Garden Design and Consultancy

Rainwater harvesting

They call water the blue gold nowadays and future wars could be fought over it. We have not reached that stage yet in the UK, but water is an issue and global warming has and will continue to affect just how much rain we get and when.

Rainwater harvesting is not new, but most of us have now heard of the term that refers to the capture of rainwater for use at a future time. The Europeans are ahead of us on this as they are on recycling in general e.g. rainwater harvesting in Germany, is about 100 times the current use in the UK. However, rainwater harvesting in the UK has increased by around 300% in the last two years.

Rainwater harvesting systems installed for typical domestic installations are able to fulfil almost all non-potable household requirements and save approximately 50% on mains water consumption.

To put it in perspective the roof on a typical four-bed house captures more than 100,000 litres of water each year and a typical family uses 70,000 litres on toilet-flushing, clothes washing and outside use annually. The costs of installation can be recouped over a period of a few years, but this will vary depending on the amount of rainwater collected and the cost of water in your area. This can be as low as 2-5 years for commercial systems and between 10-15 years for domestic systems.

Installation is between £2000 and £3000 for a good quality domestic system, depending on the size. Industrial systems are more expensive, but will deliver increased savings because of larger roof areas and capacity of water held. Once installed running expenses are low with a 1.5- 2.0 kWh pump costing 5-10p per week to pump 1 cubic meter of water (1000 litres). Maintenance is simple with a basic system requiring the filter to be cleaned by washing off the filter once a quarter is all that is required.

So how does this affect the gardener?

Well it is likely that there will be other buildings that water can be captured from, such as greenhouses, potting sheds and garages etc. This can be directed into an underground tank and then used for irrigation. Two main factors to take into account are the cost of metered water and that many areas around the UK are affected with hose pipe bans, which could mean that this will offer the only means of watering. Also, another point to consider is that rainwater is often preferred from mains water due to the absence of chemical treatments (unless you suffer from acid rain!).

Tank sizes vary from small 650 litre tanks, through to mid sized 3/4000 litre tanks, which can be sited above or below ground, through to huge 36000 litre underground tanks. When there is insufficient water in the storage tank the system will automatically draw water from the mains. The tanks have filters that remove all debris and particles from the water that is collected ensuring that the water remains clear.

Have you got room for a tank?

Here are typical dimensions for medium sized tanks:
3500 litre - 2430mm long and 1400mm high and wide
4700 litre - 2320mm long and 1700mm high and wide
6500 litre - 2370mm long and 2080mm high and wide

Each tank has a with a 700mm funnel with a 825mm manhole.

If these sizes appear too large then consider using a small tank or even linking a number of smaller tanks together, if there is insufficient room for a large tank. Narrow upright, above ground tanks could fit into a small space and then the water can be extracted using either a tap to fill a watering can or by using a pump powered hose that is self primed and activated when the hose trigger is pulled. A down pipe diverter channels the water via a filter to keep debris out of the system into the tanks, with the excess water rainwater diverted back down the existing down pipe to the drain. With larger systems a higher quality filter will remove virtually all the debris and the pump can operate at up to 4-bar pressure.

The system will generate significant environmental benefits through reducing mains water demand and reducing storm water run-off. With water bills expected to rise 10% a year for the next five years then the expected payback period will be reduced further. The ever-increasing waste of water through storm-water run-off has become a planning issue and rainwater harvesting systems can go some way to solving this. Surface water in cities is now a real issue as the drains were not built to take the large amounts of storm water that is now collected, especially with so many front gardens being built on. So if you are planning a new build, whether a house, garden feature, outbuilding or greenhouse then planners will look favourably on a rain water harvesting system that is included within your planning application.

If your garden does not have a irrigation system then this may be considered in combination with a harvesting system as it can deliver water in an efficient and timely manner and save valuable time for other tasks. For instance it would be possible to irrigate during the night, which means less evaporation and the water is than able to soak deeper into the soil encouraging deeper and stronger roots.

To summarise the case for rainwater harvesting it should be stated that this could be achieved with relatively little space and minimal cost for the smaller projects. As you climb the scale the returns are increased, as you are able to capture a valuable resource at a fixed cost which will benefit the garden for many years to come.

Teaming this system with a simple irrigation system could not only save time and money but could improve the quality of the plants in the garden as well. For those in need of water, a closer look could prove to be well worthwhile.