The most common methods for disinfecting rainwater are chlorination, ultraviolet light irradiation or heat (boiling).
ChlorinationRegular chlorination of rainwater held in domestic tanks is not considered appropriate in most cases and is generally only recommended as a remedial action. The effectiveness of chlorine is short lived and it will only act on water in the tank at the time of dosing. Fresh run-off into the tank after chlorination will probably not be disinfected.
Chlorination is effective against harmful bacteria, many viruses and Giardia but it has limited effect against Cryptosporidium. Chlorination can also remove odours from rainwater by oxidising the responsible chemicals. When chlorine is added to water, it reacts with organic matter and other impurities in the water – the amount of chlorine needed for disinfection will depend on the concentrations of these impurities.
To achieve effective disinfection, it is necessary to add sufficient chlorine to provide a free chlorine residual of at least 0.5 mg/L after a contact time of 30 minutes. This can be measured using a suitable chlorine test kit (for example, a swimming pool kit) if available.
As a general guide, the addition of 40 mL of liquid sodium hypochlorite (12.5% available chlorine) per 1000 L of water or 7 g of granular calcium hypochlorite (75% available chlorine) per 1000 L of water will give a reasonable assurance of effective disinfection. Both 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) is not effective in enclosed tanks and should not be used.
Methods for calculating the volume of water in a tank are provided in Appendix A.
The chlorine will not make the water unsafe to drink, but it will impart a distinct taste and odour that should dissipate in 10 to 14 days (depending on temperature). Boiling the water, or leaving water in an open jug in the fridge overnight, will remove most of the taste and odour associated with chlorination.
Calcium hypochlorite should be dissolved in rainwater, 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. When adding the concentrated chemical mixture to the tank, spread it as widely across the surface as possible to promote mixing (this will often be limited by restricted access) and let the water stand for at least one hour before use (or overnight if possible).
Note: When handling and storing chemical compounds, it is important to carefully read and follow the safety directions given on the package label, particularly advice in relation to the wearing of appropriate personal protective equipment (such as gloves or goggles).
Ultraviolet light irradiationUltraviolet (UV) light irradiation can be used to provide continuous assurance of water quality. UV light systems require relatively low maintenance and have the advantage of not involving the addition of chemicals. The UV light could be installed in pipework delivering water from a tank to a dwelling or selectively to taps used to supply water for drinking and food preparation. UV light systems could be particularly suitable for community supplies (see Section 11). It is important to note that typical UV doses for drinking water only provide limited inactivation of viruses.
If UV light irradiation is used, it is important to install a system incorporating a sensor that indicates when the device is or is not operational. UV lamps have a limited effective life and most need to be replaced after between nine and 12 months.
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BoilingWhile rainwater should be safe for most people to drink, at times the microbial quality may not be as high as mains water supplies. People 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, should talk to their doctor about potential risks and consider boiling the water before consumption. If gastric upsets are being experienced, boiling the water prior to use should also be considered.
Bringing water to a boil can disinfect rainwater (WHO 2008). Boiling does not have to be maintained for any length of time – kettles with automatic shut-offs are suitable for this purpose. Boiling water will kill any harmful bacteria, viruses and 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, allow water to cool then pour it back and forth from one clean container to another. Alternatively, let it stand for a few hours to increase the dissolved oxygen concentration.
Hot water supplies The use of rainwater to supply hot water systems has attracted growing interest. This water is used for household purposes such as washing and laundry. Irrespective of the source, water from hot water systems is not recommended for drinking and cooking. Vegetative bacteria, including those that cause enteric illness, are sensitive to heat and Pasteur’s original studies indicated that inactivation occurs between 55oC and 60oC.
The original low temperature method for pasteurising milk was holding it at 61.7oC for 30 minutes; this was increased in 1957 to 63oC to include inactivation of Coxiella burnetii, the causative agent of Q fever (Adams & Moss 2000).
Campylobacter, the most commonly detected cause of enteric illness in Australia, is particularly sensitive to heat with inactivation occurring at temperatures above 48oC. At 60oC (minimum storage temperature required for hot water storage by AS/NZS 3500 Part 4.2) 99% inactivation of Campylobacter, Salmonella and E. coli populations are achieved in minutes (D’Aoust et al. 1988; Feacham et al. 1983). The protozoa Cryptosporidium and Giardia are also susceptible to heat (Harp et al. 1996; WHO 2008).
In field testing in Newcastle, Coombes et al. (2000) demonstrated that although faecal coliforms, total coliforms and pseudomonads could be detected in rainwater storages, none were present in hot water samples. Total bacterial counts were also substantially reduced by heating. In one case an instantaneous heater was used. Although further investigation on the effectiveness of these units is needed, microorganisms are sensitive to rapid changes in temperature.
Where heating is used as a mechanism to reduce risks from microbial hazards, attention will need to be paid to reliability of heating systems in achieving required temperatures.
Corrective action to improve microbial quality
Uncertainty over microbial qualityAlthough there have been isolated reports of illness associated with consumption of tank rainwater, for most people rainwater from well maintained roof catchments and tanks represents a relatively low risk of illness.
Rainwater is not the most likely source of any of pathogens that cause gastrointestinal illness such as Giardia, Cryptosporidium, Campylobacter or Salmonella. Transmission of these organisms by person to person contact or contaminated food is far more common.
If it is suspected that rainwater is contaminated or if additional precautions are sought in the event of illness, water used for drinking and food preparation should be boiled or the tank rainwater should be disinfected.
Animal accessWhere a rainwater tank has become contaminated by a dead animal, such as a bird or rodent, it is recommended that the tank be drained and cleaned as soon as possible (see Section 5). If the animal is large, such as a possum or a cat, and badly decayed, impacts on taste and odour will be strong and distinct.
If it is not practical to drain and clean the tank immediately, carefully remove as much as possible of the animal carcass and then chlorinate the water as discussed above.
The point of entry for the animal should be located and repaired.