UV Disinfection Drinking Water

Drinking Water Treatment with UV Irradiation

Ultraviolet (UV) rays are part of the light that comes from the sun. The UV spectrum is higher in frequency than visible light and lower in frequency compared to  x-rays.  This also means that the UV spectrum has a larger wavelength than x-rays and a smaller wavelength than visible light and the order of energy, from low to high, is visible light, UV, than x-rays. As a water treatment technique, UV is known to be an effective disinfectant due to its strong germicidal (inactivating) ability. UV disinfects water containing bacteria and viruses and can be effective against protozoans like,  Giardia lamblia cysts or Cryptosporidium oocysts. UV has been used commercially for many years in the pharmaceutical, cosmetic, beverage, and electronics industries, especially in Europe. In the US, it was used for drinking water disinfection in the early 1900s but was abandoned due to high operating costs, unreliable equipment, and the expanding popularity of disinfection by chlorination. 

Because of safety issues associated with the reliance of chlorination and improvement in the UV technology, UV has experienced increased acceptance in both municipal and household systems. There are few large-scale UV water treatment plants in the United States although there are more than 2,000 such plants in Europe.  There are two classes of disinfection systems certified and classified by the NSF under Standard 55 – Class A and Class B Units.

Class A — These ultraviolet water treatment systems must have an ‘intensity & saturation’ rating of at least 40,000 uwsec/cm2 and possess designs that will allow them to disinfect and/or remove microorganisms from contaminated water. Affected contaminants should include bacteria and viruses  
"Class A point-of-entry and point-of-use systems covered by this Standard are designed to inactivate and/or remove microorganisms, including bacteria, viruses, Cryptosporidium oocyst and Giardia cysts, from contaminated water. Systems covered by this standard are not intended for the treatment of water that has obvious contamination or intentional source such as raw sewage, nor are systems intended to convert wastewater to drinking water. The systems are intended to be installed on visually clear water."

Class B — These ultraviolet water treatment systems must have an ‘intensity & saturation’ rating of at least 16,000 uw-sec/cm2 and possess designs that will allow them to provide supplemental bactericidal treatment of water already deemed ‘safe’. i.e., no elevated levels of E. coli. or a standard plate count of less than 500 colonies per 1 ml.  NSF Standard 55 "Class B" UV systems are designed to operate at a minimum dosage and are intended to "reduce normally occurring non-pathogenic or nuisance microorganisms only." The "Class B" or similar non-rated UV systems are not intended for the disinfection of "microbiologically unsafe water."

Therefore, the type of unit depends on your situation, source of water, and your water quality. Transmitted UV light dosage is affected by water clarity. Water treatment devices are dependent on the quality of the raw water. When turbidity is 5 NTU or greater and/or total suspended solids are greater than 10 ppm, pre-filtration of the water is highly recommended.  Normally, it is advisable to install a 5 to 20 micron filter prior to a UV disinfection system.
Principles of UV Disinfection 

UV radiation has three wavelength zones: UV-A, UV-B, and UV-C, and it is this last region, the shortwave UV-C, that has germicidal properties for disinfection.  A low-pressure mercury arc lamp resembling a fluorescent lamp produces the UV light in the range of 254 manometers (nm).   A nm is one billionth of a meter (10^-9 meter). These lamps contain elemental mercury and an inert gas, such as argon, in a UV-transmitting tube, usually quartz. Traditionally, most mercury arc UV lamps have been the so-called "low pressure" type, because they operate at relatively low partial pressure of mercury, low overall vapor pressure (about 2 mbar), low external temperature (50-100oC) and low power. These lamps emit nearly monochromatic UV radiation at a wavelength of 254 nm, which is in the optimum range for UV energy absorption by nucleic acids (about 240-280 nm).

