WATER is becoming an increasingly precious natural resource. Whilst we appear to have an abundant water supply 97% of the 1.4bn cubic kilometres of water on earth is seawater. Of the remaining 3%, 2.7% 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. There are also problems with the quality of drinking water as increasing water pollution caused by nitrates, phosphates, pesticides and other chemical substances is making purification increasingly difficult and expensive.

Stemming the flood

Surface water is currently causing flooding to 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 would make storm water a social rather than a weather driven event. 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 utilisation could be employed to help alleviate these challenges.

However, in the UK, systems are very much in their infancy as this process has only just begun.

How do rainwater 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.

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 first stage takes out larger particles, and the second takes out any remaining smaller particles.

Tanks

Rainwater flows into the tank through a calmed inlet, and oxygenates 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.

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.