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Australia has a generally hot, dry climate and outside the major towns and cities there are vast areas with very low population densities. Water is a valuable commodity and as distance from the major centers increases the supply of good quality reticulated water becomes increasingly more difficult. Many remote areas are not supplied with mains water. Rainwater tanks can provide a relatively safe, soft, clear and odourless source of water that can be used for a range of purposes including drinking, washing, bathing, laundry and gardening. Rainwater can be collected to supplement existing supplies or as the sole source of household water.

Even in areas that receive good quality mains water there is support for the use of rainwater based on conservation and a recognition of the limited fresh water resources in Australia.

The perception is that rainwater is safe to drink and this is true if it is clear, has little taste or smell and importantly that the source of the rainwater is a well maintained tank and roof catchment system. However, while the use of rainwater might be common the application of maintenance procedures is far less common. In part, this may reflect the notion that rain is a relatively pure source osf water and also it may be related to the fact that in many rural areas the availability of water is a bigger issue than quality.

It should be noted that inadequate maintenance was a common theme encountered in surveys of rainwater tanks. 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 or rainwater, particularly if used for drinking or cooking, testing may be necessary.

Roofs may be constructed from a variety of materials such as cement or terra cotta tiles, Colourbond™, zincalume and galvanized steel sheeting; asbestos/fibro cement; polycarbonate or fiberglass sheeting; and slate. Regardless of the roofing material used, with new roofs it is advisable to divert the first few substantial run-offs away from the rainwater tank. This is done to allow dust and other debris left on the roof after construction to be washed away.

Where concerns exist about the suitability of a roofing material for use as a rainwater catchment surface, advice on the matter should be sought from the manufacturer.

CEMENT BASED ON TERRACOTTA TILES

The coloured surface of cement based or terra cotta tiles will oxidise over time through natural weathering. This oxidised coating may break down slowly and be washed into rainwater tanks, thus colouring the water.

The coating has been found to be non-toxic and, if left undisturbed, will settle to the bottom of the tank. The colour may reappear after rain if settled material is stirred up by water flowing into the tank. If this becomes a problem the tank should be drained and cleaned and roof and gutters washed down and cleaned to prevent oxidised material entering the rainwater tank.

It is possible to purchase colour-through tiles that have colour impregnated throughout the tile and which may be subject to less oxidation.

PAINTS AND COATINGS

Before purchasing materials or paint to be used on roofs that will be used to collect rainwater for drinking, read and observe the manufacturer's recommendations on labels and brochures. Look for warnings. If in doubt check with the manufacturer.

Lead-based paints (including primers) - are toxic and not suitable for use in association with the collection of rainwater for human consumption.

Acrylic paint - will leach dissolved chemicals including detergents in the first few run-offs after application and these runoffs should not be collected.

PESTICIDE-TREATED WOOD

The use of pesticide-treated wood should be avoided where it may come into contact with rainwater to be collected in a domestic tank. Chemicals used in the preservation process may be washed from the treated wood into the tank. Examples of timber preservatives used in Australia are:

oil-type or oil-based preservatives such as solutions containing pentachlorophenal.

If wooden shingles have been used it will be necessary to ensure they have not been treated with chemicals that may taint the water.

As a precaution lead flashing should not be used on those parts of a roof used as a rainwater catchment area. In the case of an existing roof, lead flashing should be replaced if possible. Where small quantities of lead flashing are used or retained it can be painted over with a suitable roof paint. Coated lead flashing is also available.

Leaching of lead into roof run-off may be more of a problem on poorly maintained roofs and gutters where the process may be increased by the acidic action of water containing organic substances leached from materials such as leaf litter.

There is an increased risk of pollution by airborne contaminants in major urban centers and industrial areas. Collection of rainwater for human consumption (drinking and cooking) in areas affected by heavy traffic, industry, incinerators and smelters is not recommended.

There have been consumer reports of contamination, including tainting with creosote, of rainwater collected from roofs that incorporate flues from wood burners. While a range of hydrocarbons could be deposited on roofs in the vicinity of such flues limited testing in one survey (Victorian Department of Natural Resources and Environment, 1997) did not detect concentrations exceeding guideline values in the Australian Drinking Water Guidelines. Nevertheless, it is suggested that where possible rainwater should not be collected from that part of a roof that incorporates a flue from a wood burner. In addition extra care should be taken to operate wood burners correctly and to only use appropriate fuel.

