For the calculation below, the roof area is the flat area of ground covered by the roof – not the pitched area. Average rainfall figures for many UK locations are available from the Meteorological Office. Suppliers should be able to give estimates for system efficiency (for example a ‘WISY’ filter is 90% efficient). The run-off coefficient of a pitched, tiled roof is 0.75, but a flat roof is 0.5 or less becuase more is lost to evaporation. You also need to check that your roof is clean.

Annual collection in litres = Roof area (in square metres) x Annual rainfall (in mm) x System efficiency x Run-off coefficient of roof

A tank would normally be sized to store 5% of the annual collection. This keep the tank fuller most of the time, which prevents stagnation. 1000 litres is equal to 1 cubic metre (m³). Water should enter the storage tank just above the bottom so it doesn’t disturb sediment collecting there. Draw water from below the surface using a submersible pump to avoid picking up pollen or other floating debris.

A filter that is 90% efficient when clean can drop to below 30% if algal growth blocks the holes in the mesh, so they need to be in a position where they can be accessed and cleaned occasionally.

A basic water butt to collect water for the garden should be very low cost, and they are sometimes deals available from your water company.

A rainwater harvesting (RWH) system providing toilet flushing for a 3 or 4 bedroom house is likely to cost about £3,000 if professionally installed. Costs for retrofitting systems will generally be higher than for those built into new houses. It may be possible to put in a simpler gravity-fed system (e.g. for a ground-floor toilet) for much less.

You also need to factor in annual checks of the system. Some of these may need to be done by a professional, such as checking the tank for leaks & debris, the pump for corrosion, or the controls & sensors for accuracy.

The payback time can be very long, and some systems may never recover costs if the parts need replacing before savings are realised. Dr Judith Thornton, water treatment expert and author of CAT’s ‘Choosing Ecological Water Treatment’ book, has researched this area for many years. She writes:

“…in general they wil not decrease your sewage charge. In rough terms therefore, given an average UK bill (metered users) of around £330 per year (for water and sewage), and likely water savings of around 20-30%, the system might save you £30 per year, and that is before you account for any maintenance or capital costs. A comprehensive PhD study that modelled several thousand different configurations of system demonstrated that mains water would need to cost more than £10 per cubic metre before RWH systems made financial sense. Regional differences in rainfall and water bills mean that rainwater harvesting systems may be economically viable in some instances (for example in the south-west of England), but do your own calculations rather than relying on those from RWH system suppliers.”

Where rainwater harvesting for toilet flushing is appropriate (e.g. in a larger building), choose the equipment carefully to minimise environmental impacts.

To minimise the environmental impact of one of these systems the book recommends a polyethylene tank rather than fibreglass. The tank should be a type that does not need concrete backfilling when installed.

Pay careful attention to the energy demand of pumps and other components, and also to the durability of these. Choose an efficient, low wattage pump that is well-suited to the head & flow required.

Good controls will enable you to see how the system is performing. For example how much mains water backup is used, and how much water is in the tank at different times of year. It’s probably worth metering both rainwater input and mains backup.

In the UK, treating rainwater to a high standard for bathing or drinking is not recommended. Besides the expense, the impact of all the equipment needed outweighs the environmental benefits of reducing mains water use. Small-scale water treatment systems use lots of energy in manufacture and use, and the filters need to be regularly replaced, creating waste.

Even systems for using rainwater for non-potable uses within a house have quite a high environmental impact.

If you are not on mains water then collecting and treating rainwater is an option – but in the UK, groundwater or a stream will usually be more reliable and need less treatment.

Treating rainwater to a higher level is more suitable in dry countries (e.g. Australia), or places with very dirty groundwater (e.g. Germany). In these cases, the extra expense and energy use might be justifiable against the alternatives.

Please note that if you are not on mains sewerage, the Environment Agency may require that all grey and black water can be treated through your septic tank or similar system. This means you might not be able to save by putting in a smaller system.

Household ‘grey water’ (from showers and sinks ) is generally quite diluted and in some cases can just be used to irrigate a garden. Otherwise, the effective dispersal of grey water depends on soil porosity. You need to disperse it into the ground over as wide an area as is necessary to ensure you don’t have any surface water. For example by using a drainage field.

Where soil porosity is very poor, you’d need to clean the grey water before discharging it, possibly using a gravel filled tank such as a horizontal flow or subsurface bed. You’ll need some kind of trap at the front end of the system that will catch any solids, because hair is particularly likely to block things up and cause problems. You may also need a surge tank, depending on how much the flow rate varies.

For a simpler and lower cost option than a drainage field or a reed bed you might be able to consider a willow bank or a trench arch.

A willow bank is essentially a pile of bark peelings or wood shavings at least 1.5 metres deep and with a surface area of 5 to 10 square metres. It’s retained in some sort of frame, which can be a ‘living structure’ made from willow. Grey water is piped onto the middle of the pile. The solid matter is filtered out and humifies, while liquids will drain through and break the wood shavings down into a compost, which will itself be a good water-cleaning medium. The water emerging from the bottom will feed the surrounding willows. The willows need annual pruning, and wood shavings will need to be added less often.

A trench arch is like an elongated soakaway. It is a long gravel-lined trench capped with paving stones that can be pulled up for inspection or maintenance, or a row of pipes cut longitudinally in half. Raw greywater can rush along it until it finds somewhere nice to settle into. Worms and micro-organisms deal with solid matter and soon create a nice composty medium which treats the water very well.