In recent years medium pressure UV lamps that operate at much higher pressures, temperatures and power levels and emit a broad spectrum of higher UV energy between 200 and 320 nm have become commercially available. However, for UV disinfection of drinking water at the household level, the low-pressure lamps and systems are entirely adequate and even preferred to medium pressure lamps and systems. This is because they operate at lower power, lower temperature, and lower cost while being highly effective in disinfecting more than enough water for daily household use. An essential requirement for UV disinfection with lamp systems is an available and reliable source of electricity. While the power requirements of low-pressure mercury UV lamp disinfection systems are modest, they are essential for lamp operation to disinfect water. Since most microorganisms are affected by radiation around 260 nm, UV radiation is in the appropriate range for germicidal activity. There are UV lamps that produce radiation in the range of 185 nm that are effective on microorganisms and will also reduce the total organic carbon (TOC) content of the water.  For typical UV system, approximately 95 percent of the radiation passes through a quartz glass sleeve and into the untreated water.  The water is flowing as a thin film over the lamp.  The glass sleeve is designed to keep the lamp at an ideal temperature of approximately 104 °F. 
UV Radiation (How it Works)
UV radiation affects microorganisms by altering the DNA in the cells and impeding reproduction. UV treatment does not remove organisms from the water, it merely inactivates them. The effectiveness of this process is related to exposure time and lamp intensity as well as general water quality parameters.  The exposure time is reported as "microwatt-seconds per square centimeter" (uwatt-sec/cm^2), and the U.S. Department of Health and Human Services has established a minimum exposure of 16,000 µwatt-sec/cm^2 for UV disinfection systems.  Most manufacturers provide a lamp intensity of 30,000-50,000µwatt-sec/cm^2.  In general, coliform bacteria, for example, are destroyed at 7,000 µwatt-sec/cm^2.  Since lamp intensity decreases over time with use, lamp replacement and proper pretreatment are key to the success of  UV disinfection. In addition, UV systems should be equipped with a warning device to alert the owner when lamp intensity falls below the germicidal range.   The following gives the irradiation time required to inactivate completely various microorganisms under 30,000 µwatt-sec/cm^2 dose of UV 254 nm

Used alone, UV radiation does not improve the taste, odor, or clarity of water. UV light is a very effective disinfectant, although the disinfection can only occur inside the unit. There is no residual disinfection in the water to inactivate bacteria that may survive or may be introduced after the water passes by the light source. The percentage of microorganisms destroyed depends on the intensity of the UV light,  the contact time, raw water quality, and proper maintenance of the equipment.  If material builds up on the glass sleeve or the particle load is high, the light intensity and the effectiveness of treatment are reduced.  At sufficiently high doses, all waterborne enteric pathogens are inactivated by UV radiation. The general order of microbial resistance (from least to most) and corresponding UV doses for extensive (>99.9%) inactivation are: vegetative bacteria and the protozoan parasites Cryptosporidium parvum and Giardia lamblia at low doses (1-10 mJ/cm2) and enteric viruses and bacterial spores at high doses (30-150 mJ/cm2). Most low-pressure mercury lamp UV disinfection systems can readily achieve UV radiation doses of 50-150 mJ/cm2 in high quality water, and therefore efficiently disinfect essentially all waterborne pathogens. However, dissolved organic matter, such as natural organic matter, certain inorganic solutes, such as iron, sulfites and nitrites, and suspended matter (particulates or turbidity) will absorb UV radiation or shield microbes from UV radiation, resulting in lower delivered UV doses and reduced microbial disinfection. Another concern about disinfecting microbes with lower doses of UV radiation is the ability of bacteria and other cellular microbes to repair UV-induced damage and restore infectivity, a phenomenon known as reactivation. 

UV inactivates microbes primarily by chemically altering nucleic acids. However, the UV-induced chemical lesions can be repaired by cellular enzymatic mechanisms, some of which are independent of light (dark repair) and others of which require visible light (photorepair or photoreactivation). Therefore, achieving optimum UV disinfection of water requires delivering a sufficient UV dose to induce greater levels of nucleic acid damage and thereby overcome or overwhelm DNA repair mechanisms.

