A harvest for the world
February 10th 2006

Less than 1% of the water treated by public water systems is used for drinking or cooking. Surely there must be a better way of getting water for the other 99% of the time when it is not crucial to have water at such a high level of purity. The answer may be to use rainwater.

Water management in the UK is entering a period of considerable upheaval and change. The transposition of the European Water Framework Directive (WFD) into UK law and the progressive adoption of a catchment area approach to the way water resources are managed have wide reaching implications.

It is clear that the way we use water and how it flows through our built environment will be critical if the ambitious yet achievable objectives of legislation like the WFD are to be met.

Shockingly less than 1% of the water treated by public water systems is actually used for drinking and cooking. Yet a significant proportion of our water needs can actually be met by collecting and using rain from the roofs of our homes or places of work. This saves money, reduces water demand, reduces the pressure on the environment and makes perfect common sense.

Typically:

• Every time we flush the toilet 7.5-9.5 litres of high quality drinking water goes down the drain
• A garden sprinkler uses up to 540 litres of water an hour
• A typical washing machine cycle uses 65 litres of water

Daily each of us wastes up to 38 litres of perfectly potable water by flushing the toilet, with all the subsequent waste of effort and energy required to purify the water for this purpose. This is a massive and avoidable waste of vast quantities of an increasingly precious resource. Additionally the way we currently manage our built environment, with non-porous surfaces and drainage systems that create large amounts of excess run-off, causes unnecessary flooding during periods of heavy precipitation.

Against a background of increasing water demand, as we develop large communities in areas of existing water shortages like the Thames Gateway, the increasingly unpredictable and turbulent effects of climate change on rainfall and drought, and the widely predicted rises in water bills to consumers to pay for WFD related changes and increased infrastructure improvement costs, it quickly becomes clear that both commercial and domestic rainwater harvesting is timely, cost-effective and sensible.

So how does it work? There are a variety of systems on the market, but the general concept is always the same:

• The rainwater is first collected (usually from the building roof ), filtered and then stored
• Water is then pumped either directly to the points of use or to an internal break tank
• If the system runs low on rainwater a mains-water back up unit will guarantee continuous supply feeding mains water into the system
• When it rains again, the system changes automatically back to the rainwater supply

The components needed to achieve such a system are very robust, reliable and simple, comprising:

• Filtration, which depends upon the contamination of the water and on end-use requirements, with different methods of treatment used as appropriate; invariably, however, a pre-storage filter is used to remove solid matter.
• Storage is optimised to balance the equation between water yield and consumption; typically, plastic, GRP or concrete tanks are used
• Technical components, such as the pump and system control panel
• Overflow arrangements, usually to soak-away or storm drain, to supplement the attenuation function of the storage tank itself

Today rainwater is still often considered a problem because it can cause flooding. The perception is that rainwater needs to be drained and got rid of as quickly as possible. Rainwater harvesting aims to change this perception by retaining the water for non potable use, turning it into an asset rather than a problem.

Rainwater is free of charge and can be used for any commercial application where there is a high demand for non-potable water. Typical examples are shopping centres, sports venues, hotels, workplaces, distributions centres (vehicle washing), schools and hospitals.

The capital cost payback period for commercial use can be extremely short, with non-potable, harvested rainwater being substituted for expensive mains water in applications such as indoor and outdoor cleaning, cooling and other industrial processes, irrigation and humidification, fire-fighting, washing machines and toilet-flushing.

The economic case for investing in rainwater harvesting systems hinges upon factors such as local water supply charges (both currently and as anticipated in the future), the capital and (small) running costs, and the level of usage.

Two typical case-study examples of commercial applications illustrate the economics of installing rainwater harvesting systems:

In the first a Community Centre in Kent with a 950 sq metre roof was fitted with a 26 000 litre storage tank and associated system components at a cost of £5500; the purpose of the system is to supply washing machines and toilets. With an average annual rainfall of over 700mm, and a water harvest potential of more than 500 000 litres per year, the yearly saving on metered water is more than £1700. This gives a capital cost payback of a little over 3-years.

Similarly, an office block in Manchester with a 3200 sq metre roof and a 110 000 litre storage tank provides a water harvest potential of approaching 3.5M litres per year based on an average rainfall of around 810mm.

Used to supply the toilet flushing requirements of the 550 office employees in the building, the typical yearly saving on metered water is over £3500 per year, leading to a 3-year payback of the capital cost of around £11 000.

Government interventions which are currently helping to sway developer decisions to install rainwater harvesting systems, include increased planning system insistance on Sustainable Urban Drainage Systems (the ‘stick’), and the financial benefits resulting from application of the Enhanced Capital Allowance Scheme (the ‘carrot’).

The environmental case, meanwhile, could not be plainer with climate change and development pressures bringing in-train the unwelcome combination of too little water (droughts) for much of the time, coupled with too much water (floods) at other times. Rainwater harvesting can play a vital role in replacing up to 50% of mains water requirements at premises where it is used, and at the same time play a small but useful role in attenuating storm water.

The embedded energy wasted in bringing all mains water up to potable standard, when less than half is destined for potable use, needs also to be taken into account at the national policy level – a factor possibly to be recognised in revisions to building regulations which are being considered. These, if implemented, will make separation of potable and non-potable pipework mandatory in future new buildings. At the very least, this will make later retro-fitting of rainwater harvesting systems a practical proposition, and help to future-proof structures against the uncertainties of water supplies and charges.

The UK Rainwater Harvesting Association endeavours to influence opinion and facilitate widespread acceptance of rainwater harvesting technology and provide information to our members on a regular basis concerning standards, public enquiries, new legislation and advances in technology.

In addition we serve as an enquiry centre for the public, industry and governmental bodies. We provide information to enhance the general understanding of the advantages derived from rainwater harvesting systems.

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