Raised awareness of limited potable water reserves, frequent periods of drought, and authority imposed water restrictions has resulted in an ever increasing number of councils mandating the collection of rainwater for re-use in new developments.  This has caused much confusion on volumes of water to store, its application and suitable treatment.

As part of a DA to collect and re-use rainwater, clear direction is given on water application, but little reference is made to water quality.  Apart from the National Drinking Water Guideline, other guidelines have very little to offer with regard to the water quality.

Roof water can be used for drinking, recreational purposes, irrigation, toilets, washing machines, and cooling tower makeup.  If we come into contact with these applications, post treatment should be necessary.  Available guidelines are largely directed to applications of a single dwelling - it is not advised to use rainwater for drinking where mains drinking water supply is available.  Statistics show 11% of Australians, mainly in rural areas, use rainwater as their primary source of drinking water, and the responsibility for the quality of drinking water rests with property owners.  Community-based systems require a higher level of management due to exposure to many people.  Many forms of contaminants are found in rainwater tanks - E.coli, faecal coliforms, lead and other metals, other micro organisms, virus', parasites and bacteria.  Guidelines stipulate water should be free from all harmful contaminants, and the more contact we have with the water the better the water quality, yet post treatment is not needed for roof water supplying non-drinking water ‘subject to adequate maintenance'.  Guidelines also indicate ‘a 100-150 micron filter could be considered', however fixtures and fittings such as toilet cisterns and washing machines have in-built strainers rated at 50 micron.  

Research has found that basic maintenance and first flush diverters are not sufficient to eradicate harmful contaminants.  Other factors for consideration are:

• Location and type of above ground tanks - direct sun provides ideal temperature conditions to promote the growth of bacteria, micro organisms etc in plastic tanks;
• Roofing, guttering, and flashing materials can contaminate water with harmful metals.  Resulting electrolysis can create a water condition detrimental to Plumbing systems, fixtures and fittings;
• Collected rainwater in urban areas is generally low in pH which can also be detrimental to the tank, piping, fixtures, fittings and water heaters.

GEORGE'S TIPS

1. Ask your hydraulic services consultant to provide you with the percentage of contribution of harvested rainwater.
2. Collect rainwater from roofs, avoiding paved areas and garden beds.
3. Do not site above ground tanks in constant sun- or provide a shade structure.
4. Advise the rainwater user to clean roof gutters on a r egular basis and monitor the condition of the stored water.
5. Size of storage tanks - take into account long dry spells experienced in Australia, eg, 10,000L of stored water on a commercial development site of 10,000 m2 when used for irrigation purposes may be utilised within one week.