Table 1. Estimated Irradiation Time to 
Inactivate Microorganisms at a 
Dosage of 30,000 µwatt-sec/cm^2 of UV 254 nm


Name 100% lethal Dosage
Name 100% lethal Dosage
Dysentery bacilli 0.15 Micrococcus Candidus 0.4 ¨C 1.53
Leptospira SPP 0.2 Salmonella Paratyphi 0.41
Legionella Pneumophila 0.2 Mycobacterium Tuberculosis 0.41
Corynebacterium Diphtheriae 0.25 Streptococcus Haemolyticus 0.45
Shigella Dysenteriae 0.28 Salmonella Enteritidis 0.51
Bacillus Anthracis 0.3 Salmonella Typhimurium 0.53
Clostridium Tetani 0.33 Vibrio Cholerae 0.64
Escherichia coli 0.36 Clostridium Tetani 0.8
Pseudomonas Aeruginosa 0.37 Staphylococcus Albus 1.23
Coxsackie Virus A9 0.08 Echovirus 1 0.73
Adenovirus 3 0.1 Hepatitis B Virus 0.73
Bacteiophage 0.2 Echovirus 11 0.75
Influenza 0.23 Poliovirus 1 0.8
Rotavirus SA 11 0.52 Tobacco Mosaic 16
Mold Spores
Mucor Mucedo 0.23 ¨C 4.67 Penicillium Roqueforti 0.87 - 2.93
Oospara Lactis 0.33 Penicillium Chrysogenum 2.0 ¨C 3.33
Aspergillus Amstelodami 0.73 ¨C 8.80 Aspergillus Niger 6.67
Penicillium Digitatum 0.87 Manure Fungi 8
Chlorella Vulgaris 0.93 Protozoa 4 - 6.70
Green Algae 1.22 Paramecium 7.3
Nematode Eggs 3.4 Blue-Green Algae 10 ¨C 40
Inactivation Doses for Giardia and Cryptosporidium
UV dose is a product of UV light intensity and exposure time in seconds (IT), stated in units; mWs/cm2 or mJ/cm2. IT is analogous to the chemical dose or CT (concentration x time). Microbes show a range of sensitivities to UV as shown by the UV data. Cryptosporidium and Giardia are more sensitive to UV than bacteria and viruses are more resistant than bacteria. Similar results have been obtained using low-pressure, medium-pressure and pulsed UV irradiation- Look for a Class A UV disinfection system. UV dose required for a 4log inactivation of selected waterborne pathogens.

Table 2 .
UV Dose 4 log Inactivation

Pathogen UV dose mJcm/2
4log inactivation
Cryptosporidium parvum oocysts <10
Giardia lamblia cysts <10
Vibrio cholerae 2.9
Salmonella typhi 8.2
Shigella sonnei 8.2
Hepatitis A virus 30
Poliovirus Type 1 30
Rotavirus SA11 36
 Source: http://www.trojanuvmax.com

UV Irradiation Pretreatment
Either sediment filtration or activated carbon filtration should take place before water passes through the unit. Particulate matter, color, and turbidity affect the transmission of light to the microorganisms and must be removed for successful disinfection. 

Table 3. Recommended maximum contaminant 
levels in water entering a UV treatment device.

Turbidity 5 FTU or 5 NTU
Suspended solids
(5 to 10 micron prefiltration recommended)
< 10 mg/L
Color None
Iron < 0.3 mg/L
Manganese < 0.05 mg/L
pH 6.5-9.5
UV is often the last device in a treatment train (a series of treatment devices), following reverse osmosis, water softening, or filtration. The UV unit should be located as close as possible to the point-of-use since any part of the plumbing system could be contaminated with bacteria. It is recommended that the entire plumbing system be disinfected with chlorine prior to initial use of a UV system. 
Types of UV Disinfection Devices

The typical UV treatment device consists, of a cylindrical chamber housing the UV bulb along its central axis. A quartz glass sleeve encases the bulb; water flow is parallel to the bulb, which requires electrical power. A flow control device prevents the water from passing too quickly past the bulb, assuring appropriate radiation contact time with the flowing water. It has been reported that turbulent (agitated) water flow provides more complete exposure of the organism to UV radiation. 

