Rainwater Harvesting & Use

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Calculations need to be done to compare expected annual to proposed system cost.

Why

Save money for metered customers
Ensure enough water supply for well users

Why not

Using rainwater is not ecologically friendlier than mains water, as Britain is not short of water. We have such an excess of it we can afford to waste billions of gallons. Britain is not a dry country with a desert climate. Thames Water, with just 8 million customers, was reported to lose over 300 billion litres per annum through leaking pipes. That's almost a billion litres a day.

Many today believe Britain suffers water shortages, that water is in short supply here.

Link to uk.d-i-y thread that explains all this

Numbers seem to show hosepipe bans reduce water use by less than 1% of leaky pipe losses.

Rainwater Uses

Untreated rainwater can be used for

Toilet flushing
Garden watering

If suitably treated it can then be used for many other purposes.

How to Harvest

Rainwater downpipe
Divertor
Filter
Tank/barrel
Overflow device

A filter on the rain tank inlet will keep detritus out and reduce decomposition in the tank. This is normally enough precaution against decomposition, since the tank water is being rinsed through on an ongoing basis.

Commercially available divertors have filtering and overflow devices built in.

Adding More Storage

Sometimes there is not enough space to replace the tank with a wider one. Its possible to connect a 2nd butt or tank to the first. The connection should be made low down on both tanks, but not right at the very bottom, as we wish to keep sediment out of the connecting pipe. The connecting pipe should be kept as short as possible. Long thin pipes will give slow flow, and will cause less than 100% second tank fill during a downpour, reducing water capture.

Any additional tank should not be on a dead leg. For example with a 2 tank system one should feed water into tank 1 and take water out from tank 2 to ensure both tanks see regular water refreshment.

Keep the top of all tanks at the same level, otherwise one will overflow or not fully fill when another is full.

Tank bases don't need to be level, so using different sized tanks is workable. If doing this, water should flow into the shorter tank first, then to the taller one, so that all the water can flow out and be used.

How to Use Rainwater

System Designs

There are 2 main ways to do it.

Pumped direct to cistern

This method uses no header tank. The toilet cistern has a float switch, and when water is wanted, the pump moves water from outdoor storage tank to cistern directly.

Simpler cheaper design than a loft header tank
Cistern fill speed dependant on pump specs, an inadequate pump would mean slow fill.
Less to go wrong
Less cost
Easier access to the cistern float switch
Lower pump specs needed
Less pump power used
Good for 1 toilet

Vented loft header tank

A loft header tank provides the necessary water pressure for the loo cisterns, and this header tank has a float switch to operate the rainwater pump. When the header tank needs a topup, the pump delivers water to the header tank from the storage tank.

A 2nd float switch low down on the header tank detects low water level, and delivers mains water when the header water level gets low.

Loo cistern is unmodified and fed from this header tank, which means replumbing the cistern feed
Pump needs a higher head rating
Header tank gives a little additional water storage
Suited to multi-use setups, eg buildings with multiple flush cisterns.
Pump intake requires a filter to exlude fine grit, which could otherwise cause the cistern valve to let water dribble through continuously

Automatic mains water backup

Automatic mains backup for loo cistern fill can be provided in 2 main ways

1. With a headerless system, have the mains toilet cistern valve set to a slow fill. The cistern will then fill with rain and a little mains each time, and if rain runs out it will fill more slowly with mains.

2. For a header tank system, have a 2nd float valve in the rain header tank. Adjust it so that if the header tank water falls to 20% then mains water tops it up to that 20% mark. This system runs on all rain until rain is nearly run out, then mains is used. Fill times are not affected when running on mains.

Pump Specs

Whatever pump is used, check its head spec. Some pumps will have insufficient head to deliver any tanked water to a loft header tank.

Be aware that real world flow rate is generally far below the manufacturers claimed flow rate. Flow claims are normally based on there being no pipe connected to the pump outlet. In the real world:

The pump operates against a column of water (or head) which provides back pressure
Pipes have resistance to water flow

A better idea of real life flow can be gained by plotting head versus flow rate on x,y axes, and drawing a straight line from max flow to max head. This enables flow rates to be read off for any given head. This does not take piping resistance into account, which will lower flow rates further.

Safety

There are 2 Safety related issues with rain water:

Infection

Use a filter on the entrance to the storage tank to minimise muck
have a close fitting lid on the tank to keep insects out
only use rainwater for purposes safe for non-sterile water, eg loo flushing and garden watering.

Overflow

Float switches and fill valves do fail at times.
To reduce switch failure, use an RC snubber across the switch contacts.
Check your cistern overflow can handle the flow from the pump by holding down the float switch so the pump continues. If it can't, either improve the overflow or reduce pumping rate until it can cope.

Also Relevant

Greywater use
Rainwater and bathing
A 15 minute shower uses more water than a bath
Water charges are based on an assumed relationship between water used and waste water, and both are charged for. Rain harvesting will change this relationship.
Water treatment
Legal issues
Toilets rarely flushed may suffer cistern water stagnation, and are better supplied with treated water.