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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 to provide security of supply.

In this circumstance the size of the tank will depend on:

• the volume of water needed,
• the amount and pattern of rainfall,
• the area of the roof catchment,
• the security of supply required.

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. A number of water conservation measures could be applied including dual flush toilets, dry toilets (if permitted), water efficient devices, reduced flow shower heads, washing machines with suds savers etc. If necessary, a separate lower quality supply could be considered for uses such as toilet flushing.

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 in many rural areas there may also be storage requirements associated with firefighting. This may vary from one area to another.

Water demand will depend on:

• the number of people using the water,
• average consumption per person,
• the range of uses (drinking, cooking, bathroom, laundry, toilet etc),
• the use of water conservation devices.
• 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.

These volumes are being steadily decreased with the application of water conservation measures. Advice on water usage could be sought from the local water or water resources authority.

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.

Note that it is the flat or plan area (including eaves) that should be determined. The slope or pitch of the roof and the actual number of square metres of tiles or metal is not important.

The average roof area for a small house is about 100 m2, for a medium house about 150 m2 to 200m2 and for a large house over 250 m2.

The continued supply of water requirements under almost all conditions, including extended dry spells, will require a larger tank than that required to supply demand under normal or average conditions.

Some State Government Departments have tables of calculated tank sizes based on rainfall data. In Queensland this information has been available from the Department of Natural Resources, in South Australia from the South Australian Water Corporation or the Department of Environment, Heritage and Aboriginal Affairs and in Western Australia from the Department of Agriculture. Other departments with responsibilities for water resources or water supply may also provide this information.

Suggested Tank Size

By far the most common question faced when purchasing a water tank or looking at a new water tank design is – what size of water tank is needed?

For the average home and average water needs, a water tank around the size of 1000Lt to 3000Lt is usually more than sufficient. The most common water tank design found today is the plastic or polymer water tank, which can be found in the traditional round tank, with many water tank designers moving into rectangle or square designs.

Larger requirements such as extensive gardens and the requirement to water your lawns will require a much larger water tank, possibly up to or exceeding 10,000 litres.

Maximum Amount of Collectable Rain Water

If you wish to ensure that you don’t waste any of the rainfall you can apply known formulas to work out the maximum amount of collectable rainwater and then from that you can calculate the required tank size to ensure you collect every drop.

Once the area of roof catchment has been determined and the average rainfall has been established the maximum amount of rain that can be collected can be calculated using the formula:

Run-off (litres) = A x (Rainfall – B) x Roof Area

A = is the efficiency of collection and values of 0.8-0.85 (ie 80-85% efficiency) have been used.

B = is the loss associated with adsorption and wetting of surfaces and a value of 2 mm per month (24 mm per year) has been used

Please Note that Rainfall should be expressed in mm and Roof Area in square metres (m2)

Calculate the Size of Tank Required

The next step is to calculate the size of the tank. The tank needs to be large enough to ensure that:

• the required volume of water can be collected by the tank.
• the volume of water in the tank will be sufficient to meet demand during the drier months or through periods of low or no rainfall.

The simplest way of checking a tank size estimated to provide water throughout an average year is to use monthly rainfall data and to assume that at the start of the wetter months the tank is empty. The following formula should then be used for each month:

Vt = V t-1 + (Run-off – Demand)

Vt = theoretical volume of water remaining in the tank at the end of the month

V t-1 = volume of water left in the tank from the previous month.

Run-off should be calculated as discussed above (A = 0.8, B = 2 mm).

Starting with the tank empty then Vt-1 = 0. If after any month Vt exceeds the volume of the tank then water will be lost to overflow. If Vt is ever a negative figure then demand exceeds the available water. Providing the calculated annual run-off exceeds the annual water demand, Vt will only be negative if periodical overflows reduce the amount of water collected so that it is less than the demand.

Tank size is not necessarily based on collecting total roof run-off. If the water demand is less than this some overflow may occur while demand is still met. If water demand is to be met throughout the year the tank should be large enough so that Vt is never negative.

Calculations should be repeated using various tank sizes until Vt is = 0 at the end of every month. The greater the values of Vt over the whole year, the greater the security of meeting water demand when rainfalls are below average or when dry periods are longer than normal.

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In some rural areas there may be requirements associated with supply of water for firefighting.

Connection to the Town Water Supply

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

Contamination Risks

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

Underground Tank Location

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.

