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| UTILISING RAINWATER (Continued) |
| THE SOLUTION |
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The concept is simple. Rainwater is collected from the roof of a building and stored ready for re-use as required. In a anormal domestic situation this water can be used for flushing toilets, general cleaning, garden irrication and clothes washing. Typically this can account for at leaast 50% of the water used for most of the year. In many commercial situations and in educational establishments, this figure can be far higher, and where the mains water supply is metered, savings can be significant. |
| HOW RAINWATER HARVESTING WORKS |
| Water is filtered and collected from the roof's normal drainage system and then gravity fed to a storage tank, usually located below ground. From here it is pumped on demand to dedicated supply points - eg toilet cisterns, washing machine, garden tap, utility room, etc. A pressure switch ensures the pump only operates when water is demanded by the system. A secondary suction filter ensures that any debris that should find its way into the tank is prevented from entering and damaging the pump. The system senses when there is insufficient rainwater stored and reverts to using mains water, thus ensuring continuity of supply. It is important to be aware of the limitations of rainwater use. It is not suitable for drinking or bathing, and its use is not recommended in hot water systems. |
| SYSTEM DESIGN |
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There are several design factors which are important to the success of any rainwater harvesting system. Firstly it is necessary to calculate the correct storage volume for each individual installation. A good rule of thumb is 1000 litres (or one cubic metre) for every 25-40 square metres of roof area. A storage tank should generally not be too large; partly because you will be paying for space that never fills, but also because it is important that the tank overflows at least twice per hear in order to remove any pollens etc that float on the surface. Secondly, it is important that only rainwater is allowed to enter the system, so downpipes should not discharge into open gulleys, but into closed drains via a 'Uni-drain' connector or similar. The tank overflow should incorporate a trap to prevent any foul odours from the drains entering the tank. If the overflow connects to sewer rather than a soak away, then provision should be made to prevent backflow, or vermin entering the tank. The inlet pipe should be taken to the bottom of the tank where it should terminate in a smoothing inlet that causes minimal disturbance to the sedimentary layer. Proprietory fittings are available to suit these requirements. The tank itself should be located below ground or within a suitable building, but must not be exposed to frost or direct sunlight. |
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