Harvested rainwater can be defined as being water collected from roofs via traditional guttering, through down pipes to an underground tank.

Delivered on demand by and in-tank submersible pump direct to toilets and outside tap use. More than 50% of mains water 

can be substituted by rainwater, dependent, of course, on seasonal variations.

If it  is correctly collected and stored, rainwater can be used successfully for toilets and watering gardens without further treatment. In practice, most domestic roof are

as are too small to satisfy all this potential demand regardless of the size of the storage cistern, so it is important to evaluate the potential savings before investing in an expensive installation.

The garden water butt is the simplest way of collecting rainwater. It does not need any treatment or mains backup, and it does not have to supply water when temperatures are below freezing. Household rainwater systems are, however, much more sophisticated and their installation is quite complex. Rainwater collected from roof areas has a high organic level of bacteria and like grey water requires treatment when stores in the fabric of a home.