Many mains water supplies are dosed with fluoride to provide protection against dental caries. This practice is supported by the NHMRC (1991). Rainwater collected in domestic tanks will not contain fluoride but it is not recommended that tank water should be dosed. Alternative sources of fluoride include fluoridated toothpastes and the use of fluoride supplements. Where rainwater is used as a major source of water for drinking and cooking, advice concerning fluoride should be sought from the local dentist, school or community dental service or from the Australian Dental Association.

In those cases where a rainwater tank is to represent a supplementary source of water, the size of the tank will depend on a balance of cost weighed against the range of uses required (drinking, cooking, bathroom, laundry, toilet etc).

If the rainwater tank is to represent the only source of potable or domestic water, cost will be less important than the size of tank required. In this circumstance the size of the tank will depend on:

The amount of rain combined with the area of the roof catchment will determine the maximum volumes of water that can be collected. If this is not sufficient then either a greater catchment area will be required (eg. garage or shed) or alternatively water demand will need to be reduced.

If after the implementation of these measures the volume of rainwater that can be collected is not sufficient to meet demand, additional water will need to be obtained from another source.

Experience is always a useful guide and advice should be sought from neighbours particularly in areas where reliance on rainwater tanks is common.

It should be noted that in some areas local authorities specify minimum requirements for water storage and there may also be storage requirements associated with firefighting.

In areas supplied with reticulated water the average indoor use per household is estimated to be in the range of 400-740 litres per day or alternatively 100-200 litres per person per day.

In general the most accurate source of this information is the Bureau of Meteorology. In addition to average rainfall (annual and monthly) it is important to determine the season of the rainfall and the occurrence and length of dry spells.

Calculate the area covered by the parts of the roof from which the water is to be collected. The average roof area for a small house is about 100 m2, for a medium house about 150 m2, and for a large house about 200 m2.

How much water can be collected off the average home roof? When 1mm of water falls onto 1sq. metre of roof it collects 1 litre, so if it rains 25mm per hour for 2 hours then 50 litres per sq. metre will result. Therefore, 200 sq. metres of roof are would have collected enough water to fill a 10,000 litre tank!

Formula 1: Approximate volume (in litres) = 0.8 x water height x diameter2 ÷ 1000

Formula 2: Approximate volume (in litres) = 0.08 x water height x circumference2 ÷ 1000

Before purchasing or installing a rainwater tank it is important to establish whether there are any local health, building or planning regulation associated with rainwater tanks. The local council or regional authority(ies) with jurisdiction over these regulations should be consulted. In some areas there may be requirements associated with supply of water for firefighting.

Water authorities usually do not allow direct connection of rainwater systems with reticulated water supplies or alternatively require the use of back flow prevention devices to stop rainwater siphoning back into the reticulated supply. If in doubt information should be sought from the local water authority.

Rainwater tanks should be installed in a manner that will minimize the risk of contamination from industrial pollutants, dust, leaves, pollens, pesticide sprays, fertilizers, debris, vermin, birds, small animals and insects. Tanks should not be allowed to provide breeding sites for mosquitoes.

Underground tanks require additional protection against entry of surface run-off or groundwater, animal or human faecal material and soils. These tanks need to be properly sealed and access points need to be protected against ingress of surface run-off. Maintenance and cleaning of underground tanks may be more difficult.

Tanks should have impervious covers and all access points except for the inlet and overflow should be provided with close fitting lids which should be kept shut unless in use. The inlet to the tank should incorporate a mesh covering and/or strainer to prevent the access of mosquitoes and other insects and to prevent material such as leaves etc. that may have collected on the roof or in gutters from being washed into the tank. The overflow should also be covered with an insect-proof mesh.

Tanks should be light proof to minimize algal growth. Most algae will not make water unsafe for human consumption but can adversely affect the taste, odour and appearance of the water.

After any extended dry period it is good practice to let the first run-off of rain bypass the tank. This first rain will wash the roof catchment and may contain higher than average amounts of accumulated dust, bird and animal droppings, leaves and other debris. It has been shown for small roofs that the water quality improves once the first 5 litres of water has passed through the down-pipe from the roof guttering. For an average roof catchment it is suggested that the first 20-25 litres should be diverted/discarded.

