Disinfect water barrels and cisterns with calcium hypochlorite

Calcium hypochlorite dissolved in water creates a strong chlorine solution that kills bacteria, viruses, algae, and mold spores on interior surfaces and breaks down biofilm that rinsing alone can't reach. Same granules and measurement framework as Recipes 5.1 and 5.2.

• Calcium hypochlorite (65-70% chlorine, no algaecides)
• Freshwater source for mixing and rinsing
• Potable water for final filling
• Measuring spoons (particularly 1 teaspoon)
• Non-metallic container (glass jar or HDPE jug)
• Non-metallic stirring tool (plastic or wooden)
• Long-handled scrub brush
• Dish soap (for pre-clean of non-water residue)

1. Empty the container. If it previously held anything besides clean water (food, oil, or soap residue), pre-wash the interior with hot water and dish soap and rinse until no suds remain. Then scrub walls, seams, and bottom to break up any remaining buildup, and rinse out the debris. Organic residue consumes chlorine before it can disinfect, so this step is essential for any non-water container.
2. Make the disinfecting solution one gallon at a time. Add 1½ level teaspoons (about 5.7 to 6 grams) of calcium hypochlorite to a gallon of water in a non-metallic container. Always add powder to water, never the reverse. Stir until dissolved. You need 2 gallons of solution for every 10 gallons of container capacity (target around 200 ppm), so repeat as needed.
3. Pour the solution into the container and fill to capacity with water. If you can't fill it completely, roll or rotate the container every 15-20 minutes so the solution contacts all interior surfaces evenly.
4. Let it sit undisturbed: 6 hours minimum for a clean container, 12 to 24 hours for visible biofilm or unknown history. That long soak at 200 ppm is far more than enough to kill bacteria, viruses, and Giardia and to break down biofilm. Cryptosporidium strongly resists chlorine, so for drinking water rely on filtration or UV rather than chlorine alone.
5. Drain the chlorinated solution onto bare ground, away from plants, gardens, creeks, ponds, and storm drains, since it harms aquatic life. Then rinse 2-3 times with clean water. The container is ready to fill with safe water, or water that will be treated before drinking.

Sealed dry powder: up to 10 years if kept cool, dry, and out of direct sunlight. Check yearly for clumping or weak chlorine smell. Heat and moisture speed degradation. Best container: sealed glass jar or HDPE jug. Avoid metal. See the Containers section.