Grey Water Treatment
by Rami Elias Kremesti M.Sc., CSci, CEnv, CWEM
Kremesti Environmental Consulting Ltd.
Transmutare Substantiarum Basium In Aurum TM
Introduction
Grey water is waste water collected from showers, sinks (kitchen and toilet) and washing machines. It is normally mixed with black and yellow water from toilets but that is not a sustainable practice. Truely eco buildings and homes separate waste water at the source because this way it is easier to treat, creates less waste and this lends itself to better nutrient recovery.
What is In Grey Water?
Grey water is 99% water that is polluted with fibres from laundry, COD (Chemical Oxygen Demand) or non-biodegradable organics, hair, BOD (biodegradable organics), some bacteria (Fecal E Coli from your bum hole for example) and SS or suspended solids + TDS or dissolved solids. It can also contains scum which is the product of the reaction of hardness in the water with the organics in shampoo, soap and various detergents.
Typical Household Water Use Pie Chart
How to Recycle Grey Water?
The easiest way to recycle grey water is to wash laundry/take a shower in a tub and use the waste water to irrigate your garden or flush the toilet. Other more advanced methods involved hair straining, filtration, coagulation/flocculation, aeration/settling and disinfection. BREEAM rated eco buildings like the Bloomberg Building in London have large tanks in the basement that collect grey water from the building and treat it for recycling as toilet flushing water. The Japanese have hand washing sinks that sit on top of toilet flushing tanks and use the waste water to flush the toilet. Of course with time, the tank might need an occasional cleaning and disinfection.
Japanese Water Saving Toilet with Hand Washing Sink On Top of Flushing Tank
Technologies Used
Technologies used for grey water treatment focus on removing hair/SS, reducing BOD/COD and disinfecting the water. The following water treatment technologies can be used:
Simple Straining
Coagulation/Flocculation + Sand/Media Filters
Constructed Wetlands – Nature Based Solutions
Activated Sludge / SBR (Sequence Batch Reactor)
MBR – Membrane Bio Reactor
Biofilters
AOPs – Advanced Oxidation Processes
UV/Chlorination
Why Do We Need to Recycle Grey Water?
Water resources are stressed and large metropolitan WWTW’s are strained to capacity. Therefore, governments and municipalities need to find ways to reduce the strain on their sewage networks and central treatment works. This is one strategy that can help. Take for example an average UK home that uses 150 litres of fresh water per day for drinking, washing, laundry, showering, cooking and cleaning per capita. Out of that 150 litres of fresh water, 50 litres are wasted to the sewer as grey water. We can reduce that 50 litres of waste potable water by recycling grey water and reduce the strain on the centralized sewer network by 30%. We would also be reducing demand on the potable water network. In some water stressed regions in Africa, flushing potable water down the toilet is unthinkable !
What to Do With the Poo and Pee ?
Best to separate them from the grey water and recover valuable N and P from them.
A ton of mixed human faeces and urine typically contains a valuable amount of nutrients, though the exact concentration varies with diet and water content. On average, you can expect:
Nitrogen (N): Approximately (7 to 14 kg) per ton.
Phosphorus (P): Approximately (0.7 to 2.3 kg) per ton. A Kg og white phosphorus will cost you 100£ retail.
EAWAG which is the prestigious water research institute of the ETH in Zurich have published an open source study about urine recycling called Vuna. The word vuna means harvest in Zulu.
Statistics
While comprehensive, up-to-date, pan-EU statistics specifically focused only on household-level greywater recycling (wastewater from sinks, showers, and washing machines) are limited, available data on overall wastewater recycling suggests that Spain and Italy are the leaders in volume, while Malta and Cyprus have the highest rates relative to their total treated wastewater (100%).
The majority of data from the European Environment Agency (EEA) and Eurostat focuses on total municipal solid waste recycling, where Germany and Austria often lead. However, the picture for water is different due to varying climates and water scarcity issues.
The Situation in the UK
The UK does collect and recycle some greywater (from baths, showers, sinks, washing machines) for non-potable uses like toilet flushing and irrigation, but it’s not widespread due to relatively low mains water costs (which have increased by an average of 30-40% in 2025) and existing regulations, though it’s gaining traction as a key water security solution in national strategies. Systems need to meet strict standards (like BS8525) for safety, and while not mandatory, government plans and industry bodies increasingly promote greywater reuse to meet better sustainability goals.
How to Get to A 100% Sustainable, Circular Society?
To get to a 100% circular society we need to RECYCLE EVERYTHING. Not just water, but all resources. To achieve this we need to work together as consumers, legislators, governments, finance institutions, banks, and private sector to design an economic system that is based on infinite recycling. Earth’s resources are limited and they get depleted every year around August according to Earth Over-Shoot Day. This is not SUSTAINABLE. Pollution and Climate Change are the result of human activities that are not centered around recycling and resource recovery. When we burn fossil fuels without recycling the CO2 gas emissions, we create an imbalance in the earth’s eco system. When we dump waste water in our rivers and oceans without removing and recovering pollutants, we create disease.
Suppliers of Grey Water Recycling Systems
References/Further Reading
https://www.thegreywaterproject.org/post/how-greywater-reuse-impacts-property-values-worldwide
Conclusion
Net Zero should not be just about Zero CO2 and GHG emissions. It should mean ZERO POLLUTION as well and a 100% recycling society.
About the Author
Rami Elias Kremesti is a chartered water treatment and environmental specialist based out of the UK. He grew up in Beirut, Lebanon where he used to go fishing and snorkelling in the Mediterranean when he was not studying hard. He witnessed environmental pollution first hand as a kid when he contracted a fungus due to sewage pollution in one of the public beaches in Beirut.
Rami Elias Kremesti Portrait