Why collect rainwater in tanks?
Rainwater in most areas of Western Australia has very low salt levels and is low in other contaminants. Rainwater collected from run-off surfaces that do not contaminate the supply is a valuable resource. Obvious surfaces that meet this criteria are roofs on buildings on your property. The proportion of rainfall that can be collected from most roofing is very high.
Drinking water and other domestic uses require the highest quality water and most of the information here relates to those uses. Water of this quality can be used in households, for livestock and for spraying.
Design your roof-raintank system properly
Design roof-raintank systems to take the best advantage of the available roof area from which water can be collected and diverted into storage tanks. You can easily calculate most roof areas by measuring the ground-floor size (allow for roof overhang) of the buildings on the properties, including the house and garage, machinery sheds, hay sheds or other buildings that have a sizeable roof area.
Design guttering and downpipes to cope with expected peak run-offs. For larger buildings (in excess of 300 square metres), it may be more appropriate to install a rainwater tank at opposite corners or sides of the building rather than divert water through combined guttering to a single tank. Multiple downpipes and tanks can cope better with run-off from heavy, prolonged rainfall.
Discharge the overflow from several small tanks near buildings into a larger tank placed in a more suitable position. Design the combination of tanks to provide adequate storage to meet expected demand.
Keep your water safe
Rainwater quality starts to change as soon as it hits your roof and it can be contaminated before it comes out of the tap. For any health-related watertank concerns, please view the Department of Health's Water tanks on your property page, or contact the Department of Health.
Use safe roofing materials for household water quality
Safe materials include:
- terracotta tiles
- galvanised iron
- fibreglass sheeting
Do not use unsafe roofing materials for household water quality
Unsafe materials and conditions include:
- chemically treated wood (copper chrome, creosote or pentachlorophenol)
- bituminous products
- lead-based paints (including any gutters or flashings)
- the roof near a chimney from a wood burner
- roof with discharge pipes from roof-mounted appliances, such as evaporative air conditioners or hot water systems.
Surface deposits that remain after manufacturing will be present on most new roofs, gutters and tanks, so allow the run-off from the first few storms to run to waste.
Lime will leach from concrete tiles, asbestos cement roofing and concrete tanks, causing some increase in hardness and alkalinity of rainwater. This lime will generally disappear after the first 12 months. The lowered water quality during that time is usually safe to drink, but have the water tested if you have any concerns.
Is asbestos unsafe?
It is safe to collect rainwater from asbestos or fibro cement roofs. There is no evidence to suggest that asbestos-related diseases occur from drinking this rainwater. However, asbestos has other health risks. Please refer to the Department of Health for more information on asbestos risks.
Prevent contamination of the water
A little preventive work will save a lot of time later to keep tank water clean:
- Use gutter-guards or filters to keep leaves, animals, birds and insects out of the intake and the overflow.
- Exclude sunlight from the raintank as much as possible to minimise the growth of algae. Inspection and access points should have tightfitting lids.
- All roofs collect debris, dust and bird droppings, so use a device that diverts the first flush or run-off from the roof to drainage (not into the tank). Refer to the Department of Water's Rainwater storage and reuse systems publication for a design.
Fires and ash contamination
Divert the first runoff after fires that may have dropped ash on your collecting surface.
Maintain a clean tank
If the tank has an effective inlet strainer and lids, only a small amount of maintenance is needed:
- Keep gutters clear of leaves, dead animals and birds; check every 3 months or more often if trees overhang.
- Clean the inlet strainer whenever necessary.
- De-sludge the tank every 2 to 3 years, or as necessary. With galvanised iron tanks, do not disturb the film that builds up on the wall inside the tank, because it protects the metal from corrosion.
De-sludge your tanks regularly
You may need to de-sludge from inside the tank – this is a confined space with little ventilation and can be a health hazard. Consider using a professional tank cleaning service.
Cleaning a dry tank that collects water from an asbestos roof is potentially hazardous: get advice before cleaning (see the note above on asbestos). Keep the inside walls and the sludge wet while removing sludge to minimise the chance of inhaling dangerous asbestos fibres.
Control mosquitoes in the roof–raintank system
Mosquitoes can be carriers of diseases and they can breed in raintanks. Fully seal the tank and cover the inlet and overflow pipes with a fine mesh to prevent mosquito access. A film of food-grade liquid paraffin on the water will prevent mosquitoes breeding if they are present in the tank water. Put a tablespoon of food-grade liquid paraffin oil per 10 square metres of the water surface (about 10 millilitres per 20 000 litres of tank capacity) at the end of the winter rains and again if the water overflows. Check and add more liquid paraffin during the summer as needed.
