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Water Safety

Disinfecting a Cistern

​​​How do I disinfect a cistern?

When you’re ready to disinfect your cistern, bypass all water treatment devices and turn off the hot water heater (electric or gas).

  1. Start filling the cistern with clean, safe water.
  2. As you’re filling the cistern, add 1 litre of liquid, household, chlorine bleach (5.25%) for every 1000 litres of water the cistern can hold. Add the bleach while filling the cistern so they mix well. This gives a concentration of about 50 mg/L of chlorine.
  3. Run each tap connected to the water distribution system until you can smell chlorine, and then close the tap.
  4. Let the chlorinated water sit in the cistern and the distribution system for at least 6 hours. After 6 hours, drain and dump the water to waste (according to safe practices). Highly-chlorinated water isn’t safe to drink or give to animals. It will also cause problems if dumped in septic fields.
  5. Refill the cistern with clean, safe water.
  6. Run each tap for at least 5 minutes to flush the distribution lines.
  7. Turn on the hot water heater. Follow the manufacturer’s instructions to service any water treatment devices before you turn them on again.
  8. Test the water in the cistern and/or distribution system for bacteria. If there are no bacteria in the water sample, the water is safe to drink.
  9. If there are bacteria in the water sample, you need to find the source of contamination and fix the problem. After the problem is fixed, you need to clean and disinfect the water again. If there are no bacteria in the resample, the water is safe to drink.

Current as of: March 13, 2018

Author: Environmental Public Health, Alberta Health Services