A guide to Rainwater Harvesting
Rainwater Harvesting is the simple process of collecting rainwater run-off from a building's roof and storing it in a tank or cistern in order to supplement existing mains water supply.
What are the benefits of harvesting rain water?
There are three reasons to consider installing a rain water harvesting system:1. Financial: Reduce metered water costs
It is estimated that in a typical domestic water systems, rainwater harvesting can provide as much 50% of the household's non-potable water needs (ie washing clothes, flushing the toilet, cleaning the car, irrigating the garden, etc). Whilst many houses still do not have water meters, all new houses have them installed as standard, and it is likely that in the years to come water providers will make their installation compulsory in existing housing stock. Harvesting rainwater therefore reduces metered costs.In commercial environments where there are increased needs for water, eg in public buildings with large numbers of toilets, the pay off can be even more substantial, especially in buildings which have substantial roof areas and therefore an increased capacity for harvesting.
2. Financial: Storm Water Management
By storing water on-site, the volume of water which is released at any one time into the storm-drain or soakaway system is proportionally reduced. In new buildings there can therefore be potential savings in the size of the system which needs to be installed to handle predicted water run-off.3. Environmental
A considerable amount of energy is wasted in treatment plants which bring all water up to EU drinking standards, whereas much of it is destined to be flushed down the toilet or used to wash a car. Rainwater harvesting can therefore have a direct impact on this energy wastage by reducing the amount of water required from the water grid.In addition, by reducing the demand on the grid, water utilities need to extract less water from rivers, aquifers, which can have a beneficial effect on river wildlife, low water tables, etc.
Will a system supply all a building's non-potable water requirements?
The answer to this question obviously depends on a number of factors:- the size of the roof area
- the size of the storage facility
- the level of rainfall (especially during summer months)
- water losses from the system, eg through evaporation
A 12-month study by Severn Trent Water and the Environment Agency in 2000-2001 showed that an average domestic system supplied virtually all non-potable water requirements, and that the system only occasionally overflowed into soakaways. The system occasionally required topping up with mains water.
How clean is the water?
The water is not drinkable, due to the levels of pollution, pesticide residues, etc in rainwater, and the risks of potential contamination from the roof, eg bird droppings. However, there are systems available known as 'first flush diverters' or 'roof washers' which can reduce this contamination by sending the initial few litres through to a waste system. Once this has reached capacity, run-off is diverted to the main tank.Whilst probably not drinkable, the water is clean enough to wash clothes and certainly clean enough for other non-potable uses.
How does a system work?
Systems vary in complexity, but typically run-off from the roof will pass through a series of mechanical filters into an underground storage tank, which will itself have an overflow system leading back to the main drains. Submersible pumps then feed the water back through to inlets into the toilets, washing machines, etc.
What systems are available and how much do they cost?
There are a number of UK suppliers specialising in different types and sizes of rainwater harvesting systems. A typical domestic system for an average sized house can cost £1,500 to £2,000, plus the cost of installation. More complex systems for larger public buildings can cost considerably more, although of course the savings are proportionately greater.
Is Rainwater Harvesting the same as Greywater Harvesting?
No, it isn’t. Rainwater Harvesting collects rain water only, whereas grey water is defined as the waste water from baths, showers, hand basins, etc. There are systems which can similarly recycle this water for use in flushing toilets, external taps, etc.
If you are interesting in discussing the implications of rainwater harvesting as part of a broader scheme of building design and development, then please contact us.
