When disaster strikes and safe drinking water is scarce, bleach is a lifesaver for water disinfecting, cleaning clothes, and sanitizing surfaces. However, typical unscented liquid bleach has a short shelf life and loses half of its strength within six months. Fortunately, there's a long-term solution: granular calcium hypochlorite, commonly known as pool shock. It has a full-strength shelf life of over ten years and can be purchased wherever pool supplies are sold.

To make your own full-strength bleach from pool shock, follow these steps:

1. Purchase pool shock that contains calcium hypochlorite in a concentration range of 60% to 73%, with the rest being inert ingredients. A concentration of 68% is commonly found online and works well.

2. Wear goggles to protect your eyes and avoid breathing in the pool shock dust.

3. Use measuring cups, spoons, and a scale to measure out the granules.

4. For long-term storage, purchase a 5kg pail of pool shock. This amount will make 50 litres (15 gallons) of household-strength bleach.

5. Mix 500 grams of pool shock with 5 litres of water in a container. Stir until the granules dissolve.

6. Wait three hours for the inert ingredients to settle to the bottom.

7. Pour the clear bleach into another container, being careful not to disturb the settled material. Discard the settled material carefully, as it will kill anything it comes into contact with, including animals.

8. Test the strength of the bleach using chlorine test strips that are suitable for testing household bleach. To test the bleach, dilute it with water to bring the hypochlorite concentration to below 10,000 ppm, which is the highest concentration most test strips can measure. Use a standard kitchen 1 litre container and add exactly 100ml of bleach, then add water until the container contains 1 litre total. This is a 1 in 10 dilution that will bring the hypochlorite concentration to 6,000 ppm, which is well within the range of most test strips.

9. Stir the bleach solution in the measuring cup and dip the test strip into the solution. The test strip will come with a correlation chart that will relate the colour of the strip to the free available chlorine (FAC) level.

10. Multiply the concentration given by the test strip by 10 to get the concentration of the original undiluted bleach. For example, if the test strip indicates an FAC level of 6,500 ppm, the original undiluted bleach solution would contain 6,500 x 10 = 65,000 ppm.

By following these steps, you can make your own full-strength bleach from pool shock and have a long-term solution for safe drinking water and sanitation in case of a disaster.