A UV system housing should be of stainless steel to protect any electronic parts from corrosion. To assure they will be contaminant-free, all welds in the system should be plasma-fused and purged with argon gas. The major differences in UV treatment units are in capacity and optional features. Some are equipped with UV emission detectors that warn the user when the unit needs cleaning or when the light source is failing. This feature is extremely important to assurance of a safe water supply. A detector that emits a sound or shuts off the water flow is preferable to a warning light, especially if the system might be located where a warning light would not be noticed immediately. 

Maintenance of a UV System

Since UV radiation must reach the bacteria to inactivate them, the housing for the light source must be kept clean. Commercial products are available for rinsing the unit to remove any film on the light source. An overnight cleaning with a solution of 0.15 percent sodium hydrosulfite or citric acid effectively removes such films. Some units have wipers to aid the cleaning process. 

UV systems are designed for continuous operation and should be shut down only if treatment is not needed for several days. A few minutes for lamp warm-up is needed before the system is used again following shut-down. In addition, the plumbing system of the house should be thoroughly flushed following a period of no use. Whenever the system is serviced, the entire plumbing system should be disinfected with a chemical such as chlorine prior to relying on the UV system for disinfection. 

UV lights gradually lose effectiveness with use, the lamp should be cleaned on a regular basis and replaced at least once a year. It is not uncommon for a new lamp to lose 20 percent of its intensity within the first 100 hours of operation, although that level is maintained for the next several thousand hours. As stated previously, units equipped with properly calibrated UV emission detectors alert the owner when the light intensity falls below a certain level. 

The treated water should be monitored for coliform and heterotrophic bacteria on a monthly basis for at least the first 6 months of the device’s use. If these organisms are present in the treated water, the lamp intensity should be checked, and the entire plumbing system should be disinfected with a chemical such as chlorine.

Quick Facts about UV Water Treatment

1. UV disinfection does not add chemicals to the water. 
2. UV is effective against bacteria and viruses; and may be effective against Giardia lamblia or Cryptosporidium if the system custom designed to meet these disinfection requirements.
3. UV disinfection has no residual disinfection. 
4. Minimum lamp exposure of 16,000 µwatt-sec / cm^2 . 
5. UV often last device in a treatment train of water treatment devices.
6. UV device should have audible UV emission detector to notify user when lamp intensity is inadequate. 
7. Regular maintenance and lamp replacement is essential.

Capacity of UV Disinfection Systems

UV is an in-line, point-of-entry system that treats all the water used in the house. The capacities range from 0.5 gallons per minute (gpm) to several hundred gpm.  Since bacteria may be shielded by particles in the water, pretreatment to remove turbidity may be required. There is also a limit to the number of bacteria that can be treated. An upper limit for UV disinfection is 1,000 total coliform/100 mL water or 100 fecal coliform/100 mL. 
Special Considerations

Prefiltration is required to remove color, turbidity, and particles that shield microorganisms from the UV source. Water that contains high mineral levels can coat the lamp sleeve and reduce the treatment effectiveness. Therefore, pretreatment with a water softener or phosphate injection system may be necessary to prevent build-up of minerals on the lamp. Table 3 lists the maximum levels of certain contaminants that are allowable for effective UV treatment. 

Overall Recommendations

Installing an UV treatment system, or any other water disinfection system is not a substitute for proper well design and construction.  If you have a dug well as a supply source, replacing the well is probably a more satisfactory long-term option. If a dug well or spring is your only supply option then look at all the treatment options before you decide what to do. Make sure you get advice from an expert!  Recommended treatment process selection:
1. Obtain information about your water source.
2. Get your water tested - At least Annually  
3. Determine which problems are associated with infrastructure deficiencies, i.e., cracked casing, no cap, improper seal, poor surface drainage, etc.  Make the necessary repairs and improvements to the system.
4. Install the necessary water treatment systems.  I have provided some online links for water treatment systems, but I always recommend a preliminary water test.  

