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Water - the big picture
Rainwater - the free & available resource
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 - a precious natural resource
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. 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.
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.
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.
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.
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.
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 pumps are specific to use and made of high-grade stainless steel and brass with low noise characteristics, maintenance free, corrosion resistant and suitable for continuous operation and of low energy use to ensure water cost savings are not spent on electricity for the pump.
Drinking water supply
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.
Design Hygiene Aspects
The water quality in rainwater systems has to fulfil European regulations for recreational bathing water (76/160 EWG).
Scientific studies and reports were undertaken to determine the quality of collected rainwater from the roof. There were two main areas of examination. Firstly the rainwater in the tank was microbiologically analysed. The results showed that concentration of pathogen germs in all samples taken were of no hygienic risk for use in toilet flushing, clothes and car washing and garden irrigation.
Secondly studies were carried out comparing the quality of clothes washed in both mains water and rainwater. The results showed that there was no difference in quality between the two.
Brian Hirst, Managing Director
FREEWATER UK Ltd - The Rain/Greywater System Specialists, tried tested, proven technology
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The UK's market leaders in rainwater harvesting & greywater recycling systems technology