RWH systems provide significant environmental benefits through reducing mains water demand and reducing storm water run-off. Also, control of storm-water run-off is now increasingly a Planning issue which rainwater harvesting systems can play an important part in addressing.
The technology and principles of rainwater harvesting have been around since pre-Roman times, however the modern systems date back to Germany in the mid 1980’s, and have been in use in the UK for more than 20 years.
Having been in-use for more than 40-years, and now backed by British and European Standards, RWH technology is both simple, and very reliable.
The maintenance regime on RWH systems is very straightforward, and on domestic systems can be undertaken by the homeowner, if wished. Otherwise, the UKRMA website lists various RWH maintenance specialists, and also links through the national network of "Watersafe Plumbers", most of whom can provide a local service.
The water harvested is suitable for all non-potable purposes from watering the garden (rainwater is, unsurprisingly, particularly beneficial to plants), washing the car, flushing the toilet and running the washing machine.
This depends on the size of your collection area (usually the roof of the building) and the amount of rainfall in your area. Typically, domestic systems will reduce mains-water consumption by up to 50%, rising to more than 80% in commercial applications
Apart from "irrigation-only" applications, RWH systems are designed to maintain continuity of supply at all times to non-potable services such as toilet-flushing and clothes washing machines. During long dry spells, when there is insufficient water in the storage tank, the system automatically draws water from the mains again, so that from the point of view of the user no difference is apparent and all services continue to operate. When mains-water top-up occurs, only a limited amount of mains-water is introduced into the tank, so that the vast majority of its capacity remains available to harvest the next rainfall.
A well designed system with a good match between supply and demand will only need topping up when it has not rained for some time. Severn Trent monitored a domestic system and found that the tank only occasionally needed to be topped-up from the mains. Most of the time the tank was around 50% full (i.e. an ideal balance between having plenty of water to use, and plenty of space to accommodate the next rainfall).
The tanks have filters that remove all debris and particles from the water, so that it remains clear, when visible in toilet bowls for example.
Harvested rainwater is non-potable, and it is generally uneconomical to bring it up to a potable standard. If essential to do so, in an off-grid application for example, additional UV filtration can be added to bring up to potable standard, but its use as such would be subject to national "Private Water Supply" Regulations and regular inspections by the Local Authority.
Full RWH systems are best installed whilst buildings are under-construction or being refurbished, particularly in homes where pipework is generally inaccessible. This is less of a constraint in many commercial buildings, however, where pipework is very often accessible in service ducts. Systems providing water for external use only, such as irrigation, are also relatively straightforward to retrofit.
There are no recorded cases in the UK of Legionella or other contagions arising from a RWH installation; independent studies have shown the possibility of contagion to be very unlikely. It is recommended that the tanks used to store harvested rainwater are installed underground as they are large, and not particularly sightly. This means that the water is stored in cool and dark conditions which are not conducive to the cultivation of Legionella. Where the harvested rainwater is to be stored above ground, on small-scale irrigation-only systems for example, protection from direct sunlight is recommended.
The buried components such as the water storage tank, should last indefinitely; components such as the control system, pump and filter have an extremely long working life (typically 10 to 20 years), and are easy to replace should the need arise.
Around £2000 up to £3000 for a good quality domestic system, depending on its size, and a further c£2000 to install. Industrial systems will cost more, but deliver commensurate savings because of increased capacity. It is Government policy to keep the cost of mains-water very low (by European standards) which translates into a "payback" period usually in excess of 5-years, being dependent on local rainfall, local water tariffs and collection area. . Should future water-scarcity change future water-pricing policy, then a shorter payback period would result.
It typically takes 1.5- 2.0 kWh to pump 1 cubic meter of water (1000 litres). For a typical house using rainwater for WCs, washing machine and the garden, this translates into pumping costs are between 5-10p per week.
The benefits of a green roof relate to CO2 emissions, rather than being water-related; moreover, given predictions of future summer drought risks, green roofs would potentially need to be watered and thus add to pressures on summer supplies. Green roofs also substantially reduce the amount of water that can be collected, thus reducing their efficiency as a source of harvested rainwater; they also discolour the water being collected making it aesthetically unacceptable for in-house non-potable uses. Sedum roofs also reduce the amount of water than can be collected (but not the extent of green roofs), and also discolour the water being harvested.