINFECTION


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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


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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)


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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


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POORPOOR AVERAGE BAFFLINGAVERAGE BAFFLING

BAFFLING ARRANGEMENTSBAFFLING ARRANGEMENTS

Df = 0.3 Df = 0.5


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SUPERIOR BAFFLINGSUPERIOR BAFFLING

Df = 0.7

BAFFLING ARRANGEMENTSBAFFLING ARRANGEMENTS


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Tuam RWSS

Luimnagh Water Treatment Works

Baffling within Clear Water Tank

Df = 0.5 - 0.6


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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


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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


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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


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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


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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


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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

Module 3 Effective Contact Time for Verification of Chemical Disinfection MJoyce 23032012

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