Cistern Disinfection and Cleaning

Water in a cistern may not necessarily be clean and potable. Hauled water may already be contaminated and even if the water has been treated, it can deteriorate during storage. Over time, the loss of chlorine residual can lead to microbial re-growth; sediments/sludge can accumulate in the cistern or animals or insects can fall in.

Water from a cistern should be sampled semi-annually for bacteriological quality. If test results show the presence of coliform bacteria both the cistern and distribution system should be disinfected with chlorine2. Cisterns should also be pumped clean and chlorinated at least once per year to prevent sludge build up or biological fouling. This frequency may increase if water quality problems develop. Water testing and cistern cleaning should also follow any contamination incident (e.g. flooding, repairs) or following changes in water clarity, colour, odour or taste.

Confined Space Safety Precautions

A cistern must not be entered until you are sure the cistern’s air quality is safe. No cistern should be entered unless the person entering the tank has been trained in confined space entry and follows the appropriate safety procedures as per Part 5 of the Province of Alberta Occupational Health and Safety Code. If you have any questions regarding confined space entry safety practices, refer to the Province of Alberta Occupational Health and Safety Code at whs.gov.ab.ca or call toll-free 1-866-415-8690. If in doubt, contact a professional trained in confined space entry for information regarding cistern cleaning.

Procedure for Cleaning the Cistern

Procedure for Cistern Disinfection

Calculate how much water is in the cistern.

For a box cistern, multiply the length (L) by the width (W) by the height (h), where height equals the depth of water.

For a cylindrical cistern, use πr²h where π = 3.14, r² = radius x radius, and h = depth of water.

Water treatment devices should be bypassed during the disinfection procedure and the hot water heater power (or gas feed) shut off.

  1. Add 1 L of bleach2 to every 1000 L of water in the cistern (adding while filling the cistern will ensure good mixing). This will provide approximately 50 mg/L of chlorine.
  2. Run individual taps connected to the plumbing system until a chlorine odour is detected then close the tap.
  3. Allow the chlorinated water to sit in the cistern and plumbing system for at least 6 hours.
  4. After 6 hours, the chlorinated water can be dumped to waste in accordance with safe practices. Flush cistern by filling and draining using water from a potable source taking care not to contaminate the cistern. Highly chlorinated water is unsafe for drinking, unsuitable for domestic or livestock use, and will cause problems if discarded into septic fields.
  5. Run each tap for at least five minutes to flush the lines.
  6. If the cistern is constructed from concrete, it may be desirable to use at least three loads of water prior to drinking the water. The water may still have a ‘chalky’ appearance and have a slight ‘cement taste’.
  7. The cistern should now be ready for use and can be refilled with potable water from an approved source.
  8. Restore power to the hot water system. Service any water treatment devices according to the supplier recommendations prior to bringing them back into service.
  9. Resample if cistern was disinfected due to a failed water test. If the lab report indicates the water is still unsafe, the cistern should be emptied and cleaned.

Flooding:

Cisterns that have been contaminated by floodwaters need to be properly cleaned and disinfected. Follow the above ‘procedure for cleaning the cistern’ and ‘procedure for cistern disinfection’ prior to bringing it back into service.

Technical Advisory Committee on Safe Drinking Water, Environmental Public Health, June 2010

Cardston

Community Health
Box 1590
Cardston, AB T0K 0K0
Phone: 403-653-5231
Fax: 403-653-2926

Coaldale

Community Health
Box 1000
Coaldale, AB T1M 1M8
Phone: 403-345-3000
Fax: 403-345-2043

Crowsnest Pass

Community Health
Box 67
Blairmore, AB T0K 0E0
Phone: 403-562-5030
Fax: 403-562-7379

Fort Macleod

Community Health
Box 520
Fort MacLeod, AB T0L 0Z0
Phone: 403-553-5363
Fax: 403-553-2333

Lethbridge

Community Health
801 – 1 Avenue South
Lethbridge, AB T1J 4L5
Phone: 403-388-6689
Fax: 403-328-5934

Pincher Creek

Community Health
Box 1685
Pincher Creek, AB T0K 1W0
Phone: 403-627-1230
Fax: 403-627-5275

Raymond

Community Health
Box 251
Raymond, AB T0K 2S0
Phone: 403-752-5430
Fax: 403-752-4655

Taber

Community Health
5009 – 56 Street
Taber, AB T1G 1M8
Phone: 403-223-4403
Fax: 403-223-8733

Brooks

Community Health
220 – 4 Avenue West
Brooks, AB T1R 0E9
Phone: 403-362-2682
Fax: 403-793-2772

Medicine Hat

Community Health
River Heights Professional Centre
Suite 200, 88 Valleyview Dr. SW
Medicine Hat, AB T1A 8N6
Phone: 403-502-8205
Fax: 403-502-8256