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Chlorine, Halogens and Water Purification Involving Protozoa Cysts

Giardia cyst
Giardia cyst

A wonderful opportunity recently emerged that offered a chance to review and improve my knowledge on using chlorine for water purification. Specifically, looking at the ability for chlorine to have efficacy on protozoa cysts. This discussion will assume some prior knowledge that will be briefly reviewed before moving forward. It is important to note that each individual method has limitations and benefits.

Water purification definition: Removal or killing of harmful pathogens, chemicals and has the goal of making water safe for consumption

Sterilization definition: Complete removal or death of all microorganisms

Water pathogens: The main types of water pathogens can be divided into bacteria, viruses, parasites and protozoa. Bacteria are microscopic and generally single celled organisms. Viruses are generally smaller than bacteria and require a living cell to replicate. Parasites are multi-cellular organisms and usually visible with the naked eye. Protozoa are generally single cells that exhibit animal like behavior such as movement and predation.

Common waterborne bacteria of significance include Escherichia coli, Shigella, Salmonella and Cholera.

Common waterborne viruses of significance include Hepatitis A and Polio.

Common waterborne parasites include dracunculus, whipworm, tapeworms and Ascaris

Common waterborne protozoa of significance include Amoebias, Giardia and Cryptosporidia. Protozoal cysts are considerably resistant than bacteria and viruses largely due to their tough outer shell.

Methods of water purification: Filtration, chemical, Ultraviolet (UV) light, Heat are the mainstays of water treatment.

Filtration uses a fine mesh and relies of the size of the pathogens to be trapped by the pores of the filter. The size of the pores are measured in microns. This is an effective method against larger pathogens such as bacteria and protozoa. A typical bacteria ranges in size from 0.2 to 10 microns in diameter. Protozoa are typically in the 2 to 50 micron size. A typical virus particle is only 0.005 to 0.1 microns in diameter. A human hair is approximately 50 to 200 microns in diameter.

Chemical water treatment mainly centers around the halogen family which includes iodine and chlorine.

Ultraviolet (UV) light is utilized as either UV light from the sun or a commercially available product that generates UV light such as a SteriPen.

Heat consists of simply raising the temperature of the water to a point that kills the pathogens. Water boils at 212 degrees Fahrenheit or 100 degrees Celsius. Different pathogens have different temperatures required to be killed or inactivated. Time is a factor when heating the water and lower temperatures may be used with longer contact times. Typically bringing the water to a rolling boil will be effective for purification.

Giardia cysts and lifecycle:

Cryptosporidium and lifecycle:

Contact time: The amount of time needed to have the pathogens in contact with your chemical solution designed to purify the water.

Concentration: The amount of chemical dissolved in the water, often discussed as “parts per million (ppm)”. An example of this is 5 ppm of a chlorine solution has 5 molecules of chlorine in 1 million molecules of water.

Types of water: Potable water is water that can be safely consumed without causing illness. Non-potable water can contain harmful pathogens. Dirty water or cloudy water is water that is not clear. Cloudy water can be potable or non-potable depending on what is causing the cloudiness.

Contaminated water is water known to have harmful pathogens. Contaminated water can be clear or cloudy.

High risk water is water expected to contain multiple pathogens such as water contaminated with human waste and/or run off from fields. Expect this water to have parasites, viruses, bacteria and possible a living dinosaur. Treat this water with extreme caution.

Infectious dose: This is a measurement of the how many organisms are required to be ingested to actually cause clinical disease. Common accepted values include:

Shigella- 100 individual organisms to cause symptoms and illness

Cholera- 1,000 individual organisms

Viruses- 1 to 10 individual virus particles to cause illness

Giardia- 10 to 100 organisms

Cryptosporidium- 10 to 100 organisms


The ability to have a single-step water treatment plan is a lofty goal. Some methods are capable of this but have certain strict requirements such as extended time, can leave the water with a bad taste or require specialized devices. Often times these are impractical in a back country or austere setting. One of the methods that comes closest to this goal is the use of halogens and specifically chlorine to treat water.  But is chlorine really a safe and effective method for ALL TYPES of water treatment?

There is significant concern for chlorine’s lack of efficacy against the cyst form of Giardia and Cryptosporidium. While this can be overcome with increased concentrations of chlorine and longer contact times, is this really a practical method for individual water purification in the back country?



Giardia is a protozoa with a very difficult to kill cyst form. Filtration and heat are the common methods of eradicating this cyst form from drinking water. Research has shown efficacy of chlorine and specifically chlorine dioxide in effectively destroying these cysts but at increased concentrations and contact times. Once such study can be reviewed here: Applied and Enviromental Microbiology, Feb. 1981 by Jarroll et al. Effects of Chlorine on Giardia lamblia cyst viability

The take home message from this article is that cystacide is achievable with chlorine but is a factor of water pH, temperature and contact time. We know that cooler water and higher pH of the water adversely effect the killing ability of halogens. The solution for this is simply increasing concentrations of the halogens and/or increasing the contact time.

Temperature of water we know effects the killing ability of chlorine vs. organisms. Unfortunately, the geographic locations where Giardia is located tend to have cooler temperature water, even in summer months.



Cryptosporidium lifecycle
Cryptosporidium lifecycle

Cryptosporidium oocysts differ from other protozoal cysts and are considered highly resistant to halogens. Some data exists that demonstrate a 90% inactivation with 80 ppm of cholrine at a contact time of 90 minutes. This is roughly a 14 fold increase in resistance when compared to Giardia. This MMWR report can be found here:

It is important to note that this study above looks at Cryptosporidium survivability in swimming pools.

