Chlorine Contact Time / Virus

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Chlorine Contact Time / Virus Giardia Log
Removal Credit Calculations
The EPA Guidance Manual on the Surface water
Treatment Rule and Disinfection Profiling can be
downloaded at the following websites http//www.e
pa.gov/safewater/mdbp/lt1eswtr.html
http//www.epa.gov/ogwdw000/mdbp/pdf/profile/lt1
profiling.pdf The sample excel sheet for
calculating log removal credits can be downloaded
from the following site http//www.epa.gov/safewa
ter/mdbp/lt1/xls/profile_benchmark_calculator_shor
t.xls
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State (CDPH) Regulations on Surface Water
Treatment
The California Surface water Treatment Rule can
be found in CALIFORNIA CODE OF
REGULATIONS. TITLE 22. SOCIAL SECURITY DIVISION
4. ENVIRONMENTAL HEALTH CHAPTER 17. SURFACE WATER
TREATMENT 64652. Treatment Requirements and
Compliance Options. (a) Each supplier using an
approved surface water shall provide multibarrier
treatment that meets the requirements of this
chapter and reliably ensures at least (1) A
total of 99.9 percent reduction of Giardia
lambliacysts through filtration and disinfection
and (2) A total of 99.99 percent reduction or
viruses through filtration and disinfection.
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Data Required

• Disinfection Segments
• Type of Disinfection (Free Chlorine, Ozone etc)
• Peak Flow Rate (gpm)
• Residual Disinfectant Concentration (mg/L)
• Temperature (Celsius)
• pH (Surface water ranges from 6.5-8.5)
• Contact time Volume (Contact/Storage/Pressure
  tank volume and volume of pipe until the first
  user)

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Calculate CT (Concentration contact Time)
Total Detention Time (TDT) Lowest Operating
Volume / Peak Flow Lowest operating
volume should be taken during peak hour demand.
Contact Time TDT X Baffling Factor Baffling
Factor can found from the following table
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Baffling Factors Table
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Calculate CTcalc

• CTcalc (minutes-mg/L) C x T
• C Residual disinfectant concentration measured
  during
• peak hourly flow in mg/L.
• T Contact Time, measured in minutes, that the
  water is in
• contact with the disinfectant.

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Log Reduction

• Log reduction relates to the percentage of
  microorganisms physically removed or inactivated
  by a given process.
• 1-log reduction 90
• 2-log reduction 99
• 3-log reduction 99.9
• 4-log reduction 99.99
• Surface water treatment requires a 3-log removal
  for Giardia and 4-log removal for viruses.

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3-log CTreq for GIARDIA
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4-log CTreq for Viruses
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Inactivation Ratio

• Inactivation Ratio CTcalc / CTreq
• Actual Log Inactivation of Giardia 3 x (CTcalc
  / CTreq)
• Actual Log Inactivation of Viruses 4 x (CTcalc
  / CTreq)

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Questions and comments
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