Clean cisterns now

Cayman Islands residents who have cisterns on their properties are reminded by the Department of Environmental Health that now is a good time to perform maintenance and cleaning.

Cistern water can be safe for consumption provided it is adequately treated and the cistern is properly maintained, said a DEH press release.

The DEH recommends that cisterns be cleaned at least once yearly, usually before the start of the rainy season which runs from June to November. Cayman is currently experiencing the dry season with minimal shower activity, an ideal time to check for cracks and other defects in the walls and cover. House roofs and gutters also should be checked.

Cistern covers should be made of durable, nonporous material, and should fit tightly to prevent the entrance of insects and foreign matter. A coping or lip should also surround the cover to keep out contaminated runoff water.

Downspouts and overflows should be fitted with screens to avoid debris and insects. A downspout diverter will allow rainwater runoff, also reducing debris and contamination in a cistern. Diverters are best used during the first rains of the season when they will allow dirty roof water to go to waste. Thereafter, homeowners can collect cleaner rainwater in their cisterns.

Cistern cleaning should be thorough to minimise sediment found on the bottom. Organic in nature, it can reduce the effectiveness of disinfecting chemicals and encourage the growth of micro-organisms. Moreover, this organic matter can react with chlorine to form toxic compounds and gases, thereby compromising the safety of cistern water, so it should be kept to an absolute minimum.

Persons without experience in cleaning cisterns should not enter them. DEH officials emphasise that the space is confined and low oxygen levels and the presence of chemicals can make it dangerous.

Officials urge residents to avoid cross-connecting cisterns with wells. Some householders use well water for laundry or flushing toilets. However, if there are cross-connections between a contaminated well and a cistern, the same piping is being used for both; drinking water supply can be contaminated as a result.

As preventative measures, cistern water used for drinking, cooking, bathing, etc., should be disinfected before use. This can be done by heating water to a rolling boil for one minute or by treating it with domestic chlorine bleach – two drops of bleach per gallon of water. All containers should be sanitised.


For more information about the safe collection and use of cistern water, contact DEH at 949-6696.

How to disinfect your cistern

The Department of Environmental Health advises cisterns should be disinfected after they are cleaned; after every heavy rainfall; every time they are filled with water; and at least every other week.

If the cistern is clean – with no residue on the bottom or sides – disinfect using fragrance-free domestic chlorine bleach, such as Purex or Clorox, at a rate of 3 fluid ounces to every 1,000 gallons of water.

If, after 30 minutes, there is no residual odour of bleach in the water, it means the chlorine has been used up and additional treatment is needed. Add half of the previous amount of bleach.

A residual bleach smell in the water is an indicator that the water has been satisfactorily disinfected. Check again, 15 minutes after the second treatment, to ensure that the residual odour is present. If a little too much bleach has been added, agitation or ‘sitting’ will allow the excess to dissipate naturally.

If you don’t know how much water your cistern holds, here’s how to do the calculation. It will also help you to know how much bleach to use. Volume of water (gallons) = length x width x depth x 7.5; Amount of bleach needed = volume of water in cistern x 2.5.


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