Keep reused non-potable water separate, identifiable and protected from the wholesome supply.
Core idea
Greywater is collected wastewater from selected uses and can be treated for suitable non-potable purposes. Its quality varies and it can deteriorate during storage. A reuse system therefore needs controlled collection, treatment, storage, overflow and distribution, with robust separation from wholesome-water pipework.
What to recognise
- Define the sources and uses permitted by the design. Bathroom reuse commonly collects baths, showers and washbasins. Kitchen wastewater may be classed as greywater but has a higher grease and organic load; do not add it to a bathroom-only design. Toilet and urinal wastewater is blackwater.
- Mains backup requires fluid category 5 protection, commonly a Type AA or AB air gap with its clearance and overflow maintained. A single/double check valve or RPZ does not provide category-5 protection.
- Mark non-potable outlets and pipework to help users and future workers distinguish them from wholesome supplies; verify the actual connections as well.
- Prevent cross-connections during installation, alteration and maintenance.
- Route overflow and drain connections to the appropriate wastewater disposal arrangement. Greywater is not roof rainwater, so its overflow may not suit a surface-water drain or roof-water soakaway. Protect storage from a surcharged drain where required.
Apply it
Trace both systems from source to outlet. At every interface, ask what happens if a valve passes, a pump stops, a storage level rises or supply pressure falls. Commission by proving identification and separation, not only by showing that a WC cistern fills.
Limits
Reused water is not wholesome because it looks clear. Treatment requirements, permitted uses and maintenance follow the system design and applicable regulations. Filtration does not show that the water is safe to reuse, and WC flushing still involves possible exposure. Notify the water undertaker about proposed alternative-supply work. Do not connect a hose or temporary bypass between systems, and remember that a single check valve may not be adequate for the assessed fluid risk.