When it comes to efficiency in buildings, the focus is almost always on energy performance, thermal insulation or photovoltaic systems. However, there is another fundamental resource, indispensable and increasingly at the center of sustainability strategies and legislation: water.
Among the various integrated water management strategies, grey water recovery is one of the most effective and constant solutions to reduce domestic and business waste.
But what exactly is grey water, how are dedicated systems designed and what advantages do they bring to modern real estate?
What is grey water and why recover it
Grey water is all water discharges from showers, bathtubs, washbasins and bidets, with the exception of black water (from toilets) and kitchen water (rich in fats and complex organic residues).
They account for about 30% of the total volume of discharges from a residential or tertiary building. Unlike rainwater, which depends on seasonality and rainfall, grey water has a unique strategic characteristic: continuity of production. They are generated every single day of the year.
By treating and reusing this stream for non-potable uses — such as flushing toilets, watering greenery or washing outdoor surfaces — it is possible to reduce overall drinking water consumption by up to 30%.
Planning the recovery: the 4 pillars of the plant
The reuse of greywater is not a simple recirculation, but requires a well-defined infrastructure guided by rigorous technical standards (such as the UNI EN 16941-2 standard). A complete and efficient system consists of four key elements:
- Separate drainage networks: grey water must be intercepted and channelled separately from black water from the origin.
- Treatment and filtration system: the collected water is subjected to clarification and disinfection processes to achieve the hygiene and safety standards necessary for reuse.
- Dimensioned storage: unlike rainwater tanks, the logic for grey water is the opposite. The tank must be sized to the minimum necessary to ensure rapid replacement, preventing the bacterial load from increasing with prolonged stay.
- Dual distribution network: the treated water is returned to the building through a second line that is completely separate and isolated from the drinking water network.
The priority of the Air Gap
The coexistence of two water networks within the same building (one drinking and one non-drinking) requires maximum design attention to prevent the risk of cross-contamination.
To ensure maximum health protection, the emergency connection that replenishes the non-potable network at times of peak consumption must make use of hydraulic disconnection systems with air interruption. In addition, all non-potable pipes must be clearly identified and chromatically marked to eliminate any risk during installation, maintenance or testing.
The evolution of the market
Responsible water management is rapidly transforming from an ethical choice into a mandatory design standard.
- Procurement and public construction: the Minimum Environmental Criteria (CAM Edilizia) impose the presence of separate drainage networks, dual distribution and water-saving systems as essential requirements from the first design phase (PFTE).
- Private and residential construction: even in the private sector, national regulations and local building regulations increasingly promote and reward the adoption of dual network systems for new interventions and major renovations.
An investment that enhances the value of the property
Integrating a greywater recovery system means projecting the property into the future of sustainable construction.
- Savings on bills: drastic reduction in the purchase of drinking water from the public network.
- Environmental protection: reduction of water pressure on local water bodies and optimization of the load on municipal purification systems.
- Increase in real estate value: buildings equipped with advanced water systems meet the highest environmental standards (such as LEED, BREEAM or CASACLIMA certification protocols) and maintain a higher market value over time.
Truly sustainable architecture does not limit itself to consuming less energy, but respects and enhances every natural resource.
Last Updated on 21/08/2026
