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Rainwater Harvesting

In many areas of the country, particularly the south east, water supply has become a serious problem. What you design in rather depends on what you want to achieve. There are different levels –

  • rainwater barrel to water the garden
  • large water tank system for garden, car washing and flushing toilets. It might possibly be the case that planners insist on rain water collection in certain areas of low rainfall.

However, research by the Environment Agency in their:

shows that although water may be saved, there is an increase in the energy used by both rainwater harvesting and grey water recycling systems compared with mains water.

“Considering cumulative cradle to gate embodied and operational carbon, all rainwater and greywater systems included in the study give rise to additional net carbon emissions over their lifetimes”

The report is quite detailed and worth reading if you are considering rainwater harvesting. The problem mainly comes about because of the energy used by pumps and also the embodied energy of tanks and associated equipment.

rainwater barrel to water the plants – there are products such as the Rain-Sava which can be fitted into a rain water fall pipe to divert rain water into a barrel. When the barrel is full the rain water stays in the fall pipe. Reused plastic barrels are usually available from companies which clean and recycle barrels, very often ones used to import fruit juices.

multiple barrels – It is possible to link fall pipes where it is convenient, and save the water in barrels linked together, with an overflow to a gully. It will probably need some basic calculations for the sizing of fall pipes and barrel connector pipes. Bear in mind that fall pipes are usually oversized for the area of roof they serve and you may not need to go above the 68mm diam. pipe size. Most rainwater goods manufacturers have sizing tables on their web sites (eg. Hunter Plastics). It is usually sufficient to link the bottoms of the barrels with 22mm push fit plumbing (anything larger is difficult to find tank connectors for).

large tank system – Various tank systems are available, usually for burying in the ground near the house, so it is important to know that you can excavate sufficiently deeply. Depending on the annual rainfall in the area you may be able to quite easily supply 20% of your annual needs for water from the house roof.

Take for example a two storey four bedroom house with a total roof area of 80 m². in an area which gets rainfall of 600mm p.a. Assume there are four people in the house each using 150 lit. per day.

number of days supply p.a. = 80 x 0.6 x 1000/(4 x 150) = 80 days

However there are several other possibilities for harvesting and economising

  • collection of water from outdoor areas such as car parking and paving
  • collection from outbuildings
  • economising with water saving taps and showers
  • composting toilets which use no water
  • recycling grey water for toilet flushing

Regarding the first two ways of collecting water, drainage will probably have to be supplied for these areas anyway so it is easy to link that into the storage.

It may well be that using these extra areas for water collection could double the amount collected. Using composting toilets will immediately save 30% of water. The use of various types of water economising features could save another 20%. The equation might then look like this

number of days supply p.a. = (80 x 0.6)+(80×0.4)x 1000/(4 x 80) = 250 days

This assumes only two thirds of the water gets stored from the extra outdoor areas.

The way this type of system works is that rain water is filtered before it reaches the tank. If there is too much then the tank overflows into the drains. If the tank gets below the level of being almost empty then mains water is allowed into it to provide enough water for the house. There is a pump in the tank which supplies the house in the normal way. Drinking water cannot come from the tank because, by law, this needs to be off a rising main so that residual chlorine is present in it for disinfection.

A few points to bear in mind –

  • Correct tank sizing is very important. Too large and it will be a waste of money; too small and it will not catch enough of the water during rainy periods.
  • Collecting water from hard paved or parking areas is a useful contribution to a SUDS approach.
  • However there is more pollution on the ground than on a roof so this needs to be covered by the filtration system which comes with the tank storage system. Filtration systems can involve considerable maintenance if the collected water is of low quality.
  • There will be less water collected from a living roof, particularly a turf roof.
  • Similarly there will be less water collected from recycled polythene paving systems which use grass or recycled gravel infill.
  • Note that underground tanks can tend to float up out of the ground when they are mainly empty and the ground is flooded. They need anchoring, possibly with concrete.

4 comments to Rainwater Harvesting

  • bobthebuilder

    Hi Gary,
    yes, put a ball valve in the lower one and feed the valve from the upper one

  • gary

    do you know how to connect 2 tanks together if there on top of each other, so the bottom is alway full.


  • bobthebuilder

    Hi Drainman,
    probably not (depending on what you mean by short period, what part of the country you live in, whether it is an easy matter to install a storage tank and how much water you use). However if it makes the difference between getting planning permission or not it might be well worth it.

  • Drainman

    Is the cost of rain harvesting worth it over a short period

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