Post on 05-Apr-2018
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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Development of Ct Disinfection Policy
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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Development of Ct Disinfection Policy
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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Development of Software Tools
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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Development of Software Tools
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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Potential For Operational Cost Savings
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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Potential For Operational Cost Savings
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
48
50
52
54
56
58
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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Potential For Operational Cost Savings
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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Potential For Operational Cost Savings
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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Potential For Operational Cost Savings
Chlorine Contact TankFlow Meter
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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Potential For Operational Cost Savings
Schematic Diagram Of A Secondary Chlorine Dose ControlSystem With Automatic ECt Control
Chlorine Contact TankFlow Meter
PID
Multiplier
Multiplier
Cl2 Dose Cl2 IT
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Potential For Operational Cost Savings
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 ?