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Chlorine Contact Time / Virus & GiardiaLog Removal Credit Calculations

The EPA Guidance Manual on the Surface water TreatmentRule and Disinfection Profiling can be downloaded at the

following websites

http://www.epa.gov/safewater/mdbp/lt1eswtr.html

http://www.epa.gov/ogwdw000/mdbp/pdf/profile/lt1profiling.pdf

The sample excel sheet for calculating log removal credits

can be downloaded from the following site

http://www.epa.gov/safewater/mdbp/lt1/xls/profile_benchmark_calculator_short.xls 1

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State (CDPH) Regulations on SurfaceWater Treatment

The California Surface water Treatment Rule can be foundin

CALIFORNIA CODE OF REGULATIONS.TITLE 22. SOCIAL SECURITY

DIVISION 4. ENVIRONMENTAL HEALTHCHAPTER 17. SURFACE WATER TREATMENT

64652. Treatment Requirements and Compliance Options.

(a) Each supplier using an approved surface water shall provide multibarrier treatmentthat meets the requirements of this chapter and reliably ensures at least:

(1) A total of 99.9 percent reduction of Giardia lambliacysts through filtration anddisinfection; 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 tankvolume and volume of pipe until the first user)

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Calculate CT (Concentrationcontact Time)

Total Detention Time (TDT) = Lowest Operating Volume /Peak Flow

Lowest operating volume should be taken during peak hourdemand.

Contact Time = TDT X Baffling Factor

Baffling Factor can found from the following table

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Calculate CTcalc

CTcalc (minutes-mg/L) = C x T

C = Residual disinfectant concentration measuredduring

peak hourly flow in mg/L.

T = Contact Time, measured in minutes, that the wateris in

contact with the disinfectant.

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Log ReductionLog reduction relates to the percentage of

microorganisms physically removed or inactivated by agiven 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 forGiardia 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 ofGiardia = 3 x (CTcalc / CTreq)

Actual Log Inactivation of Viruses = 4 x (CTcalc / CTreq)

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Questions and comments

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