Provided by Robert Barker: co-director of Baca Architects
What, Why, When, How, Extras
Rainwater harvesting is a design measure to collect, store and re-use water. This takes the form of collecting surfaces such as roofs or pavements (to harvest rainwater), linked to storage vessels, such as water butts or underground tanks (to store water), sometimes combined with pumps and filtration systems (to recycle the water). There are two types of system, direct feed or header tank. Direct feed is the most common, with the tank linked directly to the use, such as garden. With a header tank harvested rainwater is pumped into a tank, typically in the loft, and mixed with mains water.
Fresh water is a finite resource that is naturally recycled during the water cycle, through evaporation and rainfall. Population growth and lifestyle changes have resulted in greater use of water on occasion, outstripping supply. Future mean annual rainfall is predicted to decrease in the south and east of the UK. Water supplied to UK homes is currently treated to drinking quality, of which 35% is used to flush toilets. Rainwater typically drains into the sewers and then sewage treatment plants, during a storm the system can overflow leading to flooding. Rainwater storage can be used to reduce run off rates during a storm, reducing flood-risk.
Rainwater harvesting is always likely to be beneficial, however, different measures are appropriate for different uses. For existing buildings it is often easier to install above ground rain water butts. For new buildings it is often preferred to install below ground and during excavation works. For larger developments communal facilities may provide economies of scale but may need metering to monitor fair use within the overall supply group. If an area is also at risk of flooding, the risk of contamination of the water supply must also be considered.
Rainwater tanks need to be located away from direct sunlight to avoid algae growth.
Underground tanks help to moderate temperature, reducing bacteria growth in the summer and frost in the winter.
Check if the area is susceptible to flooding.
Tanks larger than 5% of the water demand are sometimes found to be uneconomical.
Water run off from pavements or drives needs to be filtered, increasing cost.
- Step 2: Determine the quantity of water used overall. It is possible to calculate this using generic water consumption rates. As a rule of thumb new households use around 150 litres per person per day (l/p/d), metered houses use 136 l/p/d and older households use around 167 l/p/d. In a typical office the average use is 50 l/p/d.
BRE potable water calculator. (Click image to enlarge)
- Step 4: The size of a rainwater tank should be based on water demand, annual rainfall and the size of the catchment area (roof and/or paving). Current British Standards indicate that the tank should be large enough to contain: 5% of the non-potable water demand or 5% of the annual rainwater yield, whichever is the lesser. The average annual rainfall in England is apx 850 mm/yr (1420 mm/yr in Wales, 1650 m/yr in Scotland and 1160 mm/yr in Northern Ireland). Harvesting rainwater for domestic uses: an information guide, tank size:
effective collection area (roof) x drainage coefficient (based on roof angle, 0.8 for pitched roofs) x filter efficiency (typically 0.9) x annual rainfall (apx 850mm) x 0.05 (5% guide)
MET Office Mean Annual Rainfall. (Click image to enlarge)
- Step 5: When calculating a communal facility add up all of the buildings that will be linked to the system both for the catchment and use calculation. Communal facilities have the added benefit of being able to provide holding tanks for rainwater, to reduce flooding, see SUDS.
Communal Rainwater Harvesting as part of a zero carbon development for Hackbridge. (Click image to enlarge)
- Step 6: Determine the best location for the tank depending on the system (direct feed or header tank). Consider the quality the water needs to be treated to.
Rainwater harvesting system. (Click image to enlarge)
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