First flush diversion devices are commercially available and the installation of such a device should be considered as a means of improving water quality collected in tanks.

Run-off that is not collected in the tank or that overflows should be diverted away from tank foundations, buildings or other structures. This water should be directed onto gardens or into the storm water drain; it should not be allowed to pool or to cause nuisance to neighbouring properties or to areas or public access. Local authorities may have regulations or requirements that apply to diverted or excess rainwater flows.

Where ever possible all sections of inlet pipes should be directed down and rainwater should flow into the top of the tank. Modifications to existing down pipes should not restrict existing water flows from roof gutters. To maximize the collection of rainwater the down pipes should be of sufficient diameter to accept all of the water flow from roof gutters, even in heavy rains.

Roof catchment areas should be kept clear of overhanging vegetation to reduce the amount of leaves and debris falling onto the catchment area. Overhanging branches also provide access to the roof for rodents, cats and possums and can provide roosting points for birds.

Gutters can be shielded from large debris (bark, larger leaves, etc) with plastic mesh but smaller particles will still accumulate and require regular removal.

Gutters should have sufficient and continuous fall to down pipes to prevent pooling of water which could increase accumulation of material, lead to algal growth and possibly provide a site for mosquito breeding. A fall of one in a hundred to one in five hundred should be sufficient.

Overflow, discharge or bleed-off pipes from roof-mounted appliances such as evaporative air conditioners, hot water services, and solar heaters should not discharge onto the rainwater catchment area.

The primary focus of maintenance procedures should be to keep all components clean and to minimize the risk of contamination/rubbish either entering or remaining in rainwater tanks.

In areas subject to large amounts of wind-borne dust the roof and gutters may need to be cleaned more frequently.

Inlet screens and first flush/bypass devices should be cleaned regularly and kept in good repair.

All tanks should be examined for the accumulation of sludge every 2-3 years, or if sediment is evident in the water flow.

Sludge may be removed by siphoning without emptying the tank. To do this, use an inverted funnel in the end of a hose and move it carefully across the bottom of the tank. The sludge plus the lower portion of water in the tank can then be released to waste. If leaves and courser debris are present in the sludge, a siphon hose of approximately 50mm diameter should be used.

Sludge may also be pumped from the tank with minimum loss of water by using a suitable motor operated pump and attachments.

Finally sludge can also be removed by draining and cleaning the tank. If a drain plug is provided at the base of the tank, water can be run to waste to discharge the sludge. Once the tank is empty, the remaining sludge can be scooped up and removed through the access opening. Care should be taken not to disturb the protective film on the inside surface of steel tanks.

Professional tank cleaners (generally listed in telephone directories) may also be available to de-sludge tanks.

Organic material removed from the tank may be disposed of in the garden by spreading and digging into garden beds. Alternatively sludge should be disposed of at the license waste depot.

Where cleaning necessitates entering the tank, care should be taken to ensure adequate ventilation is provided and an additional person is in attendance. Advice on working in confined spaces should be available from Occupational, Health, Safety & Welfare authorities in each State and Territory.

It is important to check the structural condition of the tank before choosing a method of cleaning. Harsh cleaning methods may accelerate deterioration. In the case of a steel tank, removal of the protective layer on the inside walls will lead to tank corrosion.

Cleaning agents that might release hazardous fumes or adversely affect water quality after cleaning should not be used. After cleaning it is recommended that the internal walls and floor of the tank be rinsed with clean water.

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.

Where rainwater has become contaminated by a dead animal such as a bird or rodent, carefully remove as much as possible of the animal carcass. If the animal is large such as a possum or cat and badly decayed, the taste of the water will probably be affected. It is recommended that the tank be drained and cleaned as soon as possible.

Rainwater tanks can provide very good habitats for mosquito breeding. The most effective control measure is to prevent access of adult mosquitoes. If access has occurred remedial action can be taken to prevent the release of mosquitoes.

Ensure that unless in use all access points excluding the inlet and any overflows are kept shut with close fitting lids that will prevent access of mosquitoes. Inlets and overflows should be covered with closely fitting removable insect-proof screens. The screens should be made of non-rust material formed, typically, with 0.315mm diameter material and 6x7 mesh openings per cm2. The screens should be readily accessible for regular cleaning.