Harvesting rain for more sustainable water use Print this article
The water shortages caused by the extreme weather events of late 2009 and early 2010, in Ireland, highlighted to many people just how vulnerable our mains water supply network is. Tony Cain looks at water use and the role that rainwater harvesting can play in increasing efficiencies in how we manage this resource
In the past 12 months, a number of areas saw the failure or reduction of the mains water supply network, which made many people realise just how much we take for granted the water that flows from our taps. Only when it is gone do we realise how much we actually waste. Water is one of our most precious natural resources, which needs to be protected, nurtured and sustainably managed.
Potable water delivered to our homes and workplaces has been abstracted, treated and pumped through the mains supply network before it is delivered to the consumer who opens up the tap and, for the majority of the time, the water flows.
Average use
The average person uses approximately 150 litres of water per day. Studies published by the UK’s Environment Agency indicate that between 50 and 70 per cent of the potable water delivered to the home is used for non-potable uses such as:
• flushing toilets;
• showering and bathing;
• watering the garden;
• washing clothes and dishes; and,
• cleaning external surfaces and wash down areas.
This 50 to 70 per cent estimate equates to a volume of between 75 and 105 litres of water per person, per day, which is poured down the drain.
The amount of potable water we waste can be significantly reduced through the introduction of relatively inexpensive measures such as installing aerators on taps, using low-flow shower heads, detecting and fixing leaks, using high efficiency electrical appliances and installing rainwater harvesting systems.
Rainwater harvesting – what is it?
Rainwater harvesting is essentially the collection and storage of rainwater, for use at a later date. The practice has been extensively documented throughout history, but with the relatively recent introduction of the mains water supply network, the practice has all but died out in many parts of the world.
In Ireland, we are blessed with an abundance of rainfall. Rainwater can be collected and stored in a rainwater harvesting system and the stored water can be used to supplement many of the non-potable uses mentioned earlier.
Rainwater harvesting – how?
There are a number of ways in which you can harvest rainwater.
Water-barrels: These typically hold between 170 and 220 litres of water and they are usually emptied manually through the tap incorporated into the base of the water-butt.
However, there are now purpose-made pumps available on the market for use with water barrels to pump water through a hosepipe for watering gardens and cleaning down external hard areas. Water-barrels require very little maintenance and only require the occasional cleaning of debris such as leaf litter from the in-built filters and the water-barrel itself.
Rainwater harvesting systems: There are various configurations of these systems and they are more difficult to install and operate than water barrels. They require the installation of either an above-ground or below-ground storage tank, which incorporates a pump, to pump harvested rainwater either directly to water outlets (direct system), or into a header tank (indirect system), which supplies toilets and other non-potable water outlets such as washing machines by gravity. The size of the storage tank is designed to hold approximately 5 per cent of the annual average rainfall from a particular roof footprint or by providing a maximum of 18 days storage based on an analysis of the water consumption characteristics of the occupants of the property. The storage tank for a typical home would be between 3,000 and 5,000 litres.
Benefits
There are a number of environmental and financial benefits for the provision of a rainwater harvesting system in your home or workplace. The finer detail of just how water charges are going to be implemented in the domestic sector and the mechanism through which consumers will pay has not been finalised as yet.
However, the non-domestic sector has been paying for its water supply for the last number of years and the typical cost is approximately €2.40 per cubic metre of water used (which includes both a water supply and waste water treatment element). If a similar cost is applied to the domestic sector, it is foreseeable that typical family of four would use approximately 219,000 litres of water annually, with a cost of €525.
The annual cost saving in water charges through the installation of a rainwater harvesting system could be as high as €367 per year.
The estimated cost of a rainwater harvesting system would be between €2,000 and €4,000.
The payback period for such a system would be between five and 11 years. However, this assumes that there is no inflationary increase in water charges: the more the charges increase, the shorter the pay-back period will be.
Tony Cain is an Associate Member of Engineers Ireland. He has over 20 years’ experience in the water and wastewater sector in the UK and Ireland. He is a member of the Management Committee of Cork Environmental Forum (www.cef.ie). He has recently set-up his own Cork-based consultancy, H2O-Consult, to provide reliable, technical and friendly design advice in the field of sustainable water management.