Rainwater harvesting quite simply collects the rain which fall onto roofs, then stores it in a tank until required for use. When required, the water is then pumped to the point of use, thus displacing what would otherwise be a demand for mains-water. In the process, a volume of water is kept out of the storm-water management system, thereby helping to reduce flooding risks.This added to the increasing installation of mains water meters and the need to restrict our use and quantity of mains water makes this a viable system in both economical and ecological sense.

Rainwater harvesting is not a new concept; however, over the last century its use has diminished with the availability of a clean, inexpensive and reliable water source through the mains supply.

More recently, water demand has started to exceed supply, and localised flooding has become an issue. Industry experts are now recognising the important role that rainwater harvesting (also known as rainwater recycling, and greywater recycling) has to play in alleviating both these problems.

Rainwater harvesting systems overview
Rainwater is captured from the roof(s), and brought to a central point, via normal guttering and down-pipes, to enter a storage tank (frequently underground), where it is filtered on entry. A highly efficient and reliable submersible pump delivers the water to a service on demand. Where appropriate, or a special reason applies, delivery can be to a normal header tank instead.

The size of the storage tank is determined by considering the amount of water available for storage (a function of roof size and local average rainfall), and the amount of water likely to be used (a function of buildings occupancy and function)

Considerations for fitting a rainwater collection system to an existing property

• The external drainage from the roof needs to be modified to bring the water to a central point.
• Access for the tank and excavation is required.
• Internal plumbing is usually required to be seperate out the drinking (incl. bathing) water from the non-drinking water (WC, washing machine & outside tap).
• Reasons for rainwater harvesting
• Rainwater harvesting (including rainwater recycling and some versions of grey-water recycling) displaces a large proportion of the water that would otherwise be provided by the mains supply - thus reducing overall water supply costs.
• It can provide an off-mains supply for remote areas.
• It enhance a property and gains Eco-homes rating points.
• It can form part of an attenuation and rainwater management scheme, by reducing storm-water runoff and controlling the flow-rate off site.
• Increasingly planning departments are looking more favourable towards the concept of rainwater harvesting. !

Collection and use of rainwater would appear the obvious way forward in reducing mains water demand and surface water run off. Rainwater is clean, free, requires no transportation and arrives on a fairly regular basis in most parts of the UK.

Water is becoming an increasingly precious natural resource. Whilst we appear to have an abundant water supply 97% of the 1.4 billion cubic kilometres of water on Earth is sea-water, 2.7% of the remaining 3% is permanently bound up in ice at the poles. This leaves only 0.3% of the Earth’s water resources as usable fresh water. This available water moves in a permanent cycle through evaporation and rainfall. T

Worldwide
Worldwide water conservation devices are being employed and developed to combat water availability problems. The UK is no exception. There are distinct water demand, supply and disposal problems facing water companies, developers and home owners alike that may present serious short and long-term challenges for the future. Rainwater Harvesting could be employed to help alleviate these challenges. Surface water, (a large body of unwanted water to be disposed of quickly) is currently causing flooding to unprecedented levels, not previously seen. A little known fact is that for every 10% development of an area, ‘Surface Water Runoff’ increases by 50%. Therefore to collect and use significant amounts every day on new developments would make storm water a social rather than a weather driven event.

How do Rainwater Harvesting Systems work?
This separate system collects rainwater from the roof via gutters and down pipes through a gravity type filter into an underground tank. This initial filtration takes out larger particles from the rainwater. The oxygen rich water then flows into the tank through a calmed inlet, where a second biological purification takes place. There are various other components inside the tank necessary to guarantee good water quality. A submersible pump in the tank delivers rainwater to the various applications, an automatic change over to mains water in times of low rainwater prevents any discomfort to the user.

Roof material
Most common roof materials are suitable for rainwater collection, slate concrete or plastic. A pitched roof is generally better than a flat roof as this reduces evaporation. However both pitched and flat roofs are acceptable. Wire balloon gauze type filters should be fitted in the top of down pipes at the end of gutters to prevent larger debris, leaves, twigs etc from entering the tank, after which there are minimal traces of contamination in the form of organic material that flow from the roof every time it rains this material is separated out by a special gravity type filter.

Filters
Various filter systems exist but few work in a positive way with rainwater systems. The most important factors to consider are that the filter must not retain any dirt particles, be self-cleaning and dry quickly after rainfall otherwise germination may take place on the filter face. Filters that retain dirt particles such as sand, gravel and basket types require regular cleaning. Pressure type filters, charcoal etc should be avoided as they place an undue load on the pump, reducing longevity.
The latest and most advanced filter designs are integrated into the tank, the best of which is a gravity type two-step cross flow self-cleaning filter with pressure wash cleaning system, the filter collects almost 100% of the incoming rainwater. The second stage takes out larger particles, grit etc; the second takes out any remaining smaller particles. Because no wet dirt remains on the stainless steel filter face, the filter dries out quickly after rainfall ensuring that germination cannot occur.

Tanks
The quality of incoming rainwater is very good; to maintain and improve this we have certain components in the tank. The tank is installed below frost depth where the stored oxygen rich water is kept below 10º C and protected from light, ensuring non-growth of algae etc.

Water quality
Rainwater flows into the tank via a calmed inlet, oxygenating the tank water by directing the fast flowing incoming rainwater upwards towards the surface every time it rains and also prevents sediment on the floor of the tank from being disturbed. There is a natural separation of any remaining incoming dirt particles, the heavier particles sink and the lighter particles float on the surface. Studies have shown that sediment on the floor of the tank is minimal, never needing to be cleaned out and causes no risk to water quality. When we have oxygen rich tank water a further natural biological cleaning process takes place. Rainwater is now cool clear colourless and odourless.

The floating layer could be a problem and would hinder oxygen circulation. However optimal tank sizing guarantees the tank to overflow a number of times each year through a special overflow siphon with rodent barrier, skimming off this floating layer. The third in-tank component is the submersible pump with floating extraction hose connected to the suction side of the pump extracting the cleanest water from 15 cm below the surface.

Many European tank manufacturers produce specialised polyethylene rainwater tanks to meet the demands of this technology, they are made from virgin polyethylene of robust construction, heavily ribbed, have push fit pipe connections, telescopic access dome to finished levels and internal inlet, filter and overflow pipework ready fitted.

The main problem to overcome is to supply mains water to the system when rainwater is not available. As cross connection between the mains and rainwater systems is unlawful the water industry demands an air gap between the mains and rainwater supply that must be twice the diameter of the mains water pipe and not less than 20 mm. When rainwater is unavailable a small amount of mains water is automatically fed through a solenoid valve and air gap tun-dish directly into the rainwater tank to maintain a minimum level.