… water on tap, a gathering crisis

About the author

Steff Wright is the Chairman and CEO of award winning Gusto Homes, which specialises in the design and construction of sustainable homes.  He is a founder-member and Director of the UK Rainwater Management Association.

Water supplies under-stress

With widely-reported floods a regular occurrence, it is hard to think of water as a scarce commodity but the fact remains that water supplies throughout southern England remain under stress, seriously so in the highly populated south-east.

Surprisingly, however, rainfall per head of population in the south of England is lower than in the countries surrounding the Mediterranean, again particularly highlighted in the south-east where population densities are highest and average rainfall is least.  With population predicted to continue rising by a further 20-million over the next four decades, the water supply situation will clearly deteriorate critically unless effective action is taken now.

Heading the list of the most sensible ways to reduce mains-water consumption beyond the somewhat lax requirements of Building Regulations, would be to stop using drinking water for flushing toilets, and use harvested rainwater instead.

How it works

The installation of a modern rainwater harvesting system whilst a house is being built is very straightforward, with a storage tank being coupled to the normal guttering and down-pipes as part of the general drainage works. Separate pipework is also installed to serve the non-potable services so that at no time can the wholesome and non-wholesome supplies come into contact with each other.  Installation of full systems in existing properties is less straightforward, however, due to the disruption caused to internal decoration by installing separate pipework (unless coincident with other refurbishment works), making a garden-only system the preferred retrofit option.

In operation, all aspects of modern systems are fully automatic and from the user’s perspective indistinguishable from using the mains supply. The water falling on the roof is routed via the usual rain-goods and a high-quality filter to the storage tank where it remains awaiting use.  When a non-potable service, such as a toilet, is used this is sensed by a management system as a drop in pressure in the supply pipe which is immediately restored by activating an electric pump.  When the demand for water ceases, the pressure is restored and the pump de-activated.

Storage tanks are sized to take into account three main factors, namely the area of the roof, the local average rainfall, and the amount of non-potable water likely to be used by the household; a further water-quality factor is then applied to ensure that the water in the storage tank is turned round every 18-days or so.

During periods of prolonged rainfall, water will be collected more quickly than it is being used so that eventually the tank will overflow into the soak-aways or storm-drain arrangements for the property.  Conversely, during prolonged dry spells the tank will be in danger of running dry which is prevented by the management system introducing small quantities of mains water for use pending the next rainfall.  This top-up is achieved via an inbuilt air-gap to prevent contact between the wholesome and non-wholesome water supplies.

Given the right balance between roof size, local rainfall and house occupancy, a correctly sized system will provide most of the non-potable water required by a household, thereby reducing mains consumption by up to 50%.  In commercial premises and other buildings used by the public, this saving rises to well in excess of 80% given a large roof and a high demand for non-potable water.  Rainwater harvesting systems can also often be retrofitted more easily to commercial buildings as these usually afford easier access to pipe runs without disturbing decoration.

How it works

The installation of a modern rainwater harvesting system whilst a house is being built is very straightforward, with a storage tank being coupled to the normal guttering and down-pipes as part of the general drainage works. Separate pipework is also installed to serve the non-potable services so that at no time can the wholesome and non-wholesome supplies come into contact with each other.  Installation of full systems in existing properties is less straightforward, however, due to the disruption caused to internal decoration by installing separate pipework (unless coincident with other refurbishment works), making a garden-only system the preferred retrofit option.

In operation, all aspects of modern systems are fully automatic and from the user’s perspective indistinguishable from using the mains supply. The water falling on the roof is routed via the usual rain-goods and a high-quality filter to the storage tank where it remains awaiting use.  When a non-potable service, such as a toilet, is used this is sensed by a management system as a drop in pressure in the supply pipe which is immediately restored by activating an electric pump.  When the demand for water ceases, the pressure is restored and the pump de-activated.

Storage tanks are sized to take into account three main factors, namely the area of the roof, the local average rainfall, and the amount of non-potable water likely to be used by the household; a further water-quality factor is then applied to ensure that the water in the storage tank is turned round every 18-days or so.

During periods of prolonged rainfall, water will be collected more quickly than it is being used so that eventually the tank will overflow into the soak-aways or storm-drain arrangements for the property.  Conversely, during prolonged dry spells the tank will be in danger of running dry which is prevented by the management system introducing small quantities of mains water for use pending the next rainfall.  This top-up is achieved via an inbuilt air-gap to prevent contact between the wholesome and non-wholesome water supplies.

Given the right balance between roof size, local rainfall and house occupancy, a correctly sized system will provide most of the non-potable water required by a household, thereby reducing mains consumption by up to 50%.  In commercial premises and other buildings used by the public, this saving rises to well in excess of 80% given a large roof and a high demand for non-potable water.  Rainwater harvesting systems can also often be retrofitted more easily to commercial buildings as these usually afford easier access to pipe runs without disturbing decoration.

The big picture

Although primarily aimed at relieving the stresses on national water supplies, rainwater harvesting also enjoys an additional important benefit in mitigating flood risks; storing water where it falls, rather than releasing immediately it into the storm drain, helps to alleviate flood risk down-stream.  Although by itself this effect is likely to be marginal, as part of an integrated sustainable urban drainage solution a single integrated system can serve both to attenuate and manage the surface water, and provide a highly cost-effective source of water for non-potable use.  On commercial projects, the need to preserve a guaranteed source of water for fire-fighting purposes can also be incorporated into the same integrated system.

The Environment Agency predicts that by 2050, climate change and other factors will serve to reduce the amount of water available by 10 to 15%, whilst at the same time the population grows by 20-million.  Against this back-drop, river levels may fall by up to 80%, whilst harvested rainwater will become an increasingly important source of non-drinking water for homes and agriculture.

Taking an integrated approach to surface water management on new developments, aimed at reducing both flood and drought risks, produces the most cost-effective sustainable-drainage solutions, whilst reaping important commercial environmental and development amenity benefits.