Monitoring the Quality of Surfacewaters Backup

Calculating NSF Water Quality Index

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Teachers Using Our Web Portal Via Distance Learning in 2014

Water quality index is a 100-point scale that summarizes results from a total of nine different measurements when complete:

Temperature- Go to This Site to Calculate this Parameter Index
Dissolved Oxygen
Fecal Coliform
Biochemical Oxygen
Total Phosphates
Total Suspended Solids


Using the book Field Manual for Water Quality Monitoring, the National Sanitation Foundation surveyed 142 people representing a wide range of positions at the local, state, and national level about 35 water quality tests for possible inclusion in an index. Nine factors were chosen and some were judged more important than others, so a weighted mean is used to combine the values.

So that field measurements could be converted to index values, respondents were asked by questionnaire to graph the level of water quality (0 through 100) corresponding to the field measurements (e.g., pH 2-12). The curves were then averaged and are thought to represent the best professional judgment.

.Source Code for these Calculators From 
Source Code: Keith Alcock's Javascript WebMaster: webmaster@alcock.vip.best.com
Note: From Brian Oram, We have been Actively Working to Make the JavaScript Work in this New Format
if you experience a problem, please contact us and follow the link at the base of the web page.


Water Quality Index: DO sat (%)

Click to View Chart
Note: If dissolved oxygen is greater than 140%,
the quality index equals 50.

 Convert dissolved oxygen (%sat) to water quality index.

Dissolved oxygen: (%sat) Water quality index:

Water Quality Index: Fecal Coli

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Note: If the number of fecal coliform colonies is greater than 100,000,
the quality index equals 2.


Fecal coliform: (colonies/100 ml) Water quality index:  

Water Quality Index: pH

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Note: If pH is less than 2.0 or greater than 12.0,
the quality index equals 0.

Convert pH to water quality index.

pH: (units) Water quality index:  

Water Quality Index: BOD

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Note: If biochemical oxygen demand is greater than 30 ppm,
the quality index equals 2.

Convert biochemical oxygen demand (ppm) to water quality index.

Biochemical oxygen demand: (ppm) Water quality index:  

Temperature (Water Quality Index Calculator)
Based On Temperature Change from a Reference Site

Click to View Chart


Temperature change: (C) Water quality index:  

Water Quality Index: Total Phosphate

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Note: If total phosphate is greater than 10 ppm,
the quality index equals 2.

Convert total phosphate (ppm) to water quality index.

Total phosphate: (ppm) Water quality index:

Water Quality Index: Nitrate

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Note: If nitrate nitrogen is greater than 100 ppm,
the quality index equals 1.

Convert nitrates (ppm) to water quality index.

Nitrates: (ppm) Water quality index:  

Water Quality Index: Turbidity

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Note: If turbidity is greater than 100 NTU,
the quality index equals 5.

Convert turbidity to water quality index.

Turbidity: (jtu) Water quality index:  

Water Quality Index: Total Solids

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Note: If total solids are greater than 500 ppm,
the quality index equals 20.


Total solids: (ppm) Water quality index:  


Calculation of Overall Water Quality Index

Factor Weight Quality Index
Dissolved oxygen 0.17
Fecal coliform 0.16
pH 0.11
Biochemical oxygen demand 0.11
Temperature change 0.10
Total phosphate 0.10
Nitrates 0.10
Turbidity 0.08
Total solids 0.07

Based on the factors entered,
the water quality index is .

The 100-point index can be divided into several ranges corresponding to the general descriptive terms shown in the table below.


Water Quality Index Legend












Very bad


Other Resources for Educators and Students

Alkalinity Speciation Calculator

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WQI Index- Consumer Support Group Online Calculators

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Water Filtration Plant Performance Evaluations

Surface Water Filtration and Groundwater Under the Influence Investigations

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Continuing Education Courses for

Professionals Engineers, Surveyors, Geologists,
 AIA and More (Multi-State)

During the past 15 years giardiasis and cryptosproridiosis has been recognized as one of the most frequently occurring waterborne diseases in the United States.

The occurrence and detection of this parasite and drinking water source identification and protection has become a matter of urgent concern to those responsible for water utility operation in endemic areas.   Because of these concerns the Surface Water Filtration Rule was established and protocols were developed for determining if a source was characterized as surface water, groundwater, or groundwater under the influence of source water. As part of the Surface Water Filtration Rule all community water supplies identified as surface water require a minimum of filtration rather than just disinfection prior to consumption. The Surface Water Filtration Rule has lead to the development of a protocol to evaluate the performance of filtration plants. The protocol is similar to the method used for the Groundwater Under the Influence Investigations, except the risk rankings and interpretations are based on a combination of the system configuration, source water quality, degree of particle reduction, and distribution of particles.

