Fresh take on an old idea

Rainwater harvesting in its widest sense is a triumvirate. Each of the three elements can be clearly defined, like the corners of a triangle.

There is rainwater harvesting on the large scale, at the catchment level, leading to reservoir storage to provide municipal supplies. There is surface-water management, which aims to control the flow of stormwater on surfaces – permeable and impermeable – to minimise its impact on the environment. The third element of the triumvirate is the collection of rainwater from buildings to use in place of some of the potable water supplied via the municipal water-supply network.

These three aspects of rainwater harvesting are not new. They have been around for centuries.

The Romans collected water from rural areas, directed it via aqueducts to cisterns from where it was distributed to the urban population. Landowners have dug channels to drain, direct and control the flow of water on their estates and around their properties as well as to provide water for domestic and estate use. Over the years rain has been collected from building roofs, be they made from banana leaves, thatch, slates, lead sheeting or tiles, for all of the many and different uses for water in and around the home.

Supply problems

The impression that rainwater harvesting is a new and modern concept has emerged because of the development of the new age of sustainability and the prospect of water-supply problems for existing and, especially, new developments during a period of climate change. There is also awareness that the use of potable-quality water for many domestic, commercial and industrial applications is unnecessary and a waste of resource, both of the water itself and the energy used to treat and deliver high-quality potable water for all uses.

So we are turning our attention to the habits that were an accepted part of everyday life a few generations ago, even, dare I say, for the author during his upbringing in rural Cornwall. It remains de rigeur in many rural areas in developed, as well as developing, countries across the world.

Surface-water management and rainwater harvesting may be new terms for new technologies but they were regular and accepted practices, the natural ways of planning and provision water to many homes. Few if any regulations existed and pragmatic planning and personal responsibility generally ensured a sensible and safe use of the available water.

That is not to question the current development of regulations for planning requirements and quality guidelines as the necessary drivers to the development and uptake of rainwater harvesting. So, while water charges do not adequately reflect the cost of water services at a time of apparent water sufficiency, regulation will be the only driver to the development of plans and technologies to prepare for and mitigate the predictions of climate and population change.

The mention of both rainwater harvesting and sustainable drainage systems (SUDS) in planning guidance, development codes and strategies is now accepted, even expected, in all relevant legislative and regulatory developments in this era of sustainability. This is welcomed and important to ensure that the dash to be sustainable with systems does not overshadow the need to plan, install and maintain systems in a sound and technically competent manner.

There are a number of examples in other countries where rainwater harvesting at all scales has been established for some time. But the main driver was drought, leading to water shortages. Even so, this was not sufficient on its own and regulations were also required. Drought in Japan and Australia provided the impetus, whereas in Singapore politically motivated proposed cost escalation by neighbouring water-supplying countries had been the stimulus.

During a DTI-funded Global Watch Technology Mission to Japan in 2000, rainwater harvesting systems were seen in new commercial developments in Tokyo, often in conjunction with greywater reuse technologies. Not because the developers had high environmental credentials, but because they were required to do so by building regulations. Size of development was linked to the amount of rainwater from all surfaces, roofs and ground, and greywater to be collected and recycled.

The all-weather Tokyo Dome, a major baseball stadium, had a combined rainwater harvesting and greywater-recycling system to service toilet and urinal flushing. The system was designed to collect 220,000l/d of rainwater and combined it with 402,000l/d of greywater for treatment, storage and use to service toilets. Other developments were required to collect rainwater from the area surrounding the building as well as the building itself.

Modern sports stadia in the UK incorporate rainwater harvesting in their water management systems.

Paved surfaces

In Australia, the Royal Park in Melbourne has developed a recycling system that incorporates stormwater harvesting from paved surfaces, not just from buildings. The treatment system was primarily a large wetlands basin and an open water feature for storage of the treated water. This provided a significant additional community facility, as well as providing all of the water for use in the small zoological gardens within the park. In another large-scale system, reclaimed stormwater, treated in an extensive reed bed system, was used for aquifer recharge. These examples of rainwater harvesting demonstrate the seamless link between rainwater harvesting and sustainable drainage – it is primarily a matter of size, but also whether the collecting surface is a roof or the ground. Some new conurbations in Australia include rainwater harvesting and municipal wastewater treatment and reuse in their water-supply plans.

Also, building regulations require rainwater harvesting in all new developments, houses are expected to have storage tanks of 5,000-10,000 litres. The schematic of the Mawson Lakes water system (see page 10) shows the detail and complexity of a system being installed in a large development for 4,300 dwellings near Adelaide.

