An essential harvest

6th September 2010

An essential harvest

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  • Natural resources



Rainwater harvesting should be an important part of domestic water consumption, says Steff Wright.

The collection of rainwater in the UK for subsequent reuse is a historical practice that all but ceased with the introduction of a national mains water grid capable of bringing potable water direct to the home. But with mains water supplies under stress, it is time for a second coming.

Indeed, with last year's record rainfalls and resulting floods, 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, 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.

This imperative has been reflected for some time by government policy documents such as the Code for Sustainable Homes (CSH) and its counterpart for commercial buildings, BREEAM assessments.

Although aimed mainly at reducing the carbon footprint of new-build houses, the Code includes important mandatory targets for reducing mains water consumption, initially by bringing current average usage of 150 litres per person per day down to 120 litres (CSH levels 1 and 2), then in progressive stages (105 litres for CSH levels 3 and 4) down to 80 litres (CSH levels 5 and 6).

As a matter of policy, not likely to be altered following the recent change of administration, the Government is already committed to new houses being funded by public money being built to CSH levels 3 and 4, becoming levels 5 and 6 within another two or three years.

With the support of the housebuilding industry, it is also committed to progressively upgrading Building Regulations so that the private sector follows suit with all new houses being built to Code level 5 or 6 by 2016.

This is reflected in the draft updated Part-G of Building Regulations which came into force on 6 April 2010.

For the first time these introduced the concept of two water supplies in the home, namely ‘wholesome' (ie mains) water for potable use, and ‘non-wholesome' (from some other source) for non-potable applications such as toilet-flushing, clothes-washing and the outside tap.

Very helpfully, the draft updated Building Regulations also identified potential sources of non-wholesome water, of which harvested rainwater is the most readily available and straightforward. The Regulations then go on to state the need for water efficiency and identify the maximum per capita consumption permissible as identified using the Government's water consumption calculator methodology.


Could rainwater harvesting be an answer then? 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 per cent.

In commercial premises and other buildings used by the public, this saving rises to well in excess of 80 per cent 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.


Applied to meeting the water consumption requirements of the Code for Sustainable Homes (progressively being mirrored in Building Regulations), levels 1 and 2 of the Code (120 litres per person per day) can probably be met by simply using straightforward economising measures such as smaller toilet cisterns, dual-flush cisterns, aerated shower and tap heads, and use of water efficient clothes and dish washers. However, this needs to be demonstrated, using the official water consumption calculator, to the Local Planning Authority, before the house can be sold.

Levels 3 and 4 of the Code (105 litres) become more problematical as the benefit of all the above measures has already been reaped, leaving bath size (or omission of the bath altogether) as the only remaining economising measure.

Alternatively, use of harvested rainwater to reduce mains water consumption is likely to be a more popular option for most buyers of new homes. Getting down to the 80 litres per person per day required by CSH levels 5 and 6, realistically can only be achieved by substituting non-wholesome water for mains water using technology such as rainwater harvesting.

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 it immediately into the storm drain, helps to alleviate flood risk downstream.

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.

Insofar as carbon footprint considerations are concerned, recent Environment Agency studies awaiting publication indicate that on average it takes around 0.5 kWh of energy to bring one cubic metre of mains water to its point of use; conversely, the electric pumps that are central to the operation of rainwater harvesting systems are thought to use around 1.0 kWh of energy to deliver the same amount of water, meaning that a balance needs to be found between the imperative to save water, and the imperative to reduce energy consumption.

In practical terms, however, the amounts of energy used in either case are marginal in terms of total energy consumption, equating in the case of rainwater harvesting for a typical household to running a 60-watt light bulb for one hour each day.

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

Currently, the water utility companies supply around 6,000 million cubic metres of water each year, a figure that must not increase if existing stresses on supplies are not to become acute.

On the other side of the equation, around eight million new homes need to be built over the next four decades which, if fitted with rainwater harvesting systems, could reduce reliance on mains supplies by around 40 cubic metres per year per property, or 320 million cubic metres collectively.

To this needs to be added the potential for fitting/retro-fitting systems to suitable public buildings which could easily double this figure, thus meeting around 10 per cent of the country's total water needs.


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