Sustainable Water

Back in Issue 25 in 2010 Graham Hepburn  reviewed the importance of water and the sustainable process of harvesting drinking water from metal roofs. This article is an update and review of what was written then. 

Rainwater Harvesting - increasingly relevant today
Nine years later and access to potable (and indeed any) water has become even more relevant in a world in which water supply is becoming a serious issue.  Even in rain-rich New Zealand we are increasingly talking about control of water, water rationing, depletion of aquifers, and so on.

We are also looking at more limited supply of water for urban uses.   As this is written, even after what seems like heavy rain, the Hunua Dams on which Auckland relies, have significantly less than desirable levels of water. 

There’s little doubt that water is one of the planet’s most valuable commodities and maintaining water supply will become even more important as earlier predictions about the dire consequences of global warming on water supply become true.

If the planet is going to be subjected to the extremes of flooding and drought, then security of  good quality water supply will be increasingly important.

Potable water
In rural areas with no mains supply there has been little choice but to harvest rainwater from roofs and collect it in tanks, and many rural households prefer this  source,  but in recent years suburban dwellers and even businesses have begun to get in on the act.  We are now seeing, belatedly, some farmers or farming regions talking about building reservoirs to gather water when it rains for use when it doesn’t.  (Seems common sense but apparently rare). This trend is partly due to greater environmental awareness but also to the fact that water is becoming an increasingly expensive commodity.   The charges for water in urban areas steadily increase year on year.   Some Councils have  also been  encouraging  home  owners and businesses – sometimes with financial incentives - to collect rainwater because this has twin advantages: it helps to reduce stormwater flows and alleviates some of the  pressure on water supply and water infrastructure from a growing population.

Of course there is a limit to how far this can go without affecting the economics or urban water supply and e.g. Watercare Services in Auckland prevent collection of roof water for drinking purposes.   In addition, the current Drinking Water Standards New Zealand (DWSNZ) do not allow the use of rainwater for potable uses where there is a potable supply available.

The announcement of a central government Water regulator will result in some changes.  This may or may not allow for a wider use of rainwater for potable use (probably with design requirements to be met – e.g. roof type and storage tank materials) along with forcing water authorities to treat all supplies.  The cost of water for small communities in rural New Zealand is likely to increase significantly.  Certainly, if no central government funding or area-wide rate base is available, many small communities will be unable to afford to comply with DWSNZ.  Therefore, we can’t help but think that changes in the DWSNZ allowing for rainwater to be used as a potable source in smaller communities should be an option.

The best roofing material for minimisation of the risks of contaminants in long run metal roofing, painted or unpainted.

Flooding mitigation
In urban environments with their proliferation of impermeable surfaces, stormwater during heavy downpours can cause surface flooding and overwhelm sewers (where there is cross-connection between stormwater drains and sewers, as there still is in Auckland), causing foul-water discharge into waterways. 

As climate change continues we will see increasingly irregular but heavy rain falls which will exacerbate this problem.

Collecting water and storing water off roofs reduces stormwater problems by attenuating the flood peak.   In Australian cities all new properties have been required to provide short-term on-site water storage, not to provide drinking water, but to prevent overwhelming the stormwater drainage systems.  Something we should be looking at in New Zealand?  Such urban collected water, while not reducing the need for potable water (and its revenue stream) can also be used for greywater and garden watering.

Collection and storage also helps to conserve this valuable resource and will reduce  the need for councils to build more dams or find other water sources. If you are providing your own water, then that also cuts demands on treatment facilities and pumping stations, which in turn means they will need to consume less energy. The individual owner of the storage therefore also uses less water and for those in metered connections will reduce their water cost. 

Reducing demand
Just harvesting rainwater for uses other than drinking drastically cuts demand on mains supply. It has been estimated that only 5 litres per person per day  is needed for cooking and drinking while 150 litres per day is used for bathing, washing dishes and clothes, flushing toilets, in the garden or for washing down cars etc. 
As New Zealanders have known for decades,  catching water off a metal roof for drinking and other household uses is easy and safe as long as some basic precautions are taken.

BRANZ says metal roofs are safe to collect rainwater from but a check should be made to ensure there is no lead, chromium or cadmium in the roof and its flashings or in any soldering or paint. Paints used by NZ coil-coaters have been demonstrated to produce no harmful runoff. 
The roof and gutters need to be cleaned regularly with diverters in place to make sure contaminants such as bird droppings that are being washed away aren’t entering the water supply.  And,
a first-flush diverter and debris diverters should be installed – this reduces the risk of contaminants entering the storage.  Treating roof water to potable level can be as simple as coarse filtering incoming and finer filtering and UV treatment before pumping to the house. (see the illustration)

After the NZ Green Building Council (NZGBC) introduced the Green Star building rating system for commercial and industrial buildings in the 1990s, it was realised that there is also a need to encourage sustainable homes, and there are schemes for such less complex buildings elsewhere in the world.
NZGBC developed the Homestar system to fill this gap.  Then after several revisions, in 2017 Version 4 was issued.  This contains a huge expansion in ways to obtain credits and specifically some features of the materials used, and now provides credits for using metal roofing to gather rainwater for much the same reasons as listed above.
The four criteria we consider particularly relevant to metal roofing are  WST-1 Construction Waste Minimisation (up to 5 points); MAT-1 Sustainable Materials (up to 10 points); and for houses with rainwater collected from a metal roof – WAT -2 Sustainable Water Supply (up to 4 points) and STE -1 Stormwater Management (up to 4 points).
This is covered in more depth in recent Scope article and of course can be found in huge detail in the Homestar Manual, so briefly -
Particularly relevant to water are :-
WAT-2 Sustainable Water Supply - Aim is “To encourage and recognise reducing a dwelling’s demand on water supplies through the collection and use of rainwater on and around the dwelling and by promoting responsible water use behaviour” ….  Clearly, collection of roof water for drinking applies specifically to this.
STE -1 Stormwater Management.  Aim - To encourage and recognise houses/sites that reduce stormwater run-off from buildings and hard surfaces, in order to mitigate flooding, pollution and stream erosion.  Specifically points are awarded where the stormwater associated with the roof is effectively managed on site with ………… or stormwater detention tanks.

And while not related exclusively to rainwater processing points for metal roofing can also be gained for WST-1 - Waste minimisation where use of metal roofing cut to length in the factory does minimise waste on site (assuming strippable film is taken away!).  And MAT-1 includes metal roofing as sustainably produced material

Pacific Coil Coaters and New Zealand Steel have tested their painting systems for the potential to release contaminants  and have shown that there are no contaminants released of any public health concern. Therefore, excluding other environmental factors, when you use COLORSTEEL® or Colorcote® pre-painted metal roofs for the harvesting of rainwater, you can rest assured that the product you are using will not contaminate the water.

Roll-forming of metal for roofing uses no water and  the manufacture of the steel coil from which metal roofing is made uses minimal water. As an example, New Zealand Steel’s plant at Glenbrook uses about 1 million tonnes of water a day in the steel making process but this is constantly recycled – cleaned, cooled and recirculated - so that only 1% of it is discharged and what is discharged is clean enough to drink.

Homeowners collecting drinking water and greywater replacement from metal roofs can do so knowing they are risklessly harvesting a renewable resource which can also help with urban stormwater flooding mitigation. Metal is the roofing material that is arguably the best suited material for rainwater collection, and this is recognised by the NZGBC Homestar rating system.