Monday, August 4, 2014

Bleach for Water Purification

Editor's Note: Evelyn had to take a personal day, so Tim is filling in for her with an article that is both topical and relevant to preppers. She will return next week. 


I spent 18 years running a multi-million dollar industrial wastewater treatment plant. The methods of cleaning water for drinking and for discharge to public waters are the same, but they vary in their application and allowable levels of contaminants. Most of the time the water that we discharged to the local river was cleaner than what was already there, and there are hundreds of cities and towns downstream that draw their drinking water from that same river.

To be blunt about it, every drop of water that you drink has been passed through the body of countless millions of animals and people before it got to you. What comes out of your faucet or bottle of water was dinosaur urine at one time. We live on a planet where new materials are not being imported, so what we have has to be reused.


Adding household bleach to suspicious water or water of unknown quality is a staple of disaster preparations, but the recent finding of microcystin toxin in the water supply of Toledo, OH has brought up a few questions. Microcystin is a toxin produced in the cells of certain blue-green algae and is released into the water when the algae die or are destroyed. It is a liver toxin that can cause permanent damage to the liver as well as nausea, diarrhea, and vomiting. Boiling water will only concentrate the toxin by killing the algae and reducing the amount of water that it is in. The problem is not the algae, it is the chemical that they produce. Filtering out the algae (disinfection) is only the first step in cleaning up the water; any decent water filter will do that. However, getting rid of the toxin (purification) is another matter.

Purification using Bleach

You'll need a way to test for chlorine. Any swimming pool or hot tub shop will have these nifty little test strips. Quick and easy tests are the best, and they don't require much more than normal color vision. You simply follow the directions on the side of the bottle (dip the test strip into the water for 30 seconds, if I recall correctly) and then compare the colored spots on the strip to the color bars on the bottle. Where the colors match is where your chlorine level is. If you don't have any test strips, use your nose. The human nose can detect chlorine at about 0.3 mg/L in water so if you can smell it easily, it is over the 0.5 mg/L lower limit, which means it is safe to drink. 

The active ingredient in household bleach is Sodium Hypochlorite (NaClO), which is derived from common table salt (NaCl). Bleach works as a disinfectant (kills nasty microbes) because of the chlorine content. Chlorine tends to disrupt the cell walls of microorganisms, causing them to rupture and die.

In Toledo

Using bleach to treat water for microorganisms is well established, but in Toledo they're not dealing with the algae itself; they're dealing with the toxin it produces.  Fortunately, bleach will also work on the microcystin by oxidizing it into a more benign chemical. The World Health Organization has set out standards for treating water for microcystin, found here, that are fairly easy to follow if you know what you have on hand and how to use it.

From the WHO paper:
Chlorination and ozonation are effective for the removal of microcystins. A residual of at least 0.3 mg/L of ozone for 5 minutes will be sufficient for all of the most common microcystins. For chlorine a dose of 3 mg/L applied to obtain a residual of 0.5 mg/L for at least 30 minutes will be effective.
Ignore the part about ozone, that's beyond most people's capabilities. The important part is about the Chlorine- the 3mg/L and the 0.5mg/L for at least 30 minutes part. Those of you who never took Chemistry, or who just hate math, are probably looking at that as if it were written in a foreign language. I will try to explain it in a way that makes sense outside of a lab.

Translating Chemistry Terms Into English

mg/L is one of the standard ways of expressing the concentration of a substance in water. If you're familiar with the metric system, you'll read it out loud as “milligrams per liter”. Since water is the original base of the metric system, a lot of the units of measure are interconnected. One milligram of water is equal to one milliliter (or cubic centimeter, “cc”, in medical jargon) of water. That means that there are 1,000 milliliters in one liter, and since one liter of water weighs one kilogram (1,000 grams) there are 1,000,000 mg/L total. Did I lose anybody on that one?

