Water:Rainwater harvesting
From Sustainable Building Resource
[edit] Rain water harvesting
Top tips
If possible harvest the water at the high level then use gravity to feed into the system.
Use rain water for washing machines, toilets, garden watering.
Use a rain water diverter that provides some initial filtering to reduce the build up of any residues
Sizing the tank is an important element as it is better to have a store of water in hot and dry summers (which might become more prevalent in the future...)
When fitting equipment in hard to reach places always use the best quality that you can afford as these are the key places that could cost you a lot more in the future to rectify.
Experiences
[edit] How rain water harvesting works
With good planning and the right components a rainwater system can be nearly maintenance free and rainwater quality is ideal for many uses, in both the home and the workplace. A professional system will have 4 stages of cleaning the rainwater.
Only rainwater from roof areas should go to the storage tank. Smooth surfaced materials are the best (slate, tiles and glazed cement roofing slabs). The rain yield from green roofs is 30-50% and whilst it can sometimes be a bit discoloured it is still suitable for toilet flushing and garden watering.
Cleaning Stage 1 - Filter
The first cleaning step in the rainwater system is the filter. The rainwater flows from the roof to the filter. Here dirt particles and debris are separated from the water. The cleaned water flows to the tank. The dirt is washed to waste or soakaway with a small amount of rainwater.
Cleaning Stage 2 - Calmed Inlet
By using an underground tank, the water is stored in dark and cool conditions. Here the second cleaning step takes place. In the water column any fine residual particulates settle to the bottom of the tank. The rainwater calmed inlet prevents any disturbance of this sedimentation layer. At the same time the calmed inlet ensures that oxygenated water is introduced to the lower layers of the stored water in the tank. The oxygen rich water prevents anaerobic reducing conditions forming in the storage tank and ensures that the water stays fresh.
Cleaning Step 3 – Overflow Siphon
Any particles that are lighter than water (e.g. flower pollen) float slowly to the water surface. An overflow siphon, with a skimmer effect, removes the floating layer. The regular overflow from the storage tank is important to get the optimum water quality. It prevents souring of the water. The floating layer could otherwise build up over time, and so reduce oxygen diffusion at the water surface, which in turn could lead to anaerobic reducing conditions in the tank.
Cleaning Step 4 – Floating Pump Intake
The pump intake floats at all times, suspended just below the water surface where the cleanest water lies. From this position the water is extracted by the pump. A ball float, filled with air, suspends the intake, which has a further filter and a check valve.
[edit] Choosing a tank
When choosing a tank it is important to first consider the capacity of water which your household requires. It is also worth considering the average rainfall for your area and the surface area of your roof. This will ensure that you have adequate water supplies if there was to be a prolonged period without rainfall. Finally consider the space that is available for a tank, how easy it would be to install and how you would connect it to the system.
Above ground tanks
These systems are normally connected to an existing drainpipe and require minimal installation work. A filter can be placed in the downpipe to remove debris, such as leaves, from the water and as long as the container is covered there is not normally a problem with algae or insects.
Naturally a domestic system will require a larger tank which is normally a rectangular Polyurethane structure which can be placed flush against a wall. The cost of these tanks is anywhere from £100 upwards, primarily based on the volume of water which they can hold. This cost is lower than that of below ground tanks, however it is important to consider the visual impact that a large tank will have on your building.
Below ground tanks
Below ground tanks are commonly used in new build developments due to their larger capacity and low visual impact. Below ground tanks may not be the best option for retrofitting due to the requirement of large excavation works taking place. It is important to consider the type of land the tank will be in especially if there is a risk of contamination as some chemicals will attack the GRP and polyurethane tanks. Not all tanks need a concrete back-fill, however some contractors will insist on one to prevent floatation of the tank. This may occur in areas with a high water table, or when storm waters fill the voids around the tank, at times when the tank contains a low level of water.
Polyurethane
These should be placed on a firm bed of concrete or firmly compressed sand. Most tanks will require concrete back-filling; however, this depends on the design of the tank.
Advantages: -standard 25 year guarantee -lightweight -easy installation
Disadvantages: -non load bearing -requires large amounts of concrete to back-fill
Concrete
These should be placed on a firmly compacted, level base. They do not require additional concrete back-filling due to the strength of the tank.
Advantages: - load bearing (suitable for below driveways) - long guarantee - suitable for contaminated land
Disadvantages - large weight makes them harder to install
GRP (Glass Reinforced Plastic)
These should be placed on a bed, and surrounded by, 225mm of concrete. The tank should be filled with water whilst the concrete is poured to avoid the tank becoming distorted.
Advantages: - lightweight - very durable - well-known technology
Disadvantages: - requires concrete back-fill
