Not all the water consumed in a building needs to be of drinking quality. Uses such as toilet cisterns, garden irrigation, vehicle washing and general area cleaning make up a significant share of total consumption, and treated greywater or rainwater is more than sufficient for them. Greywater is the water from points such as washbasins, showers and baths that contains no toilet waste. Blackwater is collected separately and is not included in this system.
A greywater line requires a second collection line independent of the clean water plumbing. The collected water is fed into an equalisation tank, passed through coarse filtration and has its organic load reduced by biological treatment. In compact applications, membrane bioreactor (MBR) solutions are preferred; they provide high treatment efficiency in a small volume. Disinfection is carried out at the outlet, and the treated water is delivered to the points of use through an independent distribution line in a different colour and labelled.
On the rainwater side, the design is simpler. The water collected from the roof is separated from the dirty first volume by a first-flush diverter, stored in a tank and filtered and disinfected before use. The tank volume is determined by calculating the roof area, the annual rainfall pattern of the region and the target consumption together. Mains top-up automation that kicks in during dry periods keeps the system running without interruption.
In buildings with high water consumption, a recovery investment pays for itself through savings on mains water and wastewater charges. Hotels, shopping centres, hospitals, dormitories and large housing projects are the building types with the fastest return. It also earns points directly under the water efficiency category in green building certifications. The question to ask when starting the design should not be "how much water can I treat" but "what quality of water do I need for which use".