Access Points

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.

Sunlight

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

First Flow/Flush Divertors

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 (Yaziz et al, 1989). 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.

Water Tank Overflow

The inlet pipe to all rainwater tanks should be easily detachable so that when required the tank can be bypassed.

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 stormwater drain; it should not be allowed to pool or to cause nuisance to neighbouring properties or to areas of public access. Local authorities may have regulations or requirements that apply to diverted or excess rainwater flows.

Inlet Pipes

Wherever possible all sections of inlet pipes should be directed down and rainwater should flow into the top of the tank. The inclusion of rising sections will provide potential traps for sediments, biofilms and stagnant water and these should be avoided. Modifications to existing downpipes should not restrict existing water flows from roof gutters. To maximise
the collection of rainwater the downpipes should be of sufficient diameter to accept all of the water flow from roof gutters, even in heavy rains.

Roof Catchment Areas and Gutters

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 downpipes 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.

Multiple Rain Water Tanks

Tanks of different sizes and shapes can be interconnected to enable the use of one water pressure pump, resulting in a big cost saving. Additional tanks can be installed and connected at any time to an existing system.

Modular tanks are specifically made to add new tanks as your demand increases.

Compulsory Water Tank Installation

State Governments and Local Authorities are implementing legislation that will require every new home to have a water tank installed. If youre building a new home, check with your builder about local legislation.

Water Tank Stands

When considering a stand for your water tank you need to take into consideration the size of the size of the tank and how much it will weigh when full, as the tank stand has to be able to support the weight.

1 litre of water weighs 1 kilogram which means that a small 3000 litre tank will weigh 3 tonne and even a medium size 10,000 litre tank will weigh 10 tonnes when full.

With this sort of weight involved it is important to correctly install or construct your tank stand.

Different types of tank stands include:

• Concrete Slab
• Corrugated steel ring filled with crusher dust
• Crusher dust base with support edging
• Precast concrete slabs
• Timber and steel tank stand. Please note, that because of the weights involved these generally require an engineer design and building approval.

Three points of caution:

• Generally sand should not be used as a tank support.
• Before proceeding with an elevated stand, check with your local authority as regulations differ between areas.
• Consider how much weight the stand will have to support.

Where to Install a Water Tank

A water tank can be a rather large and bulky item depending on the quantity of water required, and the location of the tank affects the tanks ability to collect water. Depending on the size and shape of the selected water tank, it is best to locate it as close as possible to an existing downpipe.

A tank up to 5000 litres generally requires only one down pipe provided it drains a suitable roof area. Larger tanks perform better with multiple downpipes attached. If attaching multiple downpipes, check if access is available to the tank. Do you have an existing concrete or paved level area on which to install your water tank?

Access for the delivery of the tank also influences the type of tank installed. If your tank will be behind buildings, consider the access space between buildings and fences. If access is difficult, the tank can be delivered via a neighbouring property, and sometimes a crane will be required.

Your choice may be determined by access. Due to the many varying sizes of water tanks it is easy to find a tank size that will fit just about anywhere. Make sure your supplier has a range of options rather than offering a One tank fits all solution.

The life of a tank is dependant upon many factors and a little care in the choice of the location of the tank can often mean a difference of many years. Whenever possible tanks should be installed in a cool position, preferably on the southern side of a building, away from trees from which leaves might clog the strainer or contaminate the water.

Direct sunlight falling on a tank for long periods on a hot day can create temperature differentials in the water. This can affect the performance of the tanks. All overflow water must be piped away from the tank base regardless of the tank type.

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Water Tanks Require Cleaning

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

As per Section 6 the catchment area should be kept clear of debris. The roof should be washed clean once or twice a year and in particular should be cleaned toward the end of the dry season. Gutters should be cleaned regularly to remove leaves, collected dirt etc.

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 can provide an environment for survival and/or growth of micro-organisms and in some cases relatively high concentrations of lead have been detected in sludge even though the body of stored rainwater complied with drinking water guidelines.
Outlet taps are generally near the bottom of tanks and as sludge accumulates the likelihood of material being re-suspended and being removed with rainwater increases. Concentrations of lead exceeding the Australian Drinking Water Guidelines (NHMRC/ARMCANZ, 1996)
have been detected in water containing barely visible particulates of sludge that were resuspended by water flowing through the outlet tap.