Control microorganisms in the tank
If the tank is the only house water supply and cannot be cleaned out regularly, treat the water with chlorine, and/or boil all water for human consumption. The best arrangement is to have 2 tanks and clean them out alternately.
Chlorinate the tank if in doubt
If you suspect bacteria have contaminated the water in your rainwater tank, add chlorine to the water.
The initial dose to treat the contamination should be 7 grams of calcium hypochlorite (60–70%) per 1000 litres, or 40 millilitres of sodium hypochlorite (12%) per 1000 litres of water. Stir the water then let it stand for at least 24 hours to allow the chlorine taste and smell to dissipate. This dosage will disinfect the tank.
- Use either granular calcium hypochlorite or liquid sodium hypochlorite; do not use both at the same time.
- Calculate the amounts of chemical to add based on the volume of water in your tank at the time, not the total capacity of your tank.
- Do not use stabilised chlorine (cyanurate).
- Allow a minimum of 2 hours before drinking.
- Mix the chlorine in a plastic bucket in the open air before adding it to the tank.
- Always add chlorine to water; do not pour water onto chlorine.
- Always follow the instructions on pool chlorine containers in relation to safe handling and disposal of the product.
Do you need fluoride?
There is no fluoride in rainwater. Dental authorities recommend that children who drink only rainwater should have fluoride tablets to help prevent dental decay.
How much water can you capture and what do you need?
The amount of rain that you can capture depends on your rainfall, roof collecting area and storage capacity.
Water use varies significantly between houses, depending upon the number of people per household and the number, type and frequency of use of particular household appliances. To minimise water use, install water-saving systems: front-loading clothes washers use less water than top-loading types; dual-flush toilet cisterns often use less water than single-flush (9 litre) cisterns. Toilets and gardens can use lower quality water from dams or bores.
The calculation to estimate your reliable annual rain collection potential (reached 9 out of 10 years) is:
Decile 1 average annual rainfall (millimetres, mm) x 0.8 x roof area (square metres, m2) / 1000 = x (kilolitres [1000 litres] per year, kL/y)
For example, the reliable annual rain collection from a farm house in the Narrogin area with a roof collection area of 200m2:
354mm x 0.8 x 200 / 1000 = 56.64kL/y
Indoor water use in an average situation is about 150kL/y (assumes 2 people). This means that additional water supplies would have to be used and other water would be needed for gardens and other outdoor use. To supply all indoor water use, you would need about 424m2 of roof collection and adequate storage.
Use a spreadsheet calculator to estimate run-off and tank storage needed
The Department of Planning, Lands and Heritage has developed a simple rainfall calculator for roof catchments that uses your roof area, rainfall and expected run-off yield. This calculator estimates the run-off generated from your roof area and uses your expected water use to determine the tank storage needed. Use the calculator to evaluate the existing system's design.
The calculator uses daily rainfall data (for the period set by the user) and a simple water balance model to simulate run-off collection and drawdown based on the estimated or calculated demand. The program is not all-inclusive; it does not allow for:
- variability in demand during the year and is based on using the average daily demand
- roof design, coating (e.g. tiles, corrugated iron) or condition
- tank maintenance
- poor or inadequate guttering and piping
- other problems that might affect the yield of the roof–raintank system.
The program does provide an indication of what might be required at the designated location. It is the user’s responsibility to evaluate results and apply the knowledge gained from this tool with due consideration to expected site circumstances.
Make sure that all fittings are safe for drinking-quality water
Roof run-off for a domestic water supply can be stored in tanks constructed from several materials:
- galvanised steel
Check that all parts of the roof–raintank system are suitable for holding drinking water.
Calculate crop spray requirements
The advantage of rainwater for crop spraying is that it does not have problems with pH, salinity, hardness, muddiness or organic matter content. The disadvantage of using rainwater from a tank is that the main use will be at the beginning of the rain season over a short period, and little or no refilling is possible.
To calculate the water requirements for spraying, use this equation:
Water per spray (litres per hectare, L/ha) x number of sprays x number of hectares = total spray water requirement (L)
An example could be:
50L/ha x 3 x 1000ha = 150 000L
In the Narrogin area, this would need more than 400m2 of roof collection (probably from sheds) and a tank of that volume, collecting water from the previous season.
Caution on connecting tanks and mains water supply
Direct connections of mains water supplies to rainwater tanks are not permitted to protect public health and the security of public water supplies. However, you can use mains supply with a control valve and an approved physical air-breaktanks to connect to a raintank.
Employ a licensed plumber familiar with requirements and the need for use of authorised materials to make such connections.
Contact water-plumbing inspectors for more advice.
Information on water quality is largely from the Department of Health's Water tanks on your property publication.