Let me try again.
  • 1 liter (L) of water weighs 1 kilogram (1 kg).
  • There are 1,000 grams (g) in a kilogram (kg).
  • 1 gram (g) contains 1,000 milligrams (mg).
  • 1,000 mg (1 g) times 1,000 equals 1 kg.
  • There are a total of 1 million mg in 1 kg (which is what a liter [L] of water weighs).
So when you see something expressed as “mg/L” what they're trying to say is “parts per million” (ppm), another form you may see used. Remember, this only works for water and mixtures that are mostly water, but that's all we're interested in for now. Both ppm and mg/L are used interchangeably when measuring small quantities of substances.

This becomes important when you pick up your jug of handy-dandy bleach and look at the label and it says that it has 5% Sodium Hypochlorite (NaClO). Oh, wait. The store was out of your normal brand, so you grabbed the generic store brand. That bottle says it contains 8.25% NaClO instead!

The advent of high efficiency washing machines required that they start increasing the concentration of NaClO in household bleach. Now what? Percent means “parts per hundred”, so 1% means that there is 1 part (whatever unit you are using) per 100 parts (as long as you keep using the same units). Percent concentration is used when measuring large concentrations of a substance. Converting 1% to parts-per-million (ppm) is simple. 1 million divided by 1 hundred equals ten thousand. One percent equals ten thousand ppm, that's why it is used to measure larger concentrations. So 5% is the same as 50,000 ppm (or mg/L).

More Math

You need to dose a bottle of water in order to have something safe to drink and the WHO says that you need to add at least 3 ppm (mg/L) and after 30 minutes there needs to be at least 0.5 ppm of chlorine left. A one gallon milk jug holds about 3.78 L, so let's round it up to 4 L. How much bleach do you need to add to get 3 ppm (mg/L)? 3mg/L * 4 L = 12 mg of chlorine.

Since your bottle of bleach is only 5% (or 6% or 8.25%) NaClO, you need to know how much bleach to add to your gallon of water get it over 3.0 mg/L chlorine. You also have to figure in the fact that there is more than just chlorine in your bleach. That sodium and oxygen is taking up enough space to knock the chlorine down to only 60% of the NaClO. Let's go with the low number (5%), just in case that's all you have or your bleach has been sitting on the shelf for a year or two. Slightly overdosing with bleach is acceptable, just let the water sit in an open container in the sunlight for a few hours and the excess bleach will burn off by turning back into salt and water*.

We're starting with 5% NaClO, which is only 60% Cl, so we actually only have 3% chlorine available. That works out to 30,000 mg/L Cl in the bottle of bleach. We need to get 12 mg of Cl into the water, so we need to divide 12 mg by 30000 mg/L to get 0.0004 L of bleach. That's 0.4 mL or 0.4 cc of bleach. Again, let's round it up to 0.5 mL.

Want to try it for the concentrated bleach? If your bottle of bleach says it is 8.25% NaClO, that means that it is (60% of 8.25 =) 4.95% Cl, or about 50,000 mg/L. Divide the 12 mg Cl we need for the gallon of dirty water by 50,000 and we get 0.00024 L or 0.24 mL. Close enough to 0.25 cc for us to use.

In Practice

Now that I've got the math out of the way, how do you measure out that small of an amount of bleach? The standard in a lab or pharmacy for a “drop” is 15 drops per mL. A “drop” from an eyedropper actually has its own unit symbol or abbreviation: gtt. Since we need 0.25 or 0.50 mL of bleach to remove the microcystin that started this whole article, we need to find an eyedropper (glass will work best - bleach will eat plastic and metal ones) and slowly count out 4 drops for the concentrated bleach or 8 drops for the normal bleach for a gallon of water. Stir or shake and let it sit for 30 minutes and check for chlorine (by sniffing it, or by using test strips). If there is at least 0.5 mg/L still in your water, it is safe to drink. If not, dose it again until you do get the desired residual chlorine.

Why All the Math?

All of this is to show you how to calculate dosage rates for bleach. The charts that are available on the internet are based on the old standard of 5% or 6% NaClO, and use as a disinfectant as far as I can tell. If you're worried about blue-green algae, doubling the dose seems to be the best bet.

* Household bleach has a short lifespan; it will turn back into salt and water by as much as 20% per year, even when stored in unopened containers. Long-term storage won't work, and producing your own is a topic for another time.

The Fine Print

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