WASH 101: Chemical Alteration – Chlorine

March 6, 2019

Posted by Chip, Managing Director

Thanks for joining us again as we continue our conversation about potential solutions for the global water crisis. A quick reminder – if you’ve missed any part of our WASH 101 series – please go back and catch up with the conversation. We’ve talked about each part of the WASH acronym [Water, Sanitation, Hygiene], made sure we have a shared language, and looked into disparities for vulnerable populations. 

This month, we’re starting to look at Chemical Alteration… and since that is a pretty wide category, we’re going to break it into a couple of installments.  In today’s post, Chip will focus on using Chlorine to make dirty water clean.

As a quick reminder, here are the categories we’re working from:

  • Physical Intervention: including filtration, adsorption, sedimentation, boiling and distillation
  • Biological Intervention: including slow-sand, activated carbon, antimicrobial metals
  • Chemical Alteration: including chlorination, flocculation and iodination
  • Electromagnetic Radiation: including UV Light treatment
  • Sourcing: including rainfall, groundwater, underground aquifers, springs, and human-intervened (bottled, wells or municipal water).

First, let’s acknowledge that the primary challenge with using chemicals to make dirty water clean is that we don’t want to replace bad stuff [bacteria, parasites, etc.] with something else that is bad for human health. For example, you could dump a whole bunch of bleach into dirty water and kill off all the living organisms, but if the concentration of bleach is too strong, that water is still going to make people sick. As my mother swears she said first, “Too much of anything is a bad thing.”


The elements most commonly used to kill microbes come from the halogen section of the periodic table. All of the halogens (fluorine, chlorine, bromine, and iodine) share the common property of being oxidants (chemicals that can remove electrons from other molecules). When these halogen elements come in contact with enzymes, they steal electrons from the enzyme’s atoms. This causes the entire molecule to change shape or fall apart. When enzymes stop functioning, the cell or bacteria dies. In other words, the chemical kills the biological material and is neutralized in the process.

The WHO points out that “…the usefulness of a particular halogen as a disinfectant is not only determined by its reactivity, but also by its selectivity, chemical stability and other factors including the potential to form by-products.”  Of all the halogens, chlorine is the most useful because it best balances our requirements for strength, reactivity, selectivity and stability.

Fun fact: You probably interact with at least three forms of chlorine every day (chloride is found in salt, PVC plastics, bleach, paper, dyes, and many more). Not so fun fact: chemistry is complicated and involves complex math, and I’m bad at both, so forgive me as I treat chlorine, chloride, and chlorite in a seemingly interchangeable fashion.



Let’s start with why chlorine is a safe and effective treatment option.

Compared to filtration, chlorine actually kills the microbes whereas filtration tries to block them from passing through. You’re arguably safer drinking dead microbes than hoping you’ve managed to remove all the living ones. But, concentration matters, you want to make sure you’ve actually killed off everything.

Chlorine is plentiful, affordable, and comes in a variety of forms (gas, liquid, powder). There are several effective ways to use it to treat drinking water. Start by dissolving chloride in water to create hypochlorite. Then you can bind that to calcium (calcium hypochlorite) or sodium (sodium hypochlorite) to make bleach with great disinfection and purifying properties.

As chlorine kills microbes it also breaks down (when chlorine steals enough electrons it becomes inert), so if your concentrations (and wait times) are correct, you wind up with a solution that has very little chlorine left and no living organisms.

Chlorine (and the similar, more stable Chloramine) is widely used in water treatment systems in the developed world, so application methods and equipment are well researched and widely available.

Creating an effective chlorine solution doesn’t have to be complicated. Sodium Hypochlorite can be created from salt water in a process called chloralkali. We’ve actually built a few “chlorinators” that our volunteer teams in Gahanga and Masaka use to produce and distribute home-made chlorine. Our friends at S.W.I.M actually specialize in this treatment option.


Down Sides

There are some catches, the first of which is highlighted by this exposure table:

Disinfection time of fecal pollutants with chlorinated water
E. coli 0157 H7 bacterium < 1 minute
Hepatitis A virus about 16 minutes
Giardia parasite about 45 minutes
Cryptosporidium about 9600 minutes (6,7 days)

Yep, you need to keep a suitable concentration of chlorine in the water for more than 6 days before you can guarantee that you’ve killed all the cryptosporidium, a worm-like parasite that causes diarrhea and flu-like symptoms and can be lethal if untreated (side note: UV radiation can be used to kill cryptosporidium more reliably and at a much faster rate).

Another downside of using chlorine is that particulates within the water [things like sediment or algae] can interfere with the disinfection process. As the chlorine ions disperse, you lose concentration. So, the more biological material found in the water, the more chlorine you need. But knowing if your time and concentrations are sufficient requires testing your water pre- and post- treatment.

Shelf life can be a concern depending on the type of chlorine. Over time, sodium hypochlorite reverts back to salt water, usually in about 6 months depending on the concentration of the bleach. Calcium hypochlorite is practically stable, but is much more difficult to produce without industrial or lab equipment, and it leaves calcium deposits which are often an unwanted side effect.

Many of the other down sides are minor, but compounding. Chlorine (and iodine, even more so) creates an unpleasant taste which can lower adoption rates. Chlorines can react badly with other chemicals, creating dangerous by-products (mixing bleach and ammonia creates deadly gasses). And finally, some varieties of chlorine, like chlorine gas, are volatile and dangerous, even in small doses (chlorine gas was first used as a chemical weapon in WWII).

When we weigh the relative benefit of using Chlorine as a solution to the global water crisis, we continue to come back to this reality: When you pour clean water into a dirty container, the water becomes dirty. When you pour chlorinated water into a dirty container, the container becomes clean.

Think about that while you wait for our next post, which will focus on another commonly used chemical solution for clean water – Flocculation. Stay tuned!

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