Water treatment with Chlorine

Water treatment can be a challenge in an emergency and here is the why and how chlorine can be effective in the process of creating drinking water

Chlorine is your friend—especially in an emergency scenario where normal utilities like power, water and sanitation are compromised either for a short period (few days to weeks) or months (outages lasting for a lengthy but undetermined time). Public water supplies are usually the first to become contaminated in times of natural disasters or emergencies. Water Treatment with Chlorine for Drinking Water 1Chlorination has been in public use in the United States since 1906 when it was introduced to municipal water supplies as a disinfectant.  Up to that time, serious illness, and death due to drinking water contaminated with pathogenic microbes was common in both first world and third world countries alike. Since the introduction of chlorine to public water supplies, the mortality rate from drinking water has fallen drastically, especially in 1st world countries where tap water is deemed safe to drink.

Chlorination of water supplies for water purification has been proven safe and effective through numerous scientific studies and over the course of time.  The Environmental Protection Agency (EPA) regulates public water systems and deems water safe to drink up to 4 parts per million (PPM) of chlorine.  Chlorine concentration in water is easily measured using test strips or using inexpensive kits you can obtain online or at pool supply stores.

Chlorine for water purification is especially effective in killing bacterial and viral contaminants at the EPA recommended chlorine levels.  It is thought the mechanism of destruction of those contaminants is breaking the chemical bonds in their molecules.

Chlorine is also effective in sanitizing and disinfecting areas involved in food preparation as well as cleaning and sanitizing surfaces that may come in contact with human waste. 

One of the advantages of chlorination is not only does it kill microbes on contact, but it also has a residual effect meaning that because the chlorine persists for a time in treated water, it will continue to disinfect if any new bacteria or viruses are introduced. 

Chlorine is inexpensive and readily available to consumers.  For those who are sensitive to chlorinated water or just don’t like the taste, the concentration can be significantly reduced via filtration through activated carbon filters, through aeration of the water, and/or extended exposure to sunlight in the open air.

Chlorine is very effective in killing bacteria and viruses but doesn’t kill single cell protozoa like giardia and cryptosporidium.

Chlorine is not effective, however, in killing organisms such as giardia and cryptosporidium.  These single celled protozoa reproduce in the gut of a host (mammals and birds) and then the eggs are shed as a tough shelled cyst (which is resistant to chlorine, bromine, or iodine) when the host defecates. These cysts can persist in the environment for quite a long period.  Once ingested, they become active and reproduce this life cycle in the new host—often making it quite ill.  Because these cysts are relatively large (1 to 3 microns), micro-filtration is an effective means of removing them from a water supply.  Boiling is also effective as is ultra-violet (UV) treatment.   It is estimated that up to 80% of the rivers and lakes in the U.S. are contaminated with these microbes.

Bleach has a shelf life and chlorine concentrations are important

But there are precautions in using chlorine to disinfect your own water supply—these precautions include:

“How much chlorine do I add to disinfect the water and still be safe to drink?”

In answer to the first question of how much to add, it depends on

  1. chemistry of the chlorine source,
  2. the concentration of the chlorine source, and
  3. the age of the chlorine source

For instance, a common source of chlorine mentioned in many internet searches for disinfecting water is bleach.  Commercial bleach that you buy at a grocery store is sodium hypochlorite (NaOCl).  Numerous companies make and market this product.  Some add chemicals to make the smell more pleasant, some add thickeners to reduce the likelihood of splashing while pouring, and also make different concentrations—from 5 ½ up to 10% concentration.  It is also important to know that liquid bleach loses its potency over time.  Shelf life for liquid bleach is generally recognized as 1 year.

If using bleach as a disinfectant, you should choose a product that has no additives for smell or thickening, you should know the chlorine concentration and you should know the manufacture date to know the age of the bleach. 

The mixture formula1 is V3=V1 * C3/C1 where V3=volume of concentrated chlorine to add, V1=the volume of water being disinfected, C3=the final concentration of the mixture, and C1=the concentration of the concentrated chlorine. 

It is important that all variables are in the same units.

Let’s give an example problem—

How much bleach should be added to 5 gallons of water to treat to 4 ppm if the bleach has a concentration of 5 ½ % and is 2 months old? 

Because the bleach is only 2 months old, we can assume it is at full strength.

Here V1=5 gallons; C3=4 ppm; C1=5 ½ %

C3 and C1 need to be in the same units—lets covert C1 to ppm

1 % = 10,000 ppm

5 ½% = 5.5×10000=55000 ppm


V3 = 5 gallons x 4ppm/55000 ppm

V3 =.000364 gallons of bleach to add

Let’s change this to teaspoons where there are 768 teaspoons in a gallon

V3 = .000364 gallons x 768 tablespoons per gallon V3 = .28 teaspoons

Therefore adding ~ ¼ teaspoon of 5 ½ % bleach will treat 5 gallons of water to a concentration of 4 ppm chlorine.

1This simplified mixture formula assumes V1 is much much greater than V3

Consider a different water purification substance - Calcium Hypochlorite

Because bleach has a shelf life there is a better solution to chlorinate water to make it suitable to drinking, and this is to use calcium hypochlorite (Ca(OCl)2 ) instead of bleach (sodium hypochlorite).  Calcium hypochlorite is:Water Treatment with Chlorine for Drinking Water 2

  • a powder rather than a liquid,
  • it readily dissolves in water,
  • it has a shelf life up to 10 years (if stored properly),
  • it is inexpensive and available, and
  • it is very concentrated so a little goes a long way. 