Since as the Director for the Center for Environmental Quality has conducted research in pilot filtration plant performance and optimization, the laboratory has been involved in the evaluation of a number of bench-scale, pilot, and full-scale filtration plants in Pennsylvania, New York, and New Jersey.   The laboratory Director has worked on projects using diatomaceous earth filtration, slow sand filtration, direct filtration, submicron filters, cartridge filters and upflow filtration systems.   In addition, we have conducted bench scale and pilot scale experiments to assist in optimization of pretreatment process and filtration of water supplies and has developed protocols and conducting testing to assist in the development of solid flux loadings for secondary clarifiers for a wastewater applications and living filtration systems for on-site wastewater disposal systems.

Microscopic Evaluation Technique for Filtration Plant Performance

The purpose of this procedure is to evaluate the performance of public water supply filtration systems. A minimum of 300 gallons of both raw and finished water should be collected. The evaluation is based on the filtration plants ability to remove Giardia sized and larger particles. In addition to the MET evaluation, the evaluator should also consider the operational conditions of the plant, physical condition of the plant/ raw water, and effluent quality of the finished water.

Each analysis culminates in a filtration performance rating which reflects the overall effluent quality. The rating is always accompanied by an explanation which details the justification of the assessment. Filtration Plants with Excellent and Good ratings are reported as "Acceptable Filtration Performance". Filter which are rated Questionable or Poor are reported as "Unacceptable Filtration Performance".

The amounts of each of several specific groups of particulate matter and microorganisms are recorded. These groups include: small particulate debris, large particulate debris, cellular plant debris, diatoms, and other algae, protozoa, insects and crustaceans, nematodes, and rotifers. Also, pollen grains, Giardia, and other parasitic protozoa should be noted.

No of Specific Particles in a Category
Avg. Field at 100x

None  0
Rare  1 - 50
++  Few  51 - 100
++++ Moderate > 200


Excellent- These systems remove essentially all of the Giardia sized debris and that which is much smaller. There is no evidence of turbidity breakthrough to indicate any risk for Giardia contamination of the effluent and 300 gallons of effluent can be observed in a single sample without any Giardia sized particles reaching the rare level (+).

Good-300 gallons of effluent can be observed in a single subsample without significant amounts of Giardia sized particles present, i.e., the levels of diatoms, algae, and protozoa remain a the few (++) level.

Questionable-Giardia sized particles are at the moderate (+++) level in samples which represent 300 gallons of effluent. These systems have filtration which does not provide the obvious assurance for Giardia removal as in the above two ratings. It is not possible to comfortably predict the potential for Giardia breakthrough with this result.  Facility evaluations rely heavily on information obtained in the operations survey.

Poor-These systems are unable to remove Giardia sized particles and are therefore vulnerable for Giardia passage through the facility. Subsamples represent 300 gallons of effluent, and Giardia sized debris is in the moderated (+++) to many (++++) range. Operational changes are required to improve the effluent quality. The facility must be reevaluated to determine whether correction of the problem was achieved. Corrections must be performed within a reasonable time period and public notified if the potential for Giardia contamination remains.

 Water, Wastewater, and Stormwater Design, Operation, and Management

Monitoring the Quality of Surface Waters

Calculating NSF Water Quality Index (WQI)

teachers, learning WQI, Act 48, stream, environmental education
Teachers Using Our Web Portal Via Distance Learning in 2014

Water quality index is a 100-point scale that summarizes results from a total of nine different measurements when complete:


Using the book Field Manual for Water Quality Monitoring, the National Sanitation Foundation surveyed 142 people representing a wide range of positions at the local, state, and national level about 35 water quality tests for possible inclusion in an index. Nine factors were chosen and some were judged more important than others, so a weighted mean is used to combine the values.

