Water Purification

Using Calcium Hypochlorite

by Kevin Wilson

Calcium Hypochlorite

I hope this guide will give you all the information you need to purify your drinking water using calcium hypochlorite.

Why calcium hypochlorite?

Calcium hypochlorite can quickly kill almost everything in water. It's affordable, widely available and easy to store for long periods of time (unlike liquid bleach). Boiling is the best method to make water safe for drinking, but calcium hypochlorite can be used if boiling is not possible or practical. Calcium hypochlorite is not considered effective for killing Cryptosporidium. I'd recommend boiling your water if you're concerned about Cryptosporidium.

What is calcium hypochlorite?

Calcium hypochlorite is a form of chlorine. It's commonly known as "pool shock". It's often used to disinfect swimming pools and spas. It can be found at pool supply stores, Wal-Mart and on the internet. It's usually sold in powder form. Calcium hypochlorite is sold in various strengths, I used 52%.


Proper precautions must be taken while handling and storing calcium hypochlorite. Always follow the manufacture's instructions.

Calcium hypochlorite will slowly degrade and release corrosive chlorine gas. I do not recommend storing calcium hypochlorite in metal containers or around metallic items, canned food, electronics or paper documents. Never store calcium hypochlorite near ammonia. Store in a glass or plastic waterproof container away from heat, sunlight and liquids. Calcium hypochlorite should be stored in a well ventilated room. I recommend replacing unused calcium hypochlorite after a few years. Keep out of reach of children. Avoid contact with skin, eyes and clothing. In case of contact, immediately flush eyes or skin with plenty of water for at least 15 minutes while removing contaminated clothing and shoes. Wash clothing before reuse. Do not breathe dust or vapor. If inhaled, remove to fresh air. Use in an area with adequate ventilation. Never add water to calcium hypochlorite powder, always add powder to water. Wash hands after handling. If swallowed, DO NOT INDUCE VOMITING. Give large quantities of water.

Make sure your calcium hypochlorite does not contain any additional algaecides or other substances. Calcium hypochlorite will often be mixed with "inert" or "other" ingredients like salt. Read the entire package and verify that there are no other active ingredients (like algaecides).

Required items

You'll need the following items:

Different sized bottles and different strength calcium hypochlorite can be used, but the amounts must be adjusted.


Filter dirty or cloudy water before treatment. Fill a small bottle with 1 cup water. Add 1/4 tsp (17 grain*) calcium hypochlorite (52%). Mix for several minutes or until the calcium hypochlorite has dissolved. Do not drink this solution! Avoid contact with skin, eyes and clothing. Every 1/2 tsp of the solution added to 2 liters of water should increase chlorine by 1ppm. Allow the treated water to sit for the required contact time (see below) or a minimum of 1 hour before drinking.
*Grain is a unit of weight typically measured with a powder scale.

To remove excess chlorine you can aerate the disinfected water by pouring it back and forth from one clean container to another (after waiting the required contact time). Be sure both containers are disinfected. This process will expedite the chlorine evaporation and reduce the chlorine smell / taste. The treated water can also sit overnight in an unsealed container to allow the chlorine to evaporate out. If the container has a wide opening (like a jar or pitcher) the chlorine will evaporate faster. In my tests, an open pitcher of water with 10ppm concentration was reduced to 1ppm after 12 hours. Containers with small openings (like bottles) will require significantly more time, possibly several day even if the bottle is half empty (or half full).

How much chlorine?

Water Condition ppm
Clean, from tap or well (for storage) 1-4
Clear, from an unreliable source 10
Dirty or cloudy 20


To effectively clean water, the chlorine must be present in sufficient concentrations and must be in contact with microorganisms for a minimum amount of time. Factors that affect the required chlorine concentrations and contact time include water temperature, pH, number and types of microorganisms and concentrations of organic matter in the water.

The required chlorine concentration and contact time is refereed to as the "CT". The "CT99.9" value is the CT value required for 99.9 percent (3-log) inactivation of Giardia lamblia cysts. Giardia cysts are among the most difficult organisms to inactivate (kill) with disinfection. Colder, dirty water and higher pH will require more contact time and / or higher chlorine concentrations. The United States Environmental Protection Agency has published extensive CT tables for Giardia inactivation, for different temperature, pH, chlorine concentrations and other factors.

Low water temperature can significantly increase the required contact time. I recommend that you make every effort to warm your water before treatment. A 10 degree F increase can reduce the required CT by about 30%.

Chlorine treatment is more effective in water with lower pH. Most "raw" surface water has a pH of 6.5 - 8.5. The pH of rain varies due to its origin. On the Americas east coast, rain that is derived from the Atlantic Ocean typically has a pH of 5.0 - 5.6. Rain that comes across the continent from the west has a pH of 3.8 - 4.8. Local thunderstorms can have a pH as low as 2.0. High pH levels can be reduced by adding lemon juice or other acidic fruit juice. A bit of juice can also mask the chlorine taste and improve the flavor of the water. Low pH levels can be increased by adding baking soda.

It's very important to filter dirty or cloudy water before treating. Turbidity significantly increases the required CT. Ideally a high quality water filter should be used. If a filter is not available, several alternative methods of water filtration are possible, including coffee filters, sand, cloth, porous clay and others.

In a typical worst case scenario with water temperature just above freezing and 8.5 pH, the required contact time is about 6 hours at a chlorine concentration of 1ppm. In a more common scenario with water at room temperature and 7 pH, the required contact time is about 1 hour at 1ppm and at 3ppm the contact time is reduced to about 25 minutes.

The following table lists general notes concerning chlorine levels:

ppm Note
0.2 - 1 Typical chlorine concentration in tap water
2 Typical chlorine concentration in swimming pool water
4 USA national drinking water standards maximum residual chlorine in drinking water
5 Upper tolerable limit for long term consumption (years)
10 - 20 Upper tolerable limit for short term consumption (days)

When treating water you should take into account any chlorine that may already be in the water. I used 52% calcium hypochlorite because it was the only kind I could find. I used 2 liter bottles because they are commonly recommended for water storage. After calcium hypochlorite powder is mixed with water, it will begin to degrade and should be used within a few months. If you don't have a measuring spoon, 1/2 tsp = about 50 drops. Calcium hypochlorite does not quickly or easily dissolve into water. I found that using more than 1/4 tsp calcium hypochlorite will not easily dissolve into 1 cup water.


Improper use of calcium hypochlorite can be dangerous. Use the information on this site at your own risk. I cannot be held responsible for your actions.

Additional Information

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