Collecting Rainwater

We shouldn’t have to consider recycling domestic water in the UK, yet, as the recent droughts have shown, the cold tap can all too easily run dry. Nationally, there is enough rain to provide fresh water for everyone but, of course, the centres of population are not necessarily in the wettest parts of the country. Unlike the National Grid which distributes electricity nationwide, there is no total network for distributing water, although suggestions for using a restored canal system have been and may still be discussed. Reservoirs are needed next to cities but the siting of new reservoirs will always be met by public outcry, perhaps rightly so, for they submerge tracts of otherwise useful farmland and often houses or even villages.

You will receive no official encouragement if you do attempt to save or recycle water but since some of the means for doing both are very simple it seems a shame not to have a go. It has been demonstrated that domestic water consumption can be cut (remember those hosepipe bans and plastic bags full of water to put in the cistern?) and an effort to cut our consumption permanently could cancel out the need to build new reservoirs as insurance against the hazards of another drought. It would also be more economical if everyone could reduce their consumption of purified water.

Houses have roofs to keep the rain off, and roofs have to have a way of disposing of this water so, if you have a house, you are already in a position to intercept and catch the rainwater. It will make no difference to the final destination of the rainwater whether it goes from gutter to surface water drain and so to the river or into your storage tank.

To estimate the amount of water you are likely to collect you will first need to find out the annual rainfall in your area. This figure should be an average of the three driest consecutive years known, if such information is available, so that there is no over-estimation of the amount of rain to be collected. Detailed rainfall information can be obtained from any local institution that collects weather data (such as an RAF station). But, if such information is not available, use two-thirds of the average annual rainfall rain, the amount of rainfall collected in litres will be equal to the area of the roof, times the corrected average rainfall, times 0.9.

If you intend to use all your collected rainfall, you will need a storage tank and this should be large enough to hold at least 25 per cent of the estimated annual yield. Such a storage volume should prevent any dramatic overflowing during a storm but the storage tank should be linked to a soakaway or watercourse just in case. The estimated figure for yearly rainfall collection will be only a rough guide to the amount of water that might be obtained from a roof; but it is a help when it comes to deciding on the dimensions for a storage tank.

Before considering how to use the collected rainfall it would be well to examine the present patterns of water usage in houses. Different sources of data give slightly different figures but the following list is typical: the figures represent the average water consumption in litres per person per day.

uk-rainfall

Value for your area as shownon the map. The whole business of estimating rainfall collection is exactly that, an estimation, so worrying too much over the rainfall value is probably inappropriate.

It is worth remembering that lmm of rain provides 1 litre of water per square metre of catchment area. The catchment area is the area of the roof for a flat roof or the area of the roof in its plan form for a pitched roof. Depending on what you wish to use the rainwater for you may or may not want to collect the first run-off from the roof. This initial water flow from the roof will contain dirt, bird droppings and leaves from the roof and gutters; so, if you intend to use rainwater to wash clothes, this portion might be best avoided. If the rainwater is to be used only in the garden, you can just put some kind of screening at the entrance to the storage tank to collect the worst of the detritus. Assuming that you might lose 10 per cent of the rainfall in initial run-off, evaporation in summer and spillage over the edge of the gutter during heavy

total hot cold

  • W.C.: 43 0 43
  • personal hygiene: 55 40 15
  • laundry: 21 7 14
  • washing-up: 6 5 1
  • drinking and cooking: 4 4
  • garden and car : 2 2

Total: 131 52 79

On top of this total of 131 litres it should be noted that of the total amount of water supplied to us approximately 13 per cent is lost through leaks and bursts in the pipes: this represents not only a waste of water but a waste of purified water, and purified water costs money. Any savings made by you reducing your demand for water will be magnified because less will be wasted.

When it comes to matching rainfall to cold water demand we could imagine a household of four people, living in East Anglia under a roof with a surface area of 50m2:

yearly average rainfall=549mm

2/3 yearly average rainfall=366mm

rainfall collected =366 x 50 x 0.9=16470 litres a year.

The obvious uses for rainwater from the table of water consumption are watering the garden and washing the car. For the household in our example annual water consumption for garden and car is:

2 x4 x365 litres a year=2920 litres a year. This may be a low assumption since other sources put this figure at ten litres per person per day, which would give an annual consumption for the household of 14600 litres, which could still be met by the rainwater. You may have to use some form of pump, such as a government surplus stirrup pump, to transfer the water from the storage tank to the garden. As a result you will lose the convenience of mains pressure but you will also beat the hosepipe ban.

An excellent use for collected rainwater is flushing the loo. Rainwater itself is too impure to recommend for drinking, especially in towns where it carries not only dust, dissolved sulphur dioxide and other pollutants but also dissolved lead, the product of a motor industry that demands high octane fuels. It could be used for cold water rinsing of clothes, baths, and hair washing. But as we have become so accustomed to thinking that any water coming from a tap is pure enough to drink, it may not be worth the effort and possible risk of actually plumbing in a separate system with an indoor rainwater tap. This is accepted on trains where the wash basins are labelled ‘not drinking water’, but might be thought strange in a house.

Flushing the loo, however, presents none of these problems and the only drawback is the large quantity of cold water required by the modern cistern. For the four person household the total comes out at 62780 litres a year, a quantity that you might collect on the roof of a large house in Cornwall. However, the quantities of water used for personal hygiene and laundry in the table adequately cover the quantity used for flushing the loo; and it may be that recycling this used water would be a better solution.

The other way to save water is to change the type of appliances used in the house. A bath, depending on its size and the wallowing habits of the users, consumes between 60 and 100 litres of water whereas a shower uses only 4-5 litres per minute: it was once stated that if every person in the UK had a bath once a week there would be a serious water shortage, and this was before the drought. Not only will the use of a shower save water but it will also save hot water and that means a saving in energy, whether you are paying for it or just trying not to deplete your solar-heated water too quickly.

The Building Research Establishment has found that spray taps halve the amount of hot water used in a wash basin. Although these measurements were made in lavatories in office buildings such a saving could still be made in the house. The temptation is to wash one’s hands under running water, which is quicker than filling and emptying the washbasin, and this is where a saving in water could be made by using a spray tap. Having installed all these water saving devices you may then find that you do not have enough waste water to recycle for flushing the loo, but it can always be topped up with some of the collected rainwater.

It must be admitted that, unlike people living in other EEC countries, people in the UK have no incentive to save water as it is not metered. We pay our water rate whether we use the average 130 litres a day each or twice that. Some water authorities charge more for an outside tap or unattended hosepipe but for most of us water appears as a ‘free’ commodity. Whether it will always be so is debatable but attention to water conservation could prevent depleting the reservoirs again next time we have a real summer. This in turn would avoid the expense and inconvenience of flooding more land to store more water, so it must be a good thing. It is obvious that you will save money by installing energy saving showers and spray taps but even if your water supply is not metered, rainwater should not go straight to the drain. At the very least it should be collected and used for watering the garden. If only to grow a lusher and more palatable lettuce, bring back the water-butt.

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