ECt Presentation

7/31/2019 ECt Presentation

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The Application Of A Ct BasedDisinfection Policy To OptimiseDrinking Water Disinfection Costs


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Subject Areas That Will Be CoveredIn This PresentationDevelopment of a Ct based drinking water disinfection policy

Software tools developed to support the policy

The principle of maintaining the desired Effective Ct regardless

of the works flow

The control philosophy to make the maintenance of the desiredEffective Ct automatic.

The significant potential Opex cost savings


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Development of Ct Disinfection Policy

The Ct principle assumes that the Concentration (mg/l) multipliedby Time (min.) provides an indication of potential disinfectionefficiency.

Factors that can have a significant effect on that efficiency, for

chlorine, are:

pH - Conc. of the Disinfectant

Plug Flow Efficiency - Actual Retention Time


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pH

HOCl is the principal free chlorine speciesdisinfectant

At pH 6.0 (15C) - HOCl - 97.73 %

At pH 9.0 (15C) - HOCl - 4.13 %

Fluid Dynamics Of Vessels

Actual Retention Times (Q1) can be verydifferent than the Theoretical Retention Times


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pH and Fluid Dynamics accommodated in new Drinking WaterDisinfection Policy

Modified Ct known as Effective Ct (ECt)

Minimum ECt selected 20 mg.min/l at maximum flow

Minimum Effective Chlorine Conc. 0.3 mg/l

Giardia Inactivation Table developed from an extensive literaturesearch


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Development of Software Tools

Tailored Questionnaire/Calculation Form

Simplistic Fluid Dynamics Formula Derived

Checked by Expert in Fluid Dynamics

Satisfactory for application

Spreadsheet

Simplistic Fluid Dynamics Formula built in

HOCl Percentage Reference Module added

Conversion factors required prior to entry of certaindata


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Database developed that would:

Provide a user friendly GUI (graphical userinterface) input screens

Provide a full mathematics capability running in

the background

Allow the direct input of the details of any type ofprocess vessel or pipe/channel


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Database developed that would: (contd...)

Allow the results of tracer studies or CFD studiesto be input and thus bypass the built in

calculation.

Produce summary sheets identical to thoseproduced by the spreadsheet.

Hold records of all the benchmarking exercises thathad been carried out previously.


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Potential For Operational Cost Savings

ECt assessments normally carried out for maximum plantflows

Works flows are rarely at maximum

Thus ECt is frequently greater than assessed

Inverse relationship between flow and retention time

Direct relationship between flow and required chlorineresidual


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Assessment of potential savings:

a) Benchmark the works at maximum flow

b) Decide if the ECt is adequate/more thanadequate

c) Re-benchmark the works at maximumflow adjusting the chlorine residuals to obtain thedesired ECt

d) Benchmark the works at minimum flowplus 3 others

e) Graph the results to obtain themathematical relationship betweenthe flow and the required chlorine residual


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0.45

0.56

0.67

0.78

0.9

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

30

32

34

36

38

40

42

44

46

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60

62

ResidualChlorine-AsMeas

ured(mg/l)

Rate Of Flow (Mld)

Relationship Between Chlorine Residual (mg/l) And Rate Of Flow To Maintain An Effective Ct Of

Approx.30 mg.min/l

The slope of the line is 0.45/30 = 0.015

Thus for every Mld reduction in flow, through the plant, the

residual can be reduced by 0.015 m g/l


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Look up table useful only as a guide to demonstrate theprinciple

Requires fully automatic system

This can be achieved using:

a modified cascade control system

a modified secondary dose control loopwhere Mn removal is being practiced


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Chlorine Contact TankFlow Meter

PID

PID

Multiplier

Cl2 Dose Cl2 IT Cl2 IT

Schematic Diagram Of A Normal Cascade Control System


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PID

PID

Multiplier

Multiplier

Cl2 Dose Cl2 IT Cl2 IT

Schematic Diagram Of A Cascade Control System With

Automatic ECt Control


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Schematic Diagram Of A Secondary Chlorine Dose ControlSystem With Automatic ECt Control


PID

Multiplier

Multiplier

Cl2 Dose Cl2 IT


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The situation for a works carrying out superchlorination issome what more complex

The logic for every works needs to thought through carefully

Criteria that need to be assessed include:

Any manganese removal requirements

Any distribution chlorine residual requirements


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The Application Of A Ct Based

Disinfection Policy To Optimise Drinking

Water Disinfection Costs

Thank you for your attention

and are there any questions ?

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