An additional study looking at efficacy of Chlorine dioxide to treat Cryptosporidium with clear demonstration of viability can be read here:

RM Clark, MSivagnesan, EW Rice et al., Development of a Ct equation for the inactivation of Cryptosporidium oocysts with chlorine dioxide, Water Research 37 (2003) 2773.

Further muddying the waters is this study which draws attention to differences in commercially available Cryptosporidium parvum oocysts used in testing of chlorine efficacy. This study shows discrepancy between cysts used in testing and subsequent differences in  resistance of each suppliers’ cysts. This suggests genetic diversity and resistance among C. parvum species. An abstract of this study can be found here:

Chlorine Dioxide inactivation of Cryptosporidium parvum oocysts and bacterial spore indicators by Chauret, Radziminiski, et al in Applied Environmental Microbiology (July 2001 Vol. 67).



Chlorine has been widely used as a water disinfectant for over 200 years. An 1854 cholera epidemic saw the use of Hypochlorite and forms of chlorine are the preferred means of water disinfection by municipal water facilities, worldwide. Acute toxicity to cholrine is limited and consists of mucous membrane irritation with ingestion of concentrated solutions such as household bleach. A military study use 32 ppm for several months without adverse effects. Animal studies looking at long term chlorination in drinking water at concentrations of 100-200 ppm have not shown toxic effects according to the National Academy of Sciences Safe Drinking Water Committee.

Perhaps the greatest source of information on this topic comes from White’s Handbook of Chlorination and Alternative Disinfectants. A link can be found here to a sample of the text:

This is an excellent text that delves deeply into the subject of water purification using chlorine and also the many alternatives.

Chlorine has obvious benefits when treating water that is not expected to contain protozoal cysts. For this purpose, a simple treatment with chlorine tabs should suffice. How sure are you that there are no protozoa in the water?

The World Health Organization has wonderful guidelines on the use of chlorine for water disinfection and they can be found here:

Click to access S04.pdf

Click to access chlorine.pdf

Practical Use of Chlorine in Field Water Disinfection

Chlorine tablets are available at most outdoor and travel stores. Potable Aqua is a common brand and widely available. A goal of approximately 5 ppm is sufficient for treatment of most types of water. Treatment times using chlorine tablets vary based on temperature and clarity of water. Always ensure you are following manufacture guidelines for using your chlorine tablets for water treatment.

An example of using Potable Aqua tablets in water treatment is to simply take one tablet and add to one quart of water (0.94 liters of water) and observe a 4 hour contact time. A pro tip is to ensue you loosen the cap on your bottle allowing the solution to penetrate the threads on the screw cap which can harbor water left untreated.

To calculate contact time one should assume the highest pH and the coldest possible temperature of water expected. Below is a bsic chart showing the differences in contact time associated with water pH and temperature.

Highest pH                        Water temperature in degrees Fahrenheit

                                         >50F                    45F                    <40F

7.0                                      8min                   10min                 12min

8.0                                     16min                  20min                24min

9.0                                     24min                 30min                36min


Examining the chart above we can see the association with decreasing water temperature and the need for longer contact times. Clearly, treating colder water will necessitate longer contact times for bacterial kills.

The use of liquid chlorine bleach is another alternative. Typical household bleaches contain between 5.25 and 8.25  percent of chlorine. Using this solution concentration one can add 5 drops per liter of water and allow for a contact time of 60 minutes. 10 gallons or 37 liters may be treated with 2 teaspoons of household bleach (one gallon equals 3.78 liters).

Multi-Step Processes in Water Purification

I advocate a multi-step process with water purification and dirty water or water known to have contaminates. Filtration and chemical or UV light are my mainstays and of the two I prefer UV light due to ease. Filtration and heat are also common combinations.

The use of high risk water from sketchy sources is common in developing nations. Run off from fields, poor waste management and standing water make for dangerous combinations. These types of water will carry viruses making filtration un-safe, protozoal cysts making chemical halogens un-safe and will often be off cloudy quality. My treatment process for this water is as follows:

  1. Basic filtration through cloth or a coffee filter to remove larger bits of debris such as plant material and algae
  2. Sedimentation allowing the particulate to settle to the bottom of my container
  3. Filtration of the water at the top of the container avoiding the particulate that has collected at the bottom
  4. Second step treatment with UV light after filtration has been completed

Summing it all up

Steripen using UV light
Steripen using UV light

Chlorine is a very effective and safe method for water purification in most circumstances. Certain limitations of chlorine must be recognized. Practicality and “real world” scenarios also limit the safety of using chlorine as a single agent in water purification. Putting all of this together we see that chlorine effectively kills giardia cysts and cryptosporidia cysts. However, contact time and water temperature often make this impractical in a wilderness or remote setting. Waiting for your water to treat for four hours is often unrealistic based on the amount of water one can carry and should be consuming. Carrying 2-3 liters of potable drinking water and another 2-3 liters of water (in a separate container) undergoing a contact time is a lot of water to be carrying. Worse, remaining hydrated necessitates aggressive consumption of water when exerting yourself. Outdoor athletes can easily drink 2-3 liters of water in a four hour period.

Speed of purification and complete assurance of elimination of pathogens is necessary. Based on this I advise a two step process when dealing with contaminated water. I personally use filtration as my first step followed by UV light in the form of a steri-pen. My alternative is filtration followed by chemical use. It is important to note that having multiple methods of purification in a wilderness setting is critical. “Two is one and one is none” application is key when your filter breaks, your steripen runs out of batteries or your lose your chemical tablets.



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