Regular disinfection of rainwater held in domestic tanks is not considered necessary in most cases and is generally only recommended as a remedial action.
In the absence of any known problems and if the water is clear, essentially odourless and does not contain any suspended material it is unlikely to cause illness in most users. However the microbiological quality at times may not be as good as reticulated town water supplies.
Boiling Water
For those with lower immune responses such as the very young or very old, cancer patients, people with diabetes, organ transplants or those who are HIV positive, boiling the water before consumption should be considered.
If gastric upsets are being experienced, boiling should also be considered. If the tank water is suspected as being a possible cause of gastric illness then the rainwater tank and catchment area should be inspected and any appropriate remedial action should be taken.
Rainwater can be disinfected by heating and holding at a rolling boil for one minute or more. This will kill any harmful bacteria, viruses or protozoa including Giardia and Cryptosporidium. The water can then be cooled and stored in a clean container until use. To improve the taste of boiled water, pour it back and forth from one clean container to another, or let it stand for a few hours to increase the dissolved oxygen concentration.
Chlorine
If the water is coloured or smells the catchment area and tank should be inspected and appropriate remedial action should be taken to remove the source of any problems. If the rainwater is the only source of water it should be chlorinated. The tank should be drained and cleaned at the first opportunity. Where rainwater is the secondary source of water, the tank should be drained and cleaned prior to the next rainfall.
Chlorination is effective against harmful bacteria and many viruses but it has limited effect against protozoa such as Giardia and Cryptosporidium. Chlorination can also remove odours from rainwater by oxidising the responsible chemicals. The effectiveness of chlorine is shortlived and it will only act on what is in the tank at the time of dosing. Fresh run-off into the tank after chlorination will probably not be disinfected.
When chlorine is added to water, it reacts with organic matter and other impurities in the water and the amount of chlorine required for disinfection will depend on the concentrations of these impurities. In general, concentrations will be low in rainwater.
To achieve effective disinfection it is necessary to add sufficient chlorine to provide a free chlorine residual of 0.5 mg per litre after a contact time of 30 minutes. This can be measured using a suitable chlorine test kit if available. Such kits are used to test chlorine residuals in swimming pools.
As a general guide, the addition of 40 mL of liquid sodium hypochlorite (12.5% available chlorine) per 1000 litres of water or 7 grams of granular calcium hypochlorite (75% available chlorine) per 1000 litres of water will give a reasonable assurance of effective disinfection.
Both of these methods will provide chlorine doses of approximately 5 mg/L. Sodium and calcium hypochlorite can be purchased from large supermarkets, hardware stores or swimming pool stockists. Stabilised chlorine (chlorinated cyanurates) should not be used.
When using calcium hypochlorite this should be dissolved in a clean plastic bucket -in the open air – before adding it to the tank. Always add the disinfectant to the water rather than vice versa. After pouring the concentrated chemical mixture into the tank, stir the tank water thoroughly to provide as much mixing as possible and let it stand for at least one hour before
use.
The chlorine will not make the water unsafe to drink but it could impart a distinct taste and odour that should dissipate in one to a few days. Boiling the water will remove most of the taste and odour associated with chlorination.
Ultra Violet – UV
In some cases UV disinfection has been used to treat rainwater supplied through internal house taps. While this form of disinfection could be quite effective for rainwater it should not be used as a replacement for proper maintenance procedures.
Water Filters
Tap or outlet water filters should not be necessary for rainwater which should normally be soft, clear and free of any distinct odours. If water filters are installed they should be maintained exactly to manufacturers specifications to avoid problems associated with microbial growths.
Alternative Water Sources
In some cases it may be necessary to augment rainwater in tanks with water from other sources such as bores, dams, rivers and creeks or with carted water.
Only water that is suitable for drinking (possibly after chlorination) should be used. If there are any doubts about the suitability of a water source consult the local water or environmental health authority and if necessary have the water tested before adding to the tank.
Water should be added in one action and if from a surface supply such as a dam, river or creek the tank should be disinfected after addition is completed and the water let stand for at least one hour before use.
Water from a deep and well maintained bore will generally not require disinfection after addition to a rainwater tank.
Local authorities may be able to provide names of suitable water carriers/carters that they have approved or that they are satisfied will provide water that is suitable to drink. If local authorities cannot provide this information then only water carriers/carters that can provide evidence that water supplied will be safe to drink should be used.
This evidence could include:
- any authorisations issued for the purpose of supplying drinking water,
- the identity and quality of the source water,
- evidence that tankers used are fit for the purpose of carrying drinking water.
Rainwater
Rainwater as it falls on house roofs is soft, clear and largely free of micro-organisms and contaminating chemicals. During collection and storage however, there is a potential for chemical, physical and microbiological contamination. In most localities chemical and physical quality is relatively easy to maintain but the risk of microbiological contamination is more difficult to control.
The adoption of a sensible maintenance program for catchment areas and rainwater tanks should improve water quality including microbiological quality.
Routine testing of rainwater collected in domestic tanks should not be necessary and in most cases is not recommended. If there are doubts about the quality of rainwater, particularly if used for drinking or cooking, testing may be necessary. If rainwater is made available for public use a limited testing program focusing on microbiological quality should be performed. Rainwater used for any commercial purpose will require more rigorous testing for potability.
Advice on testing and analytical laboratories should be sought from local water or environmental health authorities.
Rainwater collected and stored in domestic tanks is likely to contain micro-organisms from one or a number of sources. While most will be harmless the microbiological safety of rainwater will depend on the exclusion of organisms that can cause infections of the gastrointestinal tract (enteric pathogens). The enteric pathogens include types of bacteria, viruses and protozoa. These organisms are typically introduced into drinking water supplies by contamination with faecal material from humans, animals and birds, with human enteric pathogens more frequently carried in human waste.
The majority of domestic rainwater tanks are installed above ground and collect run-off from roofs via guttering. Likely sources of micro-organisms include:
soil and leaf litter accumulated in gutters particularly if kept damp for long periods due to poor drainage,
faecal material deposited by birds, lizards, mice, rats, possums etc.,
dead animals and insects either in gutters or in the tank itself.
Less commonly, rainwater is collected in underground tanks. If these tanks are not fully sealed or protected against ground run-off then micro-organisms associated with human and animal excreta may also contaminate stored rainwater such as from septic tanks..
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