Collecting the rainwater …
The rainwater is usually harvested only from roofs to ensure the pollutants and hydro-carbons to be found on other hard-standings such as car-parks are avoided.
Via standard rain-goods, the drainage runs are then brought together under-ground so that all the roof-water can be collected in a storage-tank. Ideally, this should be situated underground for a variety of practical reasons, including avoiding temperature extremes on the stored water.
Filtration & overflow …
Before storage the roof water is pre-filtered to remove any solid particles or other debris; these are subsequently washed-away by the natural flow of incoming water whenever the tank approaches full. As the tank will inevitably fill on a number of occasions each year, it must be allowed to overflow into the remainder of the project’s sustainable drainage system (SuDS).
On commercial projects more specialised filters can be used to remove hydro-carbons, thus enabling water to be collected from hard-standings; depending upon system and project requirements, fine-mesh filters, carbon filters and UV filters may also be used. These are described in more detail on the “Filters” page.
Supply to services …
Water is fed to services by one of two means, as illustrated; in systems employing header-cisterns, a head of water is maintained in the cistern, keeping it topped-up with harvested rainwater whilst available. During a long dry spell the main storage tank may empty, at which point the header-cistern is kept topped-up by mains-water.
On direct systems, water is pumped directly from the main storage tank to the systems demanding a supply. This means that there must always be water in the storage tank at all times, achieved by introducing mains-water direct into the tank during long dry spells.
Water safety …
The water being supplied to the services is non-potable, and can therefore only be used for applications such as toilet-flushing, clothes-washing machines and outside taps.
Separate pipe-work is therefore needed to supply this water, which at no stage must be allowed to come into contact with the mains-water supply pipework, through cross-connections.
This means that the mains top-ups mentioned above must be via air-gaps (described later), that make it impossible for the harvested rainwater to come into contact with the mains-water supply.
Garden-only systems …
Where it is intended to only use the harvested rainwater for garden-irrigation purposes, the pattern of collection and usage is likely to be seasonal, collecting mainly in the winter and using mainly in the summer; this has a number of implications.
Usually there will be no need to disturb existing drainage, beyond intercepting the flow from one or two down-pipes, and arranging a suitable overflow for the tank.
It will also not be necessary to disturb existing mains-water pipework inside the house, or to make arrangements for mains-water backup; when the collected rainwater has all been used, it is more practical to have a secondary bib-tap connected directly to the mains supply, rather than top-up the storage tank via an air-gap.