Water don't waste a drop!

Rainwater harvesting

Forget the rising cost of mains water. Collecting rainwater from our own roofs may eventually be the only way of ensuring an adequate water supply for our buildings during a drought. Roger Budgeon and Derek Hunt, from Rainharvesting Systems Limited, report...

Domestic rainharvesting system diagram

It is likely that rainfall patterns will continue to change bringing shorter but heavier bursts of rain causing flooding and soil erosion. Household water consumption is increasing per capita, and as the climate warms other usage will increase, so new ways will have to be found to reduce, reuse and retain water within the environmental system we inhabit.

...Rainwater harvesting needs to be considered as a part of a long term and overall environmental plan towards more sustainable development. The advantages to us and the wider environment of preventing flooding and erosion of all the water courses are incalculable...

The first measure should always be to reduce the amount of water we use; water efficient appliances, taps, shower heads and toilets should be installed. Next we should look at replacing the high quality drinking water we use for low grade uses such as toilet flushing, process water in commercial applications and water for irrigation and animal husbandry, to name but a few. In our view there is no point in cleaning and adding purifying chemicals to make all the water we use every day to drinking water quality when in a domestic situation this is superfluous for about 50% of it. Commercially this percentage is much higher.

Looking back in history the Romans left evidence of rainwater harvesting systems. The Victorians developed them further and in the 1980's the Germans made the systems “high-tech” and easier maintenance. Most of the currently available systems use many German ideas and products. The global water system should be considered as a total loss system and the water should be kept “in play” for as long as possible, retaining it for later use and reuse wherever possible.

As part of this approach the water storage tanks employed in rainwater harvesting systems act as retention tanks. Not only do they store the water for reuse, but they also contribute to avoiding the flash run-off which contributes to the flooding and swelling of water courses. This will occur as a result of run-off from covered ground and sealed surfaces. Sustainable Urban Drainage Systems (SUDS) are becoming more of an issue and are often a condition of planning permission. The developer is required to attenuate the site water run-off. There are many ways of doing this but if tanks are used it makes sense to install pumps and reuse the water.

Not only is water becoming a product whose supply will no longer continue to exceed demand, the water authorities are now asking for considerable price increases. This is where rainwater harvesting systems come into the picture. This will make the payback time of installing a rainwater harvesting systems very much shorter. With water charges at the present level, the payback time on a domestic system is typically 10-20 years for domestic properties that are metered. (In the UK not all domestic dwellings are metered but all new build is. Commercial sites are all metered.) If you consider commercial properties with large roof areas and high consumption of non potable water, the payback can be very much shorter, sometimes only months. As we have said before the benefits are much greater than just financial, the environmental aspects of retaining water in the system are also very great.

The principles of collecting and storing clean water are to filter the water before storage, store in the dark and below 18 degC, then if designed correctly the rain harvesting system will supply clean water. The debris must be removed from the water before storage, as any decaying material in the tank will use any available oxygen that is in the water. The result will be smelly, putrid water. The stored water must be oxygen rich. The water is collected by the roof gutters and is directed to the rainwater filter where the water is separated from leaves and other debris. The filtered water is then transferred to the storage tank joining the other stored water at the bottom and directed upwards so it does not disturb the sediments on the bottom. The filter system should screen out all but the smallest particles, the heavier sinking to the bottom and the lighter floating to the surface. This is the second stage of the cleaning process. The light particles will be organic material such as pollens which must be flushed out as they will cause the water to become stagnant. This is done by designing the tank overflow to have a skimming effect on the surface of the water when it overflows. The system should be sized to overflow at least twice a year, and in doing so keeping itself clean. The heavy particles on the bottom of the tank accumulate at about 1-2mm/yr, a negligible amount - 50 years will result in 50-100mm. The overflow system will often include an anti-vermin trap and an anti-backflow device if the application requires. The mesh size of the pre-filter system is a compromise between maintenance intervals and water quality. The finer the filter screen the more regularly it will have to be cleaned if the collecting efficiency is to be maintained.

The rainwater stored can be supplied on demand to supply the toilet or need, or it can be pumped to a header tank, which can then in turn, feed the demand. Pumps can be submersible or they can be suction pumps, all are drawing water via floating filter, ensuring the water is taken from below the surface which will be the cleanest water in the tank. The filter on the inlet is purely to protect the pump as in normal operation the water will be clean. The control for the pump could be just a pressure switch, or a more sophisticated flow controller and pressure switch, with dry-run protection all built in (protection for the pump in case the water runs out).

