What is Grey Water? Sustainable Water Management and Recovery Guide

Today, increasing population and climate change put serious pressure on water resources. At this point, the question of what is grey water has gained critical importance in terms of sustainable living and cost management. In its most basic definition, grey water is domestic wastewater with relatively less pollution, except for black water (toilet waste), originating from showers, bathtubs, sinks and washing machines. Recovering this water, which constitutes 50 to 80% of domestic wastewater by volume, has the potential to reduce network water consumption by half. With the regulations and technological solutions that will be detailed below, grey water systems are no longer a luxury, but both a legal requirement and an economic necessity.

What is Grey Water?

Greywater is wastewater discharged from residential or commercial buildings, but does not contain toilet waste (faecal matter). It gets its name from its turbid, grey colour due to the soap, detergent, skin debris and other particles it contains. When the correct treatment methods are applied, these waters can be safely reused in areas such as reservoir feeding, garden irrigation and cleaning, even if they are not of drinking water quality. Separating grey water at source reduces the burden on wastewater treatment plants and makes a positive contribution to the natural water cycle.

Differences between Grey Water and Black Water

The two most frequently confused concepts in wastewater management are grey water and black water. This distinction is vital for the choice of treatment technology and health risks.

• Black Water: Water from toilets (toilet bowl and urinal) containing urine and faeces. It contains high levels of pathogens, bacteria and viruses. It is difficult and costly to treat; it is usually sent directly to the sewerage system or septic tanks. Water from kitchen sinks is also generally considered to be black water or "dark grey water" due to its high fat and organic waste load and is not recommended for inclusion in standard grey water lines.
• Grey Water: Comes from showers, baths, hand washing sinks and washing machines. It contains much lower levels of pathogens and nitrogen than black water. It is easier to biodegrade and treat because it has less organic load.

Components of Grey Water

The content of grey water varies according to living habits and the cleaning products used. Its main components are:

• Soap and detergents: Surfactants, phosphates and cleaning chemicals.
• Personal Care Products: Remains of shampoo, conditioner, toothpaste.
• Organic Materials: Skin rashes, hair, body oils and textile fibres.
• Microorganisms: Although at a very low level compared to black water, bacteria from the skin flora may be present.

The analysis of these components plays a key role in determining the filter capacity of the water treatment devices and systems to be installed. Especially hair and textile fibres are the most important pollutants to be considered in the physical filtration stage.

Grey Water Regulation in Turkey and 2026 Vision

Turkey is in the category of countries experiencing "water stress" and the annual per capita amount of water is 1308 m³. This situation has pushed legislators to take proactive measures. With the regulations prepared by the Ministry of Environment, Urbanisation and Climate Change, the use of grey water is gradually becoming mandatory for buildings of certain sizes.

In Which Buildings Is Grey Water System Mandatory?

The amendments made and to be made in the Zoning Regulation on Planned Areas are targeting especially large-scale buildings. According to research reports and current draft legislation, the installation of grey water recovery treatment system is mandatory or encouraged in the following buildings:

• Airports and Hotels: Hotels and large tourism facilities with a bed capacity of over 200.
• Shopping Centres: Shopping centres with a construction area of 10,000 m² and above.
• Public Buildings: 30,000 m² or more construction area of municipal and public service buildings.

These requirements become a prerequisite for new buildings to obtain a building permit.

Planned Areas Zoning Regulation Amendments

The 2026 vision and related by-laws not only impose obligations but also set technical standards. According to the regulation, buildings to be constructed on parcels larger than 2000 m² are obliged to install a rainwater collection system. Grey water systems are usually designed integrated with these systems. In the communiqués published in the Official Gazette, it is stated that these steps for efficient use of water will also be prioritised in urban transformation projects. The regulation stipulates that grey water lines should be routed through a separate installation shaft within the building and that the pipes should be purple or lilac in colour, as in international standards, to avoid confusion.

How Do Grey Water Recovery Systems Work?

A grey water system covers a series of engineering processes from the collection of wastewater to its reuse. The system does not work in a "plug and forget" manner; it requires regular maintenance and correct project design. The process basically consists of four main stages.

Collection and Filtration (Bristle Trap)

The first step in the process is the collection of grey water from black water in a separate pipeline. The collected water must be cleaned of coarse particles before entering the system.

• Hair Trap Filters: Prevents hair, feathers and coarse dirt coming from showers and sinks from clogging the pumps.
• Pre-settling: The water is allowed to settle to the bottom of the heavy solids.

It is a technical necessity for the pumps to be used at this stage (e.g. industrial brands such as Wilo) to have a certain amount of solids permeability for the system to operate without clogging.

Biological Treatment (MBR Technologies)

Physical treatment alone is not sufficient; soap and organic substances in the water must be broken down. This is where Membrane Bioreactor (MBR) technology comes into play.

Aeration: Oxygen is supplied to the water so that bacteria can consume organic matter.

Biodegradation: Microorganisms decompose detergent residues and organic pollution.

MBR technology takes up much less space than conventional systems and provides high quality effluent. This technology is the heart of modern grey water treatment systems.

Disinfection (UV and Chlorination)

The fact that biologically treated water is clear does not mean that it is hygienic. Disinfection is essential to prevent bacterial growth during the storage of water and to avoid health risks at the point of use.