In the UK, the periods of low rainfall in the decades either side of the millennium provided a stimulus for the development of advice and standards for rainwater harvesting as well as financial inducements and regulations as carrots and sticks by government. It also prompted supply chain companies to examine the opportunities and develop products to satisfy the nascent rainwater harvesting market. The main context of rainwater harvesting in the UK, especially in the consumers’ mind, is the collection of rainwater from the roofs of domestic properties. But there is a great potential for commercial and industrial organisations to develop rainwater-harvesting systems to reduce their municipal water demand, wastewater discharge volumes and associated costs. Indeed there are many systems now available to collect, treat and store rainwater for all sizes of domestic and commercial properties so that rainwater can replace municipal water supplies for outdoor and indoor uses, and in manufacturing.

Financial carrot

For commerce and industry the development of rainwater use is encouraged by the tax incentives of the Enhanced Capital Allowance (ECA) scheme. This enables companies that install rainwater harvesting technologies on the ECA list to offset the full cost against tax in the first year. It is a financial carrot for commerce and industry to improve their water efficiency and reduce water demand from the network. It is also a stimulus to the supply chain to develop suitable products that meet commercial needs and comply with regulatory aspects of the scheme.

At the domestic level, the 2007 Code for Sustainable Homes is a significant driver for the uptake of rainwater use in domestic properties. It is part of the holistic approach by government to mitigate the potential impact of climate change on water resources. It is a regulatory stick for developers and an inducement to buyers by acknowledging the developing environmental awareness of consumers.

Daily use

The code targets significant reductions in demand from the municipal supply network by requiring the use of water-efficient fittings and white goods to reduce personal daily use augmented with alternative water sources for non-potable applications. This is, of course, focused on new builds with public housing leading the way as code levels 3 and 4 are current standards with private-sector developments to follow by 2010. The next levels are to be achieved by 2013 (public sector) and 2016 (private sector).

The prime target for the Code for Sustainable Homes is new-builds, but the guidance for personal water use levels can be applied to efficiency improvements of the existing housing stock. If development targets and climate change predictions are correct then the adequacy of water resources will be challenged, leading to inevitable rises in water charges to reflect the true economic value. So pressure to minimise water use will become more necessary for all housing. This will lead to retrofitting of water-efficiency devices and rainwater-harvesting systems to the established housing stock. As water charges inevitably rise this will reduce the payback times, so making their installation more attractive.

There are many sources of information and supporting data providing guidance on all aspects of rainwater harvesting. The Environment Agency publication Harvesting rainwater for domestic uses: and information guide covers design considerations, installation and longer-term maintenance issues as well as listing references.

Risk assessment

There are as yet no water quality standards, but it is generally accepted that the standards of the Bathing Water Directive are appropriate for domestic and garden uses for rainwater.

The Market Transformation Programme has investigated and produced a report on quality standards, Rainwater and Grey Water: Review of water quality standards and recommendations for the UK, MTP (2007). The absence of regulations supports the WHO policy that the quality of water for all uses should primarily be determined by a risk assessment, this will be incorporated into the new Private Water Supply Regulations for England and Wales, which will come into effect in 2009. It is equally applicable to rainwater and greywater reuse.

EA guidance covers the obvious basic need, rainfall data for the site and the measurements and calculations required to assess the likely yield of water. Not only does surface area need to be known, but pitch and roof materials all have an impact on the volume of collectable rain.

This information is all fed into the calculations of rainwater yield which, in conjunction with the water needs of the property, will influence the optimum size of storage tank required – and a formula to calculate this is given. Sizing for industrial or commercial premises may need more detailed calculations.

There are various installation decisions to be made including siting of the collection system – inside or outside, above or below ground, direct feed or pumped to a roof tank. Once these decisions and basic calculations have been done it is then necessary to consider how to install the necessary equipment, assembly from components or proprietary systems – of which there are an increasing number available.

To ensure that systems are appropriately designed and constructed, the British Standards Institution, and the industry, is preparing a standard that is in the final stages of preparation. It is expected to be published in 2009 as BS 8515:2009 Rainwater harvesting systems – Code of practice. This standard will provide users with confidence that compliant proprietary systems will have been adequately designed and constructed, and so if maintained according to manufacturer’s recommendations, will reliably provide planned- for rainwater harvesting.

Best practice

Rainwater harvesting is fast becoming an accepted part of sustainable development politically, technologically and socially. The water-supply chain is responding to this by defining best practice and developing a range of components and systems. Consequently, rainwater harvesting is now accessible to planners and architects for developments of all sizes and applications whether commercial, industrial or domestic so contributing to making developments more sustainable.

Dr Ian Pallett is technical director at British Water. T: 020 7957 4554