Rain Water Harvesting

A Monty-Python sketch goes something like this: "Live in Bognor?  Don't waste your life throwing away beer cans, send them to Betty Flange in Oldham and she will throw them away for you".  Rain falls on distant hills and through a system of pumps, pipes and reservoirs is delivered to your house.  Whilst you are running a bath, clouds being the promiscuous things that they are, deposit a few hundred litres on your roof which is then discarded into a Victorian sewer.  If any of the Pythons had been engineers, they would have been so much more creative.

In England, the public water supply evolved in the 19th century to replace wells and other potentially polluted sources.  In urban areas whilst water was being drawn from a well, sewage was often dumped into the same aquifer.  The household economy textbooks of the time have much to say on water, drains and ventilation.  In the period between the 19th and the 21st century, the demand for water has expanded as population has grown and and devices such as lawn sprinklers, dishwashers and teenagers have proliferated.  Increasingly new public buildings like schools and libraries are using rain water for toilet flushing.

Being fascinated by environmental data, I was curious to know if there is a balance between the volume of water that falls on my roof and that flushed down the outside loo.  We have a water butt which is fed from approximately 20% of the roof area, so for a few months, I'll dip and drain this and record my findings which can be found here:

Rain Water Data

Sometime in May, a data gathering problem emerged.  Once we started using rain water in the garden, measurement became impractical.  So during the summer months, I'll make some estimates based on rain fall data, which will appear soon.......  What data is available is shown below:

Comment

By July it is apparent that:

In winter, supply exceeds demand and much water gets dumped into the drain.  In summer, the supply dwindles.

The solution to balancing supply and demand is a large storage capacity.  For a new build property, it may be practical to include 10,000 litres or more of storage capacity as part of the design.  For an existing property, it is difficult to add tanks or ponds that store large volumes of water safely at a reasonable cost.

It seems that a solution which combines bath and shower water with rain water.  A 150 to 250 litre water butt (plus pipes and pumps) is a possible solution.

Water Metering

In many areas, water companies have a legal obligation to install water meters, the objective being to stem the rise in consumption and create some awareness of patterns of consumption.  There is some anecdotal evidence that water metering increases the cost of water to the average household illustrating how hard it is to reconcile  resource management and the needs of shareholders.  The following comments are based on a single leaflet from our local supplier, readers should check what is happening in their area.

Water meter readings are used in two calculations, first for the delivery of nice clean water at a cost of approx. 95p/cubic metre, the second for the removal of nasty dirty water at a cost of approx. £1.60/cubic metre.

	Average for one Person	Average for three people
		Clean water toilet flushing	Rain and Grey water toilet flushing
Consumption	(litres/year)	(litres/year)	(litres/year)
Cooking, Drinking, personal hygiene	12,000	36,000	36,000
Showers and baths	11,000	33,000	33,000
Toilet Flushing	17,000	51,000	0
Laundry	6,000	18,000	18,000
Dishwasher	2,000	6,000	6,000
Car washing, garden etc.	2,000	6,000	6,000
Total Consumption	50,000	150,000	99,000
Cost		£	£
Clean Water - Fixed		26.00	26.00
Clean Water - Variable	0.095p/litre	142.50	94.05
Waste Water - Fixed		48.00	48.00
Waste Water - Variable	1.600p/litre	240.00	158.40
Total		456.50	326.45

Its unusual for estimated savings to fully realised in practice, but the above calculations suggest that using bath water for toilet flushing might result in savings of approx. £100/year for the consumer and 50,000 litres for the water company.  For such a scheme to be attractive, the cost of installation should be between £250 and £350 which would result in a simple payback of less than 5 years.

Water Resources

Both water consumption and energy/emissions represent the same problem.  The ideal solution is for all of us to consume less.  However, the commercial model by which we are supplied links profits to increased consumption (if I was paid by the word, this rant would be longer).  We need viable energy and water companies, but we also need a more imaginative framework for them to operate in.  Offer this problem to Monty Python, design a system where you get paid more for doing less, a less viable alternative might be to consult a bank.

Storage

It is clear that the 170 litre water butt is too small, both to harvest the available water and to provide a buffer to provide water during dry periods which might last for several weeks.  This leads to two questions.  The first is how much storage is actually required, the answer is almost certainly greater than 1,000 litres and probably closer to 2,500 litres.  As data accumulates, the answer will become clearer.  The second is how much does storage cost.  Initial research suggests an installed cost of between 10 and 50p per litre.  Reach for a calculator and do the sums and you will see that the choice is between a down-payment on a new car or an off-grid toilet.  This choice it typical of DIY planet saving.

Safety

Further research is needed, not least into the safety implications of water storage which include risk of drowning and the creation of a breeding ground for waterborne diseases.

Page Updated: 08-July-10