Rainwater harvesting

Published:  16 December, 2006

tank
The quality of stored rainwater is maintained by underground storage and efficient prior filtering.

Harvesting rainwater can meet a large part of the water consumption of commercial and domestic buildings, considerably reducing the demand on mains water. STEFF WRIGHT highlights how rainwater harvesting systems can help the environment and for what purposes the harvested water can be used.

Recent media coverage of green issues and hose-pipe bans and drought orders in the south of England during the summer months have at last triggered the process of challenging public perceptions about the UK’s water supplies. In consequence, the general public is now beginning to recognise that water is not assured, plentiful and cheap and to take seriously the part they can play in husbanding a vital natural resource.

Although Government agencies and the water-supply industry have stressed the importance of avoiding the wasting of water, partly by using better technology such as aerated taps and dual-flush toilets and partly by lifestyle changes such as substituting showers for baths, neither approach is keeping pace with the increasing demand for water nor meeting the challenge posed by a run of below-average seasonal rainfalls. Neither does it tackle the environmental impact (in the form of wasted energy) in bringing all water up to drinking standards when around half of it is destined for non-potable use.

Different approach

For many years, some of our continental cousins have adopted a quite different approach. With water an even scarcer and higher-priced commodity, reducing the use of mains water has long been the aim of both suppliers and consumers. A major plank in meeting this aim has been the widespread introduction of rainwater harvesting in countries such as Germany.

Until the development of the mains-water grid in the UK during the 19th-century, rainwater harvesting was a standard feature of most substantial new-build houses — freeing the occupants from the need to share the village pump. Very simply, rain falling on the roof is channelled to a storage tank, from which it can be recovered when needed.

On modern buildings, using modern building materials, harvesting rainwater is extremely straightforward, with the quality of the stored water being maintained by underground storage and efficient prior filtering. High-quality, non-potable water is then automatically fed on demand to services such as toilet-flushing, washing machines for clothes and outside taps. In a typical family home, such usage accounts for about half of all water-usage, reducing the use of mains-water correspondingly.

Gusto Industries (the parent company of Freerain) first became involved with the use of rainwater harvesting systems in 1998 when it designed its award-winning Millennium Green development near Newark in Nottinghamshire. Subsequently, using experience gained with the systems then available in the UK, a separate specialist operation was established under the Freerain brand to bring to the market systems designed to meet developers’ prime need for ease of installation, and reliability in-service.

The decision to incorporate rainwater harvesting as standard in nearly all subsequent Gusto Homes has proved extremely popular with buyers, who cite them as one of the factors that swayed them in their purchase. Recent publicity will have heightened this customer preference — particularly amongst garden enthusiasts who by reserving their stored rainwater for irrigation purposes both beat hose-pipe bans and provide their plants with un-chlorinated water.

win-win

Completing the win-win-win picture, many local authority planning and Building Regulation officers also take into consideration the potential attenuation provided by the storage tanks on sites where storm-water management or soakaway performance is an issue.

Given the attractions of rainwater harvesting, and its lack of down-side (even with extremely widespread use they would have no perceptible impact on ambient water levels available to the water supply companies), it comes as no surprise that the UK market for systems has trebled in the last two years, with a further 100% growth forecast for this year.

Independent monitoring by Severn-Trent Water and the Environment Agency on installations on the Gusto Millennium Green development showed that for the most of the year rainfall keeps storage tanks around half-full, with only occasional need to overflow into soakaways or top up with mains-water. It also showed that around half of all the water used by the householders was non-potable rainwater.

The water economies are even more marked in the case of commercial or public buildings, with a combination of large roof areas and a high demand for non-potable water — typically for toilet-flushing or fleet washing purposes. This means that commercial, industrial, leisure and public-sector developments (such as schools, hospitals, fire, police and ambulance stations, and military projects) will substantially add to the market demand for rainwater harvesting systems over the coming years.

Re-dawning

Meanwhile, it is already clear that there is a re-dawning of the age of rainwater harvesting, reflected in the 120 systems currently being installed by Cornhill Estates and Miller Homes in developments at Upton, near Northampton. Scaled-up across the UK, particularly in the south of England, consistent specification of rainwater harvesting systems in all new developments will help provide the medium-term solution to what otherwise will be a growing water-supply and storm-water management problem.

In short, rainwater harvesting provides developers with a potential solution to their site water supply and storm-water management issues. Where relevant, it will also assist them with their Eco-Homes rating. In the process, rainwater harvesting in speculative developments also provides a highly cost-effective sales incentive in a consumer market increasingly preferring environmental benefits to the traditional free carpets.

Data collected by Severn Trent Water for a year’s water usage for a house on the Millennium Green project shows clearly the relative use of mains water (blue) and rainwater (grey).

Incorporating rainwater harvesting into speculative housing developments presents few practical problems for the builder. It typically poses no great demands upon the overall design, other than to ensure that the roof-water is brought together in a single run for collection in the storage tank. Invert-levels also need to be designed so that the tank can overflow into soakaways or the storm-drain, as appropriate.

Automatic operation of the system is controlled by a management panel that needs to be mounted in a frost-free environment such as the garage or utility room; electricity and mains-water supply for topping up in prolonged dry spells. Connections also need to be made to the management panel. Inside and outside the house, separate pipe-runs need to be installed for the potable and non-potable supplies — with no contact allowed between these two different water supplies.

In operation, roof water is simply filtered to remove solid impurities, then stored in a large collection tank — usually located underground to maintain water quality. When non-potable water is required, such as when a toilet is flushed or the outside tap operated, the pressure-drop is sensed by the management panel, which then activates the in-tank pump to meet the demand.

Data collected by Severn Trent Water for a year’s water usage for a house on the Millennium Green project show that a tank of just over 3000 l capacity never ran dry, and only occasionally overflowed. (There is a short period of no data, rather than the tank running dry.)

During lengthy periods of rain, when the storage tank is full, water overflows automatically into the storm-drain or soakaway — cleaning the top of the filter in the process. During prolonged dry periods, low water level is sensed by the management panel, which then automatically tops-up the tank with a small amount of mains-water to ensure continuity of supply to the services pending the next rainfall. Commercial systems work on exactly the same principles, but are specially designed for the project concerned.

From an end-user’s perspective, using harvested rainwater for non-potable applications is indistinguishable from using mains-water. The water supplied is clear, soft and free of chemical additives. Exceptionally, if specifically required, the water can easily be brought up to drinking-water standard by using simple but effective additional treatments, although this is would usually only be necessary for off-mains projects.

Reliable

In summary, rainwater harvesting provides builders and developers with a simple, reliable, easily-installed and cost-effective solution to site water-related issues. It is also an excellent inexpensive marketing tool, increasingly in demand by today’s environmentally-conscious client.

Steff Wright is chairman of the Gusto Group of companies, Millennium Green Business Centre, Rio Drive, Collingham, Newark NG23 7NB.



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