Rainwater harvesting
Using this free, and plentiful, resource at home is good for the planet and your wallet. Nigel Griffiths discusses the essentials
Eco expert Nigel Griffiths explains the benefits of harvesting and reusing rainwater to create a sustainable home that makes use of this free and natural resource
Integrating some kind of rainwater harvesting system into a self build or renovation project has been growing in popularity for years, not only for the economical benefits, but also the ecological impact. But how can you maximise yield, reduce your demand for mains water and still achieve those all important financial savings?
To start you must understand the basics. As the name suggests the practice centres on the gathering and use of rainwater in the home instead of a mains supply. You can use it to flush toilets, run the washing machine and irrigate the garden. In most cases, the primary collection system is the roof – most already have a way to accumulate this thanks to guttering and downpipes, which can be used to convey water to a reserve. Although the exposed tops of buildings are not perfectly clean, they do not have all the contaminants that are present at ground level, and foreign bodies such as leaves can easily be removed.
In terms of storage, tanks can be situated above or below ground, but the former will require protection from
freezing and may need to be disguised in some way as they are often unsightly. In order to use the water, it needs to be pumped from the unit, either directly to the points of use in the property or to a secondary header tank located elsewhere in the home, usually in the loft.
Mains backup
There are several ways of arranging a contigency in case of dry spells. The most common for direct-pumped systems is to use a float switch inside the designated tank. This is able to sense when the water level reaches a critically low marker, and then opens the mains feed to replenish the supply as necessary, up to a limited point.
As the tank only partly refills there is still plenty of room for rainwater to be collected when the wet weather returns, so the system will always favour the use of the harvested resource. It’s important to note that the backup must be correctly installed in accordance with water regulations to prevent back contamination of the mains supply. There are also legal standards regarding how pipes must be marked up to identify rainwater pipes clearly – you can find out more at www.wras.co.uk.
Maximising use
To get the most out of harvesting rainwater, you need to use the alternative to displace as much mains water usage
as possible. Rain can be used for a variety of domestic and non-domestic uses without any disinfection.
The majority of systems are used for supplying WCS. Flushing toilets accounts for about a third of water use in the average home, so a rainwater harvesting system can make a significant difference to both consumption and metered bills. Be aware that water aggregated from the heavens can sometimes appear slightly coloured in the pan, so it may be useful to advise people unfamiliar with this system to avoid them flushing the loo twice.
Rainwater can also be used in washing machines because it is softer that its mains counterpart. As hardness in water comes from the ground not the air, it is ideal for laundry, eliminating the need to use fabric conditioners. It can sometimes be used for hand washing, too.
Optimising storage
If it rained every day for an hour, tank sizes could be very small as you could use and refill them easily. However, even in the UK, it is not uncommon for two weeks or more to pass without rain, so a larger unit is normally required to ensure that you are still using harvested water rather than the mains after a prolonged dry spell.
Tank sizing is of course subject to the law of diminishing returns. The larger the tank, the fewer the occasions when mains backup will be required, but the effect decreases as tank size is increased. A container catering for 1,500 to 2,000 litres (1m3 = 1,000L) will supply most of the WC and washing machine requirements in the average house.
As a general rule of thumb, 5% of the available annual rainfall is a good starting point for storage capacity.
Assessing the resource
To work out the amount of water that could be collected from a domestic property, you must first calculate the area of the roof in place – this is independent of the slope, so it should be similar to the ground floor area. Next, find out the expected annual rainfall per square metre – the Met Office publishes all of these figures across the UK on its website – www.metoffice.gov.uk/climate.
Consider this example: a house in the West Midlands would receive an average rainfall of 669mm per annum. If the ground floor measured 5m x 10m, the available water would be calculated as follows:
Average rainfall (m) x area of roof (m²) = available water (m³)
Ground floor area = 5m x 10m = 50m² Available rainwater = 50 x 0.669 = 33.5m³
Some of this is lost to evaporation and overflow, so not all of the water is collected. In order to get an approximate idea of the yield, the general rule is to multiply by 80% − in this case 0.80 x 33.5 = 27m3.
For many houses, the roof shape can only permit water to gather on one side of the property. One solution is for additional pipework to be installed to ferry this differently, through or under the building, but this could be costly.
Sustainable drainage
As a result of climate change, spells of heavy rain are expected to increase in the future. Widespread harvesting and storage of rainwater could reduce the risk of local flooding. When it rains heavily, even an empty tank will fill within a relatively short space of time, but every litre of water that goes into your storage system will prevent a litre being discharged into the public drains (or your own soakaways) at peak periods of rainfall.
Obviously, the larger (and emptier) your tank, the greater your contribution to this process of delaying rainwater discharge – so if you live in an area where your surface water drainage could contribute to a local flooding risk, a larger tank might be more appropriate. As you are usually excavating in any case, and the rest of the equipment does not change, then a larger tank should add only a little to the cost while increasing the usefulness of the system.
Communal systems
Economies of scale continue as the storage system gets larger – from a small tank in the garden, all the way to reservoirs and treated water. This is why the mains system is actually a very energy-efficient way of delivering the utility to our homes and businesses – which has led to some questions around the environmental economics of rainwater harvesting at a domestic level.
In between domestic and mains water, in terms of size, you have communal systems (usually underground) that pool the rainfall from several properties – these are slightly more efficient than individual systems. They benefit from the averaging effects of multiple uses; a 10m³ tank serving a group of 10 properties should require less mains backup in prolonged periods of low rainfall than a 1m³ tank serving a single home. As there are economies of scale in terms of installation, it should be possible to double the size and fit a single 20m³ tank for less than the cost of 10 separate 1m³ versions. So, a communal system offers a greater cost saving, an efficiency gain and, theoretically, overall further reduction in the demand for treated water.
As an aside, the same is of course true of sustainable drainage systems shared throughout the neighbourhood, and as these become larger and more commonplace they become ponds or lakes, contributing to vegetation, wildlife, leisure and ultimately to everyone’s quality of life.