A common use for calcium hypochlorite is pool shock—used to control an algae bloom in swimming pools.  You can purchase it either online, from pool supply sources or sometimes from your local preparedness store.  It is a potent chemical so take care how you store it and use it.  If you follow the precautions on the label, it is perfectly safe. Just remember it eats metal and fumes can be strong so store in a plastic or glass container, in a cool location, out of direct sunlight and use in a well-ventilated area. This brand “drytec” claims to be safe for treating drinking water.

Because Calcium Hypochlorite is so concentrated (usually 65 to 70% chlorine by weight), I first mix my own “bleach solution” (say 1 gallon at 600 ppm) and then use this lower concentration solution for final treatment of my drinking water.  This two-part approach ensures a better dilution accuracy and ensures that my “bleach” solution is at full strength. 

For instance, if you want to treat 5 gallons of water to a chlorine concentration of 4 ppm using powdered calcium hypochlorite at 68% concentration, you add only .01 teaspoons of powder—hard to measure accurately.  If you want to treat that same 5 gallons of water using the 600-ppm mixture to a chlorine concentration of 4 ppm you would add ½ cup—a much easier and more precise dosage. 

To make up one gallon of 600-ppm bleach using powdered calcium hypochlorite, the calculation uses the same formula, but the unit corrections are a little more complicated because this is a solid_liquid mixture and not a liquid_liquid mixture as is the previous case.  

Here is an example problem:

How much calcium hypochlorite do I add to 1 gallon of water to make a solution of 600 ppm chlorine if calcium hypochlorite has a concentration of 68% chlorine by weight?

Here V1=1 gallon; C3= 600 ppm and C1=68 %

Converting C1 to a ppm=68 *10000 or 680000 ppm and making other units consistent

V3 = 1607 * V1 * C3/C1                         

Note: the constant 1607 results from conversions to make measurement units consistent

V3  = 1607 * 1 * 600/680000

V3 = .24 cc (cubic centimeters)

Therefore adding ~ ¼ cc of 68% calcium hypochlorite will treat 1 gallon of water to a concentration of 600 ppm chlorine.

The 600-ppm solution is also good for disinfecting surfaces that may come in contact with harmful microbes.  A 200-ppm solution is recommended for sanitizing dishes and eating utensils.  It is easy to convert the 600-ppm solution to 200-ppm by adding 2 parts water to 1 part of the 600-ppm bleach solution.

Chlorine can always be removed or significantly reduced prior to drinking by filtering the chlorinated water through an activate charcoal filter. 

When should I treat my water with chlorine?

The next question is when should you chlorinate the water to make it suitable for drinking?  The figure below shows a schematic of a typical municipal water treatment plant.  You will note that chemical treatment (chlorination) is the final treatment step before the water is piped to the consumer.

The reason for this is three-fold

  1. The source water may have a high concentration of organic particulate that reacts with the free chlorine. If the water is treated with chlorine before removal of this particulate, it requires more chlorineand there may not be enough active chlorine left to completely disinfect the water to drinking water standards.
  2. A high concentration of particulate matter adds significant uncertainty to the correct chlorine dosage to make the water suitable for drinking. This is because most of the chlorine will be used up reacting with the organic particulate rather than disinfecting the pathogenic microbes.
  3. Chlorination of water with a high concentration of organic particulate can result in a higher concentration of byproducts such as trihalomethanes, which can present long term negative health effects.  Trihalomethane is an EPA regulated drinking water contaminant.

For these reasons it is best practice to chlorinate after the water has been filtered for removal of visible particulate.  This practice:

  • minimizes the amount of chlorine necessary to add,
  • improves the accuracy of the dosage calculations,
  • minimizes the creation of potentially harmful byproducts, and
  • provides a residual chlorine concentration for disinfection of any microbes in your storage containers.

In summary:

In an emergency scenario where, clean drinking water no longer comes out or your tap, you have exhausted your supply of stored water and you must turn to available surface water to survive:

  1. A combination of microfiltration (PortaWell®) followed by chlorination should be considered, especially if source water is suspected of viral contamination or heavy concentration of disease-causing microbes (slow moving stream, pond or other heavily clouded water).
  2. Mixture concentrations are easy to calculate using the simplified formulas in Appendix A
  3. Powdered calcium hypochlorite is a better storage option for chlorine treatment of water or disinfection, sanitation, and drinking than common bleach (sodium hypochlorite) because it:
    1. Has a much longer storage shelf-life
    2. Better ability to predict and calculate more precise dosage because of known concentration
    3. The powder is very concentrated andoccupies a smaller storage footprint
    4. Is economical and readily available
  4. Chlorination (if needed) should be applied after filtration to:
    1. Minimize and amount of chlorine required
    2. Minimize to formation of potentially harmful chlorinated by-products
    3. Provide residual chlorine in stored water to reduce potential for algae buildup

can always be removed or significantly reduced prior to drinking by filtering the chlorinated water through an activate charcoal filter.