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

For purposes of water purification, 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 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. Chlorination 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 first 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. 

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 as 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 a water filtration system using an activated charcoal filter, 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 especially in humans.  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.

This is the reason both micro filtration and chlorination should be considered in making surface water suitable for drinking, especially when the water is suspected of significant viral contamination or heavy concentration of disease-causing microbes (slow moving stream, pond or other heavily clouded water).

Commercial chlorine bleach has a shelf life and chlorine concentrations are important

There are precautions to be taken when using chlorine to disinfect your own water supply.  An important question is:

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

In answer to this question, it depends on:

  • chemistry of the chlorine source,
  • the concentration of the chlorine source and
  • the age of the chlorine source.

For instance, a common source of chlorine mentioned in many internet searches for disinfecting water is household liquid 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 they also make different concentrations—from 5% up to 10% concentration. 

If using household 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.

It is important to know that liquid bleach loses its potency over time.  Shelf life for liquid bleach is generally recognized as 1 year.

Because liquid chlorine bleach has a short shelf life consider a different water purification chlorine source

There is a better solution to chlorinate water to make it suitable for drinking, and this is to use calcium hypochlorite (Ca(OCl)2 ) instead of household bleach (sodium hypochlorite).  

Water Treatment with Chlorine for Drinking Water 1

Calcium hypochlorite is:

  • 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 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 it in a closed plastic or glass container, in a cool location, out of direct sunlight and use in a well-ventilated area. The brand “Drytec” (figure 1) claims to be safe for treating drinking water.  Once it is in liquid format it also has a 1-year shelf life.

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

Table 1 below represents the two-part mixing strategy.

Water Treatment with Chlorine for Drinking Water 2

Note: A 600 ppm solution can be used for disinfecting food prep surfaces and a 200-ppm solution for sanitizing eating dishes and utensils

Appendix A below gives the mixture formula for developing your own custom table and Appendix B and C gives example problems on how to use the formulas.

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 stored and delivered to the consumer.

The reason for this is two-fold.