Greywater is the collective term for household wastewater that has not been contaminated by toilet waste. In a typical Italian residence, the main greywater sources are the bathroom basin, shower, bathtub, and washing machine. These streams represent a portion of daily water consumption that — after appropriate treatment — can be reused for purposes that do not require potable quality water.
What Distinguishes Greywater from Rainwater
Unlike harvested rainwater, greywater contains surfactants from soap and detergents, skin cells, hair, trace pharmaceuticals, and — in the case of washing machine effluent — synthetic fibres and potentially pathogens from soiled laundry. This higher contamination load means treatment requirements are correspondingly more complex, and the permitted uses under Italian regulation are more restricted.
Kitchen sink and dishwasher effluent is generally excluded from residential greywater reuse circuits in Italian practice because of its higher organic load and temperature, which complicate biological treatment at small scale. The standard approach is to limit greywater collection to bath and shower sources for toilet flushing reuse, and to treat washing machine effluent separately for garden irrigation where permitted.
Italian Regulatory Context for Greywater
Greywater reuse at the residential level occupies a less clearly defined regulatory space than rainwater harvesting in Italian law. The national Environmental Code (D.Lgs 152/2006) addresses reclaimed water primarily in the context of treated municipal wastewater, not domestic greywater. Regional water plans and municipal building regulations are therefore the operative instruments for residential installations.
Lombardia and Emilia-Romagna have published specific technical guidance for building-level water reuse that includes greywater. Other regions have not yet developed analogous frameworks, which means homeowners in those regions must obtain clarification from the local water utility (the gestore del servizio idrico integrato) before installation.
One consistent requirement across regions where guidance exists: the greywater circuit must be physically separated from the drinking water distribution with an air gap or comparable backflow prevention device, and all non-potable pipework must be identified as such throughout the building.
Treatment Technologies at Residential Scale
Several treatment approaches are available for residential greywater reuse. Selection depends on the volume of water to be processed, the intended reuse application, the available installation space, and the maintenance capacity of the household.
Simple Diversion for Garden Use
The most basic intervention is diverting shower and basin drainage to an outdoor soakaway or subsurface irrigation system. This requires no treatment equipment but limits reuse to subsurface garden irrigation of non-edible plants. Direct surface application — sprinkler irrigation of vegetable gardens or lawns — is not recommended without treatment, due to potential pathogen transfer and surfactant accumulation in shallow soil layers.
Physical-Chemical Treatment Systems
Package units designed for residential greywater treatment typically combine a settling chamber, a fine filter (50–100 microns), activated carbon adsorption (to reduce detergent residue and odour), and UV disinfection. These units are available in capacities of 100–500 litres per day, which is appropriate for the greywater output of a family of three to five. Italian and German manufacturers — including units by Huber Technology and Graf — have products in this segment.
Treated water from these systems is generally suitable for WC flushing and subsurface irrigation. Some systems are certified to UNI EN 16941-2 parameters for WC use.
Constructed Wetlands and Reed Beds
Subsurface constructed wetlands use gravel beds planted with common reed (Phragmites australis) or other wetland species to treat greywater through physical filtration, microbial decomposition, and plant uptake. They require more land than package units — a minimum of 2–5 m² per person treated — but have minimal energy input and lower maintenance requirements once established.
They perform well in the warmer parts of Italy where temperature supports year-round biological activity. In northern Italy, cold winters reduce biological treatment efficiency and may require a covered or insulated variant. Effluent from a properly functioning reed bed is typically suitable for subsurface garden irrigation.
Membrane Bioreactor Systems
Membrane bioreactor (MBR) units combine biological treatment with ultrafiltration membranes. They produce consistently high-quality effluent suitable for WC flushing in a compact footprint. The main limitations for residential use are higher capital cost, energy consumption (typically 0.4–0.8 kWh per m³ treated), and the need for periodic membrane cleaning and eventual replacement.
MBR systems are most commonly specified for residential developments where local regulations require a documented treatment standard and where space prevents a reed bed installation.
Practical Reuse Applications
Toilet Flushing
WC cisterns account for a substantial proportion of household water use in Italy. Connecting a treated greywater supply to toilet cisterns reduces demand on the drinking water network proportionally. The installation requires a separate distribution pipe from the treatment unit to each WC, a properly sized holding tank between the treatment unit and the distribution circuit, and automatic switching to mains supply when the greywater reserve is depleted.
The holding tank prevents supply interruption during periods of low greywater generation (absent occupants, dry weather). A standard approach uses a 200–400 litre buffer tank with a mains top-up valve set to activate when the level drops below 20%.
Garden Irrigation
Treated greywater is suitable for subsurface drip irrigation of non-edible ornamental plants and lawns. Drip emitters buried 15–20 cm below the surface eliminate contact with the surface and reduce evaporation. Surface sprinkler irrigation using greywater is not recommended due to aerosol risk and potential leaf contact with surfactant residues.
Vegetable garden irrigation with greywater requires caution. Root vegetables and low-growing crops with edible parts near the soil surface should not be irrigated with greywater under any treatment level commonly available at residential scale. Tall-growing crops with inedible foliage or fruit not in contact with the soil — tomatoes trained on supports, for example — may be irrigated subsurface, but this is at the homeowner's discretion and not endorsed by Italian food safety guidance.
| Source | Treatment level needed | Permitted use |
|---|---|---|
| Bath / shower | Physical-chemical + UV | WC flushing, subsurface irrigation |
| Washing machine | Settling + fine filter + UV | Subsurface garden irrigation only |
| Basin (bathroom) | Physical-chemical + UV | WC flushing, subsurface irrigation |
| Kitchen sink | Not recommended for reuse | — |
Installation Considerations in Italian Homes
Older Italian residential buildings — particularly apartment blocks (condomini) built between 1950 and 1980 — typically have combined drainage stacks where bath, basin, and WC effluent run in a single pipe. Separating greywater from this combined stack for reuse requires replumbing within the apartment, which affects common building structure and requires approval from the condominium assembly.
New construction and major renovation projects offer a cleaner opportunity to install separate grey and black water drainage from the outset. The marginal cost of installing dual stacks during construction is significantly lower than retrofitting an existing building.
Detached rural houses (case rurali) and recently built villette with accessible roof voids and crawl spaces are typically the most practical candidates for residential greywater reuse retrofits.