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.
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.
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.
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
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