Benefits outweigh costs of rainwater harvesting

Published: Thu 29 Nov 2007 10:45 AM
Thursday, November 29, 2007
Benefits outweigh the costs of rainwater harvesting
With a long, hot summer forecast and drought conditions already in some regions, New Zealanders will increasingly depend on collecting their own rainwater as the effects of climate change become more apparent, says roof water researcher Stan Abbott.
Mr Abbott, Director of the Roof Water Research Centre at Massey's Wellington campus, predicts the number of New Zealanders either partly or fully reliant on water collected from their home's roof will increase.
Currently 400,000 – or one in 10 of us – rely on roof water for drinking. Most live in rural areas, beaches and offshore islands. The trend towards coastal living and lifestyle blocks will increase that percentage closer to that of rural Australia, where three million people rely on roof water as their sole source of water.
“The use of alternative water sources such as roof-collected rainwater is definitely part of the solution to diminishing water resources,” Mr Abbott says.
Local authorities are now encouraging householders in urban areas to install domestic rainwater tanks, not only as a mains-water saving measure but also to reduce the adverse effects of storm water runoff and to reduce flood risks.
City Councils such Waitakere, North Shore and Rodney offer rebates to householders who retrofit rainwater tanks to existing houses. Rainwater can be used as a secondary source for toilet flushing, washing clothes or in water heating systems, and outdoors for garden watering, car washing, or filling swimming pools, spas and ornamental ponds.
In Australia, which has been plagued by worsening droughts, there is huge demand for roof-collected rainwater, Mr Abbott says.
“Rainwater is becoming an important supplement to mains water supplies in urban areas. Authorities are encouraging more Australians to use rainwater, and in the past 18 months 147,000 rainwater tanks have been installed in Queensland alone.
“In some parts of Australia building consents for renovations or new houses are issued only if rainwater tanks are installed.”
Although the microbiological quality of rainwater collected in tanks will generally be poorer than that of many public mains water supplies, the health risks associated with contaminated roof-collected rainwater consumption are not well defined or quantified, Mr Abbott says.
“While occasionally there are reports of illness, the risk of disease arising from roof-collected rainwater consumption can be low, providing that the water is visibly clear, has little taste or smell and, most importantly, the storage and collection of rainwater uses a properly maintained tank and roof catchment system.
“Over the past decade there has been considerable research into how to keep rainwater safe. There is a wide range of products on the market that can prevent rainwater from contamination. These include devices such as gutter guards, downpipe debris screens, first flush diverters and filtration systems.”
Mr Abbott says design and installation preventive measures are vital, as well as ongoing maintenance. "Well-designed systems are low maintenance and will generally prevent problems.”
Preventive measures and corrective actions for safe rainwater harvesting include:
- Use a clean, impervious roof made from non-toxic material. Keep roof clean and clear of moss, lichen, debris and leaves.
- Remove any items containing lead products, such as paints, flashings or nails. Replace with approved materials.
- Keep roof clear of overhanging vegetation, as branches provide roosting points for birds and can provide access for rodents, cats and possums.
- Inspect gutters regularly and clean if necessary. Disconnect the pipe(s) that feed the water tank before cleaning the gutters, or install downpipe diverters. Be careful when cleaning gutters: make sure the ladder is secure and avoid going anywhere near overhead power lines, or better still have the power disconnected before cleaning the gutters.
- Install gutter guard screens to prevent gutters becoming blocked with debris or leaves.
- Ensure that chimneys near roof water collection areas are sufficiently high to minimise the settlement of ash or residues.
- Use a downpipe debris screens (rain heads) and first foul flush diverters to prevent contaminated water entering the tank.
- Clean gutter, tank inlets and screens every three to four months.
- In the event of any weed or chemical spraying nearby, advise the contractor that the roof is used to collect drinking water, and that there must be no over-spraying. Obtain a guarantee from the contractor that persistent organochlorine pesticides will not be used.
- Prevent access by animals, birds and mosquitoes into rainwater storage tanks by screening all tank inlets as well as overflows. Keep access hatches closed.
- Prevent entry of surface run-off from areas other than roof catchment into below-ground tanks. Tank roofs must be secure and the sides and bottom of the tank should be sealed.
- Inspect tanks annually. If necessary have tanks cleaned out professionally.
- If tank contamination is apparent the water may have to be disinfected or boiled before consumption and food and drink preparation.
- Ensure that tank taps or draw-off pipes are at least 100 mm above the tank floor. Alternatively use a floating arm draw off valve.
- Depending on the circumstances, additional water purifying equipment may need to be installed. These include a 20 µm washable cartridge filter, a UV steriliser, and a 1 µm activated carbon under-bench filter.

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