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GROWing the next generation of water recycling plants


A vegetated rooftop recycling system has been developed that allows water to be used twice before it is flushed into the communal waste water system.

The Green Roof Water Recycling System (GROW) uses semi-aquatic plants to treat waste washing water, which can then be reused for activities such as flushing the toilet.

GROW is the brainchild of Chris Shirley-Smith, whose company Water Works UK is collaborating with Imperial College London and Cranfield University. The researchers are funded by the Engineering and Physical Sciences Research Council.

So-called grey water from washbasins, baths and showers is pumped up to the GROW system, which is constructed on the roof of an office or housing block. It consists of an inclined framework of interconnected horizontal troughs. Planted in these troughs are rows of specially chosen plants that gently cleanse the grey water. Trickling through the GROW framework, the plants’ roots naturally take up the dissolved pollutants, leaving ’green water’. Green water is not drinkable and will be dyed with a vegetable colour to signify this, but it can be used to flush toilets or water the garden.

More than half the water used in the home and workplace does not need to be of drinkable quality yet it comes from the same pure source as our kitchen taps. Using GROW, much of the water that enters a building can be used twice before being placed into the national wastewater management system.

“We had to carefully choose which semi-aquatic plants to use. One of the most successful is water mint, whose roots have disinfectant qualities,” says Professor David Butler, who oversees the project at Imperial College. The other plant species include the yellow flag iris, marsh marigold, and the common reed. They are chosen to be resistant to the pollutants they absorb. By planting more than one species, the engineers guard against an unusually dirty batch of water exceeding a particular species’ tolerance level. Should one species die off, there will still be others there to continue the job until the dead plants can be replaced.

The beauty of the system is that it is not ’high-tech’ in the traditional sense. “It does not require sophisticated maintenance, just tending, like any garden,” says Butler.

The next aim for GROW is to see if it can be reduced in size to sit above a household water butt, making it serviceable for individual households. The team will also investigate whether the addition of an ultraviolet light can enhance the disinfection of the water. They hope to market GROW commercially in the second half of 2006.

GROW is one project in a much larger EPSRC-funded Sustainable Water Management programme (WaND) that Professor Butler oversees at Imperial. “Our overall aim is to contribute towards sustainable water management in new developments. We hope that GROW will be one of the tools that can help us achieve that goal,” says Butler.


Notes for editors:

GROW won the 2004 Chartered Institution of Water and Environmental Management (CIWEM)/Black&Veatch World of Difference award. It was also exhibited on the CIWEM stand at the Chelsea Flower Show, where it gained a bronze medal. It was also a finalist in the Environment Agency’s 2005 sustainability awards.

The Chartered Institution of Water and Environmental Management (CIWEM) is the leading professional and examining body for scientists, engineers, other environmental professionals, students and those committed to the sustainable management and development of water and the environment. Its website is:

Chris Shirley-Smith, whose company Water Works UK is working to commercialise GROW can be found on the web at:

The WaND programme (Water Cycle Management for New Developments) can be found on the web at:


Professor David Butler, Imperial College London, tel: 020 7594 6099 / 01525 754694, e-mail:

Chris Shirley-Smith, Water Works UK, tel: 020 8888 7259, e-mail

An image of GROW is available from the EPSRC press office. Contact Natasha Richardson,, tel: 01793 444404.

Suggested caption details for GROW.jpg: “The Green Roof Water Recycling System, GROW, will bring a splash of colour to the rooftops of office blocks and flats in cities across the UK. It will also help recycle water. The green membrane protects the system from rain, which itself could be collected and reused.”


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