Rainwater harvesting: The Basics
Rainwater harvesting is simply collecting the rainwater that would normally run off your roof and down the drain and putting it to better use. Harvested rain water can be used to flush toilets, water gardens and wash your car instead of using treated mains water that you are paying for and which requires energy to produce.
Rainwater is most easily collected from roofs or potentially other surfaces like driveways, and then channelled to a tank for storage. Before it enters the tank it is filtered to remove any debris. In the tank it is treated and stored until it is needed.
Rain water is non-potable, which means that it is not drinking water. It can be used for flushing toilets, washing cars, watering the garden and for washing machines.
How Rainwater Harvesting Works
It is as simple as it sounds, in principle. There are a few things that you need to think about however:
- The Collection Area - the size of this will determine how much water you will be able to collect from it. Also bear in mind that if the collection area is not just your roof you will need to ensure that the system will be able to filter out contaminants such as oil or animal faeces.
- Sizing your tank - This is the most expensive part of your system so it is well worth getting this right. Your choice of tank will depend on a number of factors;
- The size of the collection area
- The amount of annual rainfall in your area
- Water losses; water that is lost before it gets to the tank. The type of guttering on your roof and the type of filter the system has will affect this
- The volume of water that you need - as a rule of thumb this will be around 5% of the water that can be collected
- Start small - There are cheaper and simpler water conservation solutions, such as low flow taps, aerated showers, rainwater butts and low flush toilets that can offer short payback periods and should be considered before going ahead with rainwater harvesting
- Rainwater is collected and enters the tank - it is collected from a roof or area of hard standing. A filter stops leaves and other large particles from entering the holding tank. A smoothing inlet stops sediment at the bottom of the tank from being disturbed by the rainwater that enters the tank
- Water is pumped out of the tank for use - Another filter prevents the uptake of floating particles when the water is pumped out for use. The water is non-potable, so it has to travel through a separate set of pipes to your mains supply. You can use a submerged pump or a suction pump fitted to the outside of the tank to pump the water out
- Automated control system - A control unit monitors the water level in the tank and provides information on whether the system is operating properly. If the water level in the tank drops too low the control unit switches the system over to mains water supply so that you don't run appliances dry. There is an air gap fitted so that rainwater cannot flow back into the mains
- Keeping the tank clean - There is an overflow trap which allows floating material to be skimmed off into the storm drain when the tank overfills - this should happen twice a year ideally without you having to do anything. Rodent barriers and a non-return valve stops unwanted foreign bodies getting back into the tank this way
- Collection from surfaces other than roofs - Optionally rain water can be collected through a permeable pavement but in addition to a filter, an oil trap should also be fitted. Collection from the pavement increases the potential for oil and animal faeces contamination of the rainwater stored in the tank, but additional filtration and disinfection will solve this problem
Installing a Rainwater Harvesting System:
There are 3 basic types of rain water harvesting system and you will need to choose the most suitable one for you:
- The water is pumped directly to points of use from the storage tank(s)
- The water is fed by gravity to points of use from the storage tank(s)
- The water is pumped from the collection tank(s) to an elevated cistern and fed by gravity to the points of use
Sizing the tank is the most critical part of the installation, as this is the most expensive part of the system. A simple calculation can be made to size the tank:
Annual rainfall (mm) x effective collection area (m²) x drainage coefficient (%) x filter efficiency (%) x tank sizing for demand (5%)
Annual rainfall (mm) - the annual rainfall can vary quite considerably across the country; in the west and in the highlands rainfall can be as high as 6000mm, whereas in East Anglia and London rainfall can be as low as 500mm per year.
Effective collection area (m²) - This is the area available to catch the rainwater, which is similar in size to the base of your roof.
Drainage coefficient (%) - This describes the loss of water from the system and the collection area that doesn't reach the tank, for example due to evaporation or overflowing down pipes. Typically values would be:
- Pitched Roof - 90%
- Pitched Roof with tiles - 80%
- Flat roof with gravel layer - 80%
Filter efficiency (%) - This is reflects the ability of the filter to separate the debris and water and feed the filtered water into the tank. The manufacturer will supply this information, however if no specific value is given 90% should be used.
Tank sizing for demand (5%) -This is the percentage of the annual rainwater supply that it is possible to collect that will be used by your household. Typically 5% is used as a general rule.
BS 8515 gives guidance on the design, installation and maintenance of rainwater harvesting (RWH) systems for the supply of non-potable water in the UK, and applies to both retrofitting and new builds. The only Building Regulations in place for Rainwater Harvesting Systems are Building Regulations Part H and Building Regulations Part G.
Ensure that any external taps fed by untreated rainwater have their handles removed and are clearly labelled to prevent this water from being used for drinking.
Where possible, different coloured pipe work must be used to distinguish between rainwater and potable pipe work. This is to make sure there is no accidental cross connection. Suppliers can provide marking tape and warning labels.
The tank is the most expensive part of the system and therefore selecting the right size is the most important part of planning. There is a trade off between cost and the size or capacity of storage. Also the tank ideally needs to overflow twice per year so that it can flush out any floating debris.
Costs of domestic rain water harvesting systems can vary considerably; the Environment Agency estimate that complete system installation costs can vary from £2,500 to £6,000, depending largely on the size of the system. Ideally a RWH system should be fitted when completing other building work as this will help to reduce the cost significantly, particularly as it will be necessary to retrofit plumbing from the tank to the point of use, such as the toilet.
The payback for the system will depend on the ongoing maintenance costs and the current cost of your water. Maintenance can be minimal, usually just washing the filter once every three months. If you live in a hard water area you will find that installing a rainwater harvesting system will mean that white goods, such as your washing machine, will often last longer as rainwater is soft and therefore lime scale is not such a problem.
The rainwater storage tank is designed to keep the rainwater oxygenated as this discourages algae growth and any sediment that gets in is kept at the bottom of the tank. This water is not suitable for drinking. If there is a long dry spell and the rainwater level falls to a low point, the tank is connected to the treated mains water supply so the tank can be topped up and not allowed to dry out.
It is 'good practice' to take a number of measures to maintain the quality of the water:
- Remove floating material with an overflow siphon
- Prevent animals getting into the tank with a rodent barrier
- Taking water from just below the surface, where it is at it's cleanest, is possible with floating extraction and an additional filter
- Clear guttering regularly: Bird and animal faeces and leaf litter on roofs or guttering can pose a health risk if they are washed into the RWH system
Benefits of Rainwater Harvesting
On average we all use approximately 150 litres of water per day. Substituting rainwater for the mains supply when flushing the toilet could potentially save you approximately 39 litres of mains water per day (26%). Further savings can be made by substituting clothes washing and outdoor uses.
Measured total England and Wales micro component domestic water use 2009-10 (%). Source: Environment Agency.
Rainwater harvesting systems can save up to 50% of the treated mains water you use in your home with commercial buildings saving as much as 80% of the mains treated water they use.
Grants for the installation of these systems are not yet available for private households but are available for commercial buildings in the form of Tax Relief Schemes (ECA) for suitable approved equipment on the Water Technology List.
You might like to go to our video section on Rainwater harvesting to watch a film on what is involved in installing a system in your home. There is also a video on how to set up a water butt in your garden.