Grey water recycling: treatment technologies and practical applications

Why and how to reuse grey water

Grey water recycling and reuse refers to a set of solutions designed to collect, treat and reuse greywater generated inside a building, reducing the demand for potable water in applications where it isn’t required. For facilities with high or continuous water consumption—such as hotels, gyms, residential complexes, and large commercial buildings—installing a grey water recycling system can become a technical and economic advantage, in addition to an environmental one.

Idro Group designs and supplies wastewater treatment and reuse systems and, specifically, provides solutions such as IDROCELL, designed for filtration and purification of greywater intended for non-potable uses.

What is grey water and how it differs from black water

In practical terms, grey water refers to wastewater that does not include sewage from toilets. In residential and commercial settings, it typically comes from showers/bathtubs, bathroom sinks and often washing machines. It’s important to clarify one key point right away: greywater is not “clean” water. It contains typical everyday contaminants such as surfactants and detergents, cosmetic residues, particulate matter, hair, textile microfibers and a variable microbial load that can increase with stagnation and higher temperatures.

Because its origin is generally domestic and does not include fecal waste, grey water tends to have a more manageable contamination profile than black water. As a result, it is better suited to a fit-for-purpose approach—meaning that treating grey water for reuse can be tailored to the final application.

Black water, on the other hand, comes from toilets and contains significantly higher organic and microbiological loads, along with increased sanitary risks. For this reason, when discussing grey water recycling systems in buildings, the first “technology” is often the separation of grey and black water plumbing networks. This improves process stability and increases treatment effectiveness for greywater alone.

In real-world design, certain “borderline” sources are handled conservatively. For example, kitchen wastewater is often excluded or treated separately because it may introduce fats, oils and food residues that increase the overall load and complicate system operation (odors, scaling, fouling).

Treating grey water for reuse: filtration, membranes, UV, and the “multi-barrier” approach

When the goal is reliable greywater treatment for reuse, the most effective strategy is the multi-barrier approach: each treatment step removes a portion of the contaminants—first solids, then biodegradable substances, and finally microorganisms—so the treated water becomes suitable for non-potable uses such as toilet flushing, irrigation, and cleaning.

The grey water recycling systems offered by Idro Group are designed precisely with this progressive “multi-barrier” logic, using structured treatment sequences. In properly sized systems, the typical configuration includes:

• Coarse filtration + peak flow management (storage/equalization): this stage removes hair, fibers, and suspended particulate matter to prevent clogging and reduce process instability. It also stabilizes incoming flow through an equalization tank.

• Membrane treatment (UF / MBR): ultrafiltration membranes create a physical barrier that significantly improves water quality and makes reusing grey water safer and easier. To give a concrete example: in Idro Group solutions, these stages are designed to produce much clearer, more stable water with very low suspended solids and turbidity, reduced organic load, and a strong reduction in microbiological components. This makes the water suitable for planned non-potable applications (such as toilet flushing, irrigation, or cleaning), in compliance with the requirements applicable to the installation context.

To ensure operational continuity, these systems typically include backwashing and automated process management functions (also referenced in technical documentation), maintaining performance over time without unnecessarily increasing maintenance.

• Polishing + final disinfection (activated carbon + UV): when further water quality improvement is required—especially for odors, light micropollutants, and detergent residues—and to enhance microbiological safety, it is common to include activated carbon filtration followed by UV disinfection. Idro Group uses this configuration in grey water recycling systems: activated carbon treatment and subsequent UV disinfection before distribution to end users.

This “multi-barrier” structure follows the same risk-based logic recommended by the World Health Organization (WHO): first identify risks based on the source water and intended use, then define the technical and operational measures to reduce those risks, and finally monitor performance over time. In practice, treatment level and monitoring are not identical in every case—they are adapted based on the final destination of the recycled grey water, such as toilet flushing or irrigation.

How to reuse grey water: practical applications and safety requirements

In most building applications, greywater reuse is permitted only for non-potable uses and requires a dedicated distribution network fully separated from potable water lines. The most common applications include toilet flushing, irrigation (depending on site conditions and treated water quality), and surface cleaning/washing of common areas where allowed. In some contexts, it may also be possible to evaluate greywater reuse for washing machines, strictly as a non-potable use and only after verifying water quality and material compatibility.

To ensure safe and compliant operation, systems must include:

• Physical separation of networks and clear labeling of piping and draw-off points
• Backflow prevention and measures to avoid cross-connections
• Proper storage management to limit deterioration in water quality
• Maintenance plans and scheduled performance checks
   

Ultimately, the purpose of grey water recycling is to reduce potable water consumption in uses that do not require drinking-quality water, optimizing overall building efficiency.

When does it really make sense to install a grey water recycling system?

A grey water recycling system delivers maximum value when two conditions are present: a regular production of greywater (showers, sinks, bathtubs, and—where included—laundry) and a continuous demand for non-potable water, such as toilet flushing and irrigation.

• Hotels and hospitality facilities

Greywater availability is typically steady, especially from showers, and toilet water demand is continuous. Proper system sizing depends on seasonality, occupancy rates, and available technical space.

• Residential buildings and condominiums

Potential increases with the number of users and with the presence of plumbing networks that are already separated—or can be retrofitted—to segregate grey and black water flows. The benefit becomes greater when gardens or landscaped areas require irrigation.

• Communities and shared facilities (campuses, sports centers, complexes with showers): concentrated and predictable volumes make treatment easier to stabilize and allow reuse across multiple end uses.

A real-world example in the civil/residential sector is an Idro Group installation for a private customer in Ragusa: a greywater treatment plant designed for domestic reuse (toilets, irrigation, and cleaning), based on a dedicated treatment line with filtration and UV disinfection, with a reported capacity of 20 L/min (approximately 5 population equivalents).

Another reference case involves an ultrafiltration plant for greywater recovery, where treated water is sent through activated carbon and then disinfected with UV before distribution. The case study highlights that solutions have been developed with different capacities, both underground and above-ground installations.

If you want to determine whether grey water recycling is feasible for a new building or a retrofit project, contact us. Idro Group can support you with a preliminary technical assessment and a properly sized proposal based on users, volumes, and intended reuse applications.