Cisterns and Other Rain Catchment Systems

1. If the cistern is connected to any water treatment units, close valves to those units. Follow manufacturer recommendations for treatment system disinfection.
2. Add 3 cups (a little less than 1 liter) of 5%-8.25% unscented liquid household bleach for every 100 gallons (about 380 liters) of water in the holding cistern. This should give you a chlorine concentration of about 100 parts per million (ppm)or mg/L.. Table 1, below, shows cistern capacity volumes.
3. If the cistern is connected to interior plumbing, open each faucet and run the water until you smell chlorine (bleach).
4. Turn off all faucets and allow the solution to remain in the cistern and plumbing for at least 12 hours.
5. Drain all water from the cistern.
6. Refill the cistern with safe drinking water.
7. Open each faucet and run the water until you do not smell chlorine (bleach).
8. Add 1 tablespoon of 5%-8.25% unscented liquid household bleach for every 100 gallons of water in your cistern to prevent microbial growth.

Maintaining water quality in cisterns can be difficult. Consider installing a treatment system to improve the quality and safety of cistern water. Contact your local health department or a water treatment specialist for more information.

Measure the chlorine residual regularly; chlorine test kits can be found at most swimming pool supply stores. Maintain a free chlorine residual between 0.2 ppm to 2.0 ppm to prevent microbial growth.