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 coarser debris are present in the sludge, a siphon hose of approximately 50 mm 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 a licensed 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 and 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. Rinse water and sediment should be run to waste.

Discarded water should be diverted away from tank foundations, buildings and other structures.

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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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Colorbond, zincalume and galvanised steel sheeting; asbestos/fibro cement; polycarbonate or fibreglass 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.

NOTE: Under AS 2180 – 1986; Section 1.11 – “on completion of the installation of all guttering and roofing, all debris such as cement mortar and in particular, metal clippings and filings shall be removed”.

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.
Asbestos/fibro-cement roofing

Please refer to our seperate guide on Asbestos Roof and a Water Tank

Cement based or terracotta tiles

The coloured surface of cement based or terracotta 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 manufacturers 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 run-offs should not be collected.

Bitumen-based (tar) materials – are generally not recommended as they may leach hazardous substances or cause taste problems.

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:

water-based preservatives such as copper chrome arsenate compounds and boron compounds
oil-type or oil-based preservatives such as creosote
light organic solvent preservatives such as solutions containing pentachlorophenol. If wooden shingles have been used it will be necessary to ensure they have not been treated with chemicals that may taint the water.
Lead flashing

Lead is a cumulative poison that can have a number of adverse effects including serious impacts on the central nervous system. In utero exposure of fetuses and exposure of infants are of greatest concern.

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.

Industrial pollution

There is an increased risk of pollution by airborne contaminants in major urban centres 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.

In Port Pirie, South Australia, for example, there is a long history of smelting with associated airborne lead pollution and analyses of rainwater collected in domestic tanks have detected concentrations of lead exceeding drinking water guidelines.

When in doubt about the possible impact of local industry advice should be sought from the local authority, environmental health authority or environment protection authority.

Agricultural pollution – pesticides

Although the use of pesticides is coming under increasing scrutiny and control, rainwater collected in some agricultural areas may be subject to contamination by pesticides or other chemicals. Aerial spraying provides a greater potential for contamination.

In surveys of rainwater quality in rural areas pesticides were not detected in most samples. Endosulfan, profenofos, chlorpyrifos and dieldrin have been detected in some samples but all at concentrations below Australian Drinking Water Guidelines. There was evidence that tanks included in the surveys were poorly maintained. The risk of pesticide contamination could be reduced by appropriate maintenance and by the use of first flush diversion devices.

If in doubt about the use of pesticides in the general area, advice should be sought from the local authority, Department of Agriculture, environmental health authority or environment protection authority.

Wood burners

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.

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To prevent mosquito breeding and contamination, its vital guttering and pipework should be self-draining or fitted with drainage points.

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 0.315 mm diameter material and 6×7 mesh openings per cm2. The screens should be readily accessible for regular cleaning.

Water should not be allowed to pool under the overflow outlet or tap of the tank as these can become mosquito breeding sites. The tank should be a sealed unit with the lid preventing sunlight from reaching the water. Sunlight encourages the growth of algae that may taint the water.
A good tank design prohibits vermin from entering the tank by ensuring all the inlets are covered with screening too small to allow them in, however holes and spaces created by you after or during installation could allow mosquitoes to enter, so be careful.

Mosquito larvae (wrigglers) found in rainwater tanks indicate the presence of an opening through which the female mosquito can enter and lay eggs on the water. The opening should be closed. This will prevent further entry and will also prevent the escape of any hatched mosquitoes.

Larvae present in rainwater can be treated by the addition of a small quantity of domestic kerosene (5 mL per 1000 litres). Commercial or industrial kerosenes (eg power kerosene for tractors etc) should not be used for mosquito control.

Kerosene should not be used in tanks coated with AquaplateTM. In addition, kerosene may not be suitable for use in tanks constructed or lined with plastic. If in doubt consult the manufacturer.

Used carefully kerosene will not provide any risk to human health but excessive amounts can taint the water and it is a human poison. Kerosene should not be added when water levels are low. Kerosene added to the surface will not mix through the body of rainwater in the tank and it will either be washed out of the tank by overflow or it will evaporate.

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Where a rainwater tank has become contaminated by a dead animal such as a bird or rodent, carefully remove as much as possible of the animal carcass and then disinfect the water.

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.

It is also vitally important that the point of entry for the animal should be located and repaired/sealed.

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