So that field measurements could be converted to index values, respondents were asked by questionnaire to graph the level of water quality (0 through 100) corresponding to the field measurements (e.g., pH 2-12). The curves were then averaged and are thought to represent the best professional judgment.  The updated calculator allows you to enter the latitude and longitude for the site or pick this location from the Google Earth Map.  The calculator completes the individual and group calculation and permits you to generate a customized report.  (Please like and share!)


Other Monitoring Resources for Private Well OwnersEducators and Students
Training Courses for Professionals


More References Related to:
Alkalinity Speciation

WQI Index- Consumer Support Group Online Calculators
emperature Only


Path Finder Science

Water Quality Index Stream Monitoring Program


C-T Contact Time and Inactivation Calculations for Chlorine Disinfection

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Boil Water Advisories - Surface Water Treatment Rule - GWUDI


Chlorine is the primary disinfectant used in the United States. In order to be effective, the chlorine must be given time to react with the microorganisms.

The time required depends on the temperature and the pH of the water. Chlorine works best in water with a low pH and a high temperature. The concentration and contact time required to inactivate Giardia using chlorine is approximated by the following formula.

CT=.2828 * ( pH^2.69 ) * ( Cl^.15 ) * (.933^(T-5)) * L

  • CT = Product of Free Chlorine Residual and Time required
  • pH = pH of water
  • Cl = Free Chlorine residual, mg/l
  • T = Temperature, degrees C
  • L = Log Removal

The PDFs below use this formula to solve for any desired parameter.
- CT Made Simple
- CT Lookup Table


The CT concept was developed specifically for surfacewater, with the assumption that water suppliers would be trying to inactivate both Giardia and viruses. Since the CT required to provide 3 log inactivation of Giardia is at least enough to provide the required 4 log inactivation of viruses, the EPA just set the standard for Giardia and ignored viruses.

If a well tests positive for e. coli bacteria, it is very likely that it will test positive for viruses as well.  It seems reasonable, therefore, that the disinfection of a well that tests positive for coliform bacteria should be effective in inactivating viruses.

Just in case the Applet is not working - Here is a summary table with some inactivation and CT data as a function of pH, chlorine concentration, and log inactivations.


Water pH 6.0 at 0.5 C

Chlorine Conc  1.0 log 2.0 log 3.0 log 
0.4 mg/L 46 (CT value) 91 137
1.0 mg/L 49 99 148
2.0 mg/L 55 110 165

Water pH 7.0 at 0.5 C

Chlorine Conc 

1.0 log

2.0 log

3.0 log 

0.4 mg/L

65 (CT value)



1.0 mg/L




2.0 mg/L





Water pH 8.0 at 0.5 C

Chlorine Conc 

1.0 log

2.0 log

3.0 log 

0.4 mg/L

92 (CT value)



1.0 mg/L




2.0 mg/L





The next question is how do know how much contact time you are providing? 

Unfortunately, it is not as easy as dividing the storage volume by the flow rate. In order to count at all, there must be a separate inlet and outlet to the tank, widely separated. Even with this provided, the volume would then be discounted by one of the following baffling factors.


Baffling Condition Factor Description
Unbaffled 0.1 No baffling, low length to width ratio. Also applies to agitated basins (e.g. flocculation tanks)
Poor 0.3 unbaffled inlet/outlet. No baffles inside basin.
Average 0.5 Baffled inlet or outlet. Some inter-basin baffles.
Superior 0.7 Baffled inlet and outlet, serpentine inter-basin baffles.
Excellent 0.9 As above. Very high length-to-width ratio.
(plug Flow)
1.0 Used for pipe flow

Shock Well Disinfection Protocols

Water Quality Help Guides
Glossary of Water Terminology
Cryptosporidium Website

EPA Enhanced Surface Water Treatment Rule - EPA


Note: (This formula was provided by Peter Martin, an associate Engineer with the Contra Costa Water District, and was published in the AWWA Journal AWWA 85:12:12 Dec 1993).The applet above uses this formula to solve for any desired parameter. The applet may not be conservative for high pH, high chlorine residuals, and high temperatues.



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