• Ultrafiltration / Membrane: Water is passed through filters with pores too small for bacteria and viruses to pass through.
• UV (Ultraviolet) Rays: It neutralises microorganisms in the DNA structure of water.
• Chlorination: Low doses of chlorine are added to prevent recontamination of the water waiting in the tank.

Usage Areas of Grey Water

Treated grey water does not qualify as "drinking water", but it meets the standards of "water for use". It has a wide range of use in domestic and industrial areas.

Toilet Cisterns and Siphons

Approximately 25-30% of the water consumed in a household is spent in toilet reservoirs. The use of drinking quality mains water in toilet flushing is the biggest example of wasting resources. The most common and efficient use of treated grey water is in toilet cisterns. In this way, recycled water is used instead of mains water every time the toilet is flushed.

Garden Irrigation and Landscaping

Grey water can be nutritious for plants thanks to the small amounts of nitrogen and phosphorus it contains. However, drip irrigation or subsoil irrigation systems should be preferred over spraying methods that directly contact the leaves. This both prevents the spread of bacteria through aerosols and increases the efficiency of water saving at home by reducing evaporation.

Cooling Towers and Industrial Use

Cooling towers in large commercial buildings, hotels and factories consume enormous amounts of water. The water obtained with the grey water recovery treatment system can be used as booster water in cooling towers. In addition, car wash stations, fire extinguishing lines and construction site dust suppression processes are among the industrial uses of grey water.

Advantages and Saving Potential of Grey Water System

Investing in grey water systems contributes directly to both the individual budget and the ecosystem.

Up to 50% Savings in Water Bill

A family of 4 people produces 400-600 litres of grey water daily. Recycling this water and using it for reservoirs and cleaning provides a reduction of up to 50% in the mains water bill. In addition, since many municipalities calculate "Waste Water Fee" (KSUB) based on municipal water consumption, the reduction in municipal water use also reduces the waste water fee. The return on investment (ROI) period for businesses has decreased to as short as 2-4 years with the increase in water prices.

Protection of Natural Water Resources

In water stressed countries like Turkey, it is vital to protect ground and surface water resources. The use of grey water reduces the amount of water withdrawn from dams. This reduces the carbon footprint by saving the energy consumed for transporting and treating water. In sustainable building certification systems (LEED, BREEAM), grey water systems are one of the highest scoring items.

Integration of Grey Water and Rainwater Harvesting

Rainwater and greywater are different sources but complement each other perfectly. Rainwater is seasonal and depends on the rainfall regime, while grey water is produced regularly every day as long as there is life in the building.

The most efficient systems are hybrid systems. The tank level is supported by rainwater. In periods when there is no rain, grey water is activated. In emergencies where both sources are insufficient, the system automatically switches to mains water. This integration provides uninterrupted water savings 365 days a year.

Water Quality and Technical Standards

The quality of water for grey water reuse is determined by international standards. Treated water must be clear, odourless and pathogen-free. The TDS value, which gives information about the conductivity of the water and the amount of dissolved solids in it, is one of the parameters used to monitor the treatment performance (but the main determinants for grey water are BOD and LOD values).

Treated Grey Water Limits According to TS EN 16941-2 Standard:

• pH: 6 to 9
• Turbidity: < 2 NTU
• Biological Oxygen Demand (BOD5): < 10 mg/L
• Suspended Solid Matter (SSM): < 10 mg/L
• E.Coli Count: 0 (per 100 ml)

These values are critical to ensure that water does not odour during storage and does not pose a health risk at the point of use.

Frequently Asked Questions (FAQ)

The questions and answers that users are most curious about grey water systems:

What is the grey water project?

It is an engineering project that collects the wastewater (excluding toilets) of a building at the source through separate pipes, filters it in an on-site treatment plant and converts it back into reservoir or irrigation water. Projects such as Istanbul Airport and Kuzu Effect are examples.

How is grey water made (obtained)?

Greywater is not "made", it is recovered from existing wastewater. The process is as follows: Separate collection -> Bristle trap filtration -> Biological treatment (MBR) -> Ultrafiltration -> UV Disinfection -> Storage and Reuse.

Is rainwater grey water?

No, it is not. Rainwater is atmospheric water collected from roofs and surfaces. Greywater is wastewater from domestic use (shower/sink). Greywater has a higher pollution load than rainwater and requires biological treatment.

Is there a grey water bill?

No, you do not pay a separate bill for the use of grey water. On the contrary, since you will use less mains water, your water bill and waste water fee (KSUB) will decrease and you will save money.

Can you drink treated grey water?

No, treated grey water is never used as drinking water. It is only suitable for toilet tanks, garden watering, car washing and rough cleaning. The drinking water network must never be connected to the grey water network (risk of cross-connection).

Does the system smell?

Properly designed, properly ventilated (oxygen supply) and regularly maintained (filter cleaning) MBR systems do not cause odour. Odour usually occurs as a result of anaerobic (oxygen-free) decomposition caused by prolonged storage of water without treatment or lack of ventilation.

Can the grey water system be installed later?

Installing a grey water system in existing buildings is costly and difficult because it requires crushing-casting operations and a separate installation line. However, it is much easier and economical to integrate into the system in extensive renovations or urban transformation projects. In new buildings, it is the most accurate method to plan at the project stage.