The system must include some back-up in case there is no rainwater in the tank. Toilets need to be flushed and clothes washed. When water level in the storage tank falls to about 10% of its capacity, adding a small percentage of mains water to the tank keeps the system going until it rains again. This can also be achieved by changing over to a small mains-fed cistern, as in a module system. In a header tank system the change over can be achieved by just supplying the header tank with a mains supply, activated when the rain water tank is low. Any back-up or top-up system must comply with the regulations separating the rainwater from the drinking water supply so there is no risk of contact or contamination. The rainwater tank should never be allowed to empty as this would have a detrimental effect on the beneficial bacteria that have established themselves in the tank, keeping the water clean. The incoming rainwater brings oxygen with it keeping the system working.

As described earlier the water collected has many uses without further treatment, it will be particle free so uses that do not involve human consumption or skin contact, such as toilet flush, garden irrigation, and washing machine use, are acceptable without further treatment. If a wider range of uses are required, such as drinking or bathing, or the application is more sensitive, such as a hospital, then a risk assessment strategy should be employed to assess what further purification is required. Particle and UV filtration is often employed in such circumstances. This extra filtration uses more energy so an evaluation of operational cost/benefit also needs to be considered.

A well designed system should only require regular pre-filter cleaning, there will be no other regular maintenance required. Systems with extra particle and UV filtration will require maintenance as scheduled for these components. Commercial systems can be more sophisticated with double pump, duty standby, or multi-pump systems, all sized for the specific application and often with control systems that link to a Building Management System.

The tank itself could be designed to perform more than one function. The top half could have porous sides or have a slow water release system such as a floating overflow, to slow the release of the storm water into the soak-away, the lower half fulfilling the rainwater harvesting function. In another system the bottom half could be an emergency service reserve water supply, so the normal reusable rainwater for the rain harvesting function of the tank would be from 50% to full.

Design calculations

Water supply
It is important that a rainwater storage tank is sized correctly, so that it over flows and flushes itself clean regularly. This requires a simple calculation:

Rainwater collection roof area (plan view m2) x annual rainfall (mm) x filter collection efficiency x co-efficient of collection of the roof = total annual rainwater collection (litres).

Roof materials and pitch affect the collection co-efficient, profiled steel sheeting would transfer the water to the gutters quickly and be good (0.9), a flat roof would suffer more from evaporation and wind blown losses (0.8).

Most pre-filters are very efficient 0.8 to 0.95 when clean but time reduces this so a regular cleaning programme should be instigated, about four to six times a year. This is probably the only regular maintenance the system will need.

So a terraced house with a pitched tiled roof of area 70m2 (co-efficient of collection about 0.8) and a rainwater pre-filter (efficiency 0.9) in an area of the country that has 900mm of rainfall:

70 x 0.8 x 0.9 x 900 = 45,360 litres - would yield 45.36m3 of rainwater each year.

From this figure a proven and reliable calculation that gives an optimum tank size that will give about 18 days storage, is to take 5% of the annual yield. This gives a capacity of 2268 litres. As this takes in account the usable rainwater volume, the 10% “wet” volume at the bottom and dead space at the top will lead you to chose the next standard size tank above this, say 2500 litres. Choosing too large a tank can give a longer number of day's storage but can increase the cost significantly and perhaps risk a decrease in water quality if the tank failed to overflow.

Water demand

Using the same house occupied by two people using say 150 litres/day each, with about 46% (69 litres/person/day) being used for toilet flush, washing machine and gardening results in an estimated annual consumption of:-

69 x 2 x 365 = 50,370 litres (50.37m3).

This figure works out to be of the same order as the rainwater supply possibility, so a significant part of the total water consumption of the house can be met (about 40%).

In a commercial situation the roof areas and potential uses for non-drinking water are higher so the volume of water saved is much greater. A commercial example might be:

A factory with a roof area of 1000m2 and 900mm of rainfall employing 40 people, using rainwater only for toilets. 1000 x 0.8 x 0.9 x 900 = 648,000 litres potentially to collect. 40 x 3 (visits to the loo) x 6 litres x 5 days x 52 weeks = 187,000 litres required.

This example illustrates that more than enough water can be collected off only half the available roof area. Rainwater harvesting needs to be considered as a part of a long term and overall environmental plan towards more sustainable development. The advantages to us and the wider environment of preventing flooding and erosion of all the water courses are incalculable.


About the authors
Rainharvesting Systems Limited, UK leaders in rainwater collection and reuse, have been designing rainwater recycling systems since 1994. As well as developing rainwater harvesting technology and the UK rainwater harvesting market, they have designed, specified and supplied countless trouble-free rainwater harvesting systems. Rainharvesting Systems is a member of the Greenshop Group of companies and founder members of the UK Rainwater Harvesting Association.

This article* has been reproduced with the kind permission of the Green Building Press and Rainharvesting Systems Limited. It first appeared (2004) in the Green Building Magazine (formerly Building for a Future). The Green Building Press is dedicated to promoting sustainable and environmentally responsible construction. Copyright © Green Building Press
*updated April 2010