Rainwater Harvesting & Use
Calculations need to be done to compare expected annual to proposed system cost.
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
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
- 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 bigger one. In such cases, connect a 2nd butt or tank to the first. The connection should be made low down on both tanks, but not right at the 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.
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.
How to Use Rainwater
There are 2 main ways to do it.
Pumped direct to cistern
- 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.
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.
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.
There are 2 Safety related issues with rain water:
- 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.
- 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.
- 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.