The reuse of wastewater and rainwater has become an increasingly interesting and important topic. While most countries have yet to make such systems mandatory, several nations in southern Europe — facing potable water shortages — are now compelled to conserve fresh water and seek alternative drinking water sources. The cost of treating certain types of used water is generally lower than that of desalinating seawater. For this reason, many modern office buildings and hotels are being designed in line with energy-efficient building standards that incorporate greywater and rainwater reuse systems.
The main reason for recycling greywater (bath and shower water) is that it is relatively low in contamination, easier to treat, and retains usable heat energy. Additionally, the environmental impact of a building can be evaluated using certification systems such as LEED (Leadership in Energy and Environmental Design) and BREEAM (Building Research Establishment Environmental Assessment Method). These rating systems help architects and designers identify solutions for reducing water consumption alongside heating and cooling demand.
Treated greywater is often combined with harvested rainwater for broader reuse applications.
What is Greywater?
The term greywater originates from its appearance and, according to the European EN 12056 standard, refers to wastewater from sinks, washing machines, bathtubs, showers, and kitchen sinks that does not contain feces or urine.
Treated greywater — especially water collected after bathing — can be reused as “white water” for applications such as:
Flushing toilets and urinals
Car washing facilities
Irrigating green spaces and landscaping
This approach not only conserves valuable drinking water but also contributes to sustainable building design and reduced environmental impact.
From the data, it is clear that more than 50% of household wastewater is greywater. In some countries, the amount of greywater generated ranges from 57–75 liters per person per day.
In service-sector facilities such as hotels, swimming pools, saunas, and restaurants, large volumes of water are wasted. Water consumption varies widely — from 150 L/person/day in a three-star hotel to 1000 L/person/day in a five-star hotel — depending on the establishment’s infrastructure, such as wellness centers, saunas, pools, and kitchen facilities.
Chemical Composition of Greywater
The COD/BOD₅ ratio is around 4:1, indicating a high content of organic matter that is difficult to biodegrade.
(For regular potable water, this ratio is about 2:1.)
This 4:1 ratio also applies to shower water containing soaps and shampoos.
pH values:
Potable water: pH 7–8
Laundry wastewater: pH 9–10 (alkaline)
Kitchen wastewater: More acidic
Temperature: Greywater from baths, showers, and washing machines ranges between 18–38 °C, reflecting the predominance of hot water used for hygiene purposes.
Typical Greywater Parameters
ParameterTypical Range
BOD₅ (Biochemical Oxygen Demand, 5 days)Moderate to high depending on source
COD (Chemical Oxygen Demand)High, ~4× BOD₅
COD/BOD₅ Ratio~4:1
pH – Potable Water7–8
pH – Laundry Wastewater9–10 (alkaline)
pH – Kitchen WastewaterSlightly acidic
Temperature (°C)18–38
If you want, I can also prepare a combined chart showing the proportion of greywater sources, their daily volume, and key quality parameters for easier visual comparison.
Secondary Water Sources Beyond Potable Water
High-quality drinking water should be reserved for essential uses only. For applications such as toilet flushing or irrigation of green areas, treated water can be used instead. This approach is particularly important in regions where potable water resources are limited.
Advantages of Using Treated Water
Economic savings – potable water prices continue to rise
Lower environmental impact – reduced ecological footprint of discharged compounds
Lower carbon emissions – due to reduced energy demand for potable water production
Examples of Secondary Water Sources
Treated water supplied via a separate, independent distribution network from drinking water
Non-centralized or on-site produced treated water
Rainwater collected from roofs and other impermeable surfaces
Treated greywater
Greywater Treatment
Characteristics
The quality of greywater is influenced mainly by residents’ lifestyle.
Lowest contamination load: water from showers and baths
Highest contamination load: kitchen sink and dishwasher water — rich in organic residues and suspended solids
Thus, wastewater from sinks, bathtubs, and showers is the most suitable for reuse, whereas water from kitchen sinks and dishwashers requires stricter treatment.
BS 8525-1 Standard
The British BS 8525-1 standard specifies technical and health requirements for white water (treated greywater), including microbiological quality parameters:
Parameter (per 100 ml)Spray UseNon-Spray UseHigh-Pressure Washing, Garden Irrigation, Car WashingToilet FlushingGarden/Green Area Irrigation A)Laundry Use
Escherichia coliNot detected≤ 250≤ 250Not detectedNot detectedNot detected
Intestinal EnterococciNot detected≤ 100≤ 100Not detectedNot detectedNot detected
Legionella pneumophila≤ 10Not applicableNot applicableNot applicableNot applicableNot applicable
Coliform bacteria≤ 10≤ 1000≤ 1000≤ 10≤ 10≤ 10
A) If water is used for irrigating edible crops (fruit, vegetables), consumers must be informed in writing, and it is recommended to wash produce thoroughly with potable water.
B) In addition to the above, visual quality should be monitored — treated water should appear clear, transparent, free of sediments, and colorless.
Greywater Treatment Methods
Greywater treatment technologies are categorized as:
Physical – filtration, sedimentation
Physico-chemical – coagulation, flocculation, membrane filtration
Biological – biological reactors, aerobic or anaerobic treatment
In Europe, natural treatment methods (settling ponds, soil filtration) are mainly used in rural or remote homes.
For large-scale facilities, biological treatment combined with suspended solids removal and disinfection is essential.
Membrane Bioreactor (MBR) Systems
Most modern manufacturers offer biological reactors with membrane separation (MBR). In some cases, integrated disinfection is included.
A typical treated greywater system consists of:
Storage tank for collected greywater
Reactor (with MBR) for treatment
Distribution unit for supplying treated water to secondary-use pipelines
If you want, I can also prepare a detailed diagram showing this MBR greywater treatment system with labeled components and flow directions for visual clarity. This would make it easier to understand the process for presentations or technical documents.
Diagram Description
Wastewater first passes through a mechanical debris filter before entering the stabilization tank. In this tank, various types of solid impurities settle and accumulate. From the stabilization tank, water is pumped into the aeration/activation tank, which contains a membrane module. At the bottom of this tank, an aeration system is installed to supply oxygen — both for biological treatment and to clean the membrane surface.
Above the membrane module, membrane filtration draws treated water upward, directing it into a treated water storage tank. From there, water flows to an automatic pumping unit with a membrane pressure vessel, which supplies the non-potable water distribution system.
After the pump, the system includes:
A membrane pressure tank (to maintain constant supply pressure)
UV disinfection unit (ultraviolet light) to ensure microbiological safety
All storage tanks are equipped with safety features to prevent overflows and malfunctions. If greywater levels are insufficient, the treatment tank can be supplemented with potable water to maintain supply.
Conclusion
Considering the limited availability of potable water and the economic benefits, installing a white water (treated greywater) reuse system is a highly effective solution for facilities with high water consumption and significant greywater generation — such as hotels, sports complexes, swimming pools, hospitals, and car washes.
Greywater generation typically ranges from 55–100 liters per person per day, and its reuse can reduce daily potable water demand by up to 50%. Studies indicate that such systems can achieve a payback period of 4–10 years, making them both environmentally and financially sustainable.
