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Recycling water in buildings by microbes

August 02, 2000

New information about how households and businesses can recycle and re-use their own water suggests that a submerged membrane bio-reactor could be the best method. The aim is to make water a more sustainable resource by reducing the amount that needs to be treated by large, central facilities.

The work is being carried out in Cranfield University’s School of Water Sciences with funding from the Engineering and Physical Sciences Research Council.

The population of the UK living in urban areas is projected to increase significantly over the next decade, requiring around 1.5 million new houses to be built. Many sectors of the water industry are investigating new ways to reduce the demand for water, both by domestic and commercial users. While some industries do recycle and re-use their own water, little is known about the practicalities, acceptability and benefits for households.

One aim of the research has been to identify which existing wastewater treatment methods would be sufficiently compact for recycling water in residential accommodation or office buildings while achieving the appropriate degree of treatment.

“There are a range of existing technologies for wastewater treatment, but they have not been tested systematically for the kind of data that is needed,” says Dr Simon Judd, one of the research team.

The team decided at the outset that a biological-based system, rather than chemical treatment, would be required. “Chemical systems can give problems with by-products and the chemicals themselves need to be stored, which can be a potential hazard,” says Dr Judd. “In a biological system, where the waste is degraded by microbes, the process can be carried out at ambient temperature and is a more sustainable system.”

The researchers examined a number of biological treatment systems, concluding that the most appropriate was a submerged membrane bioreactor, or MBR. Here microbes break down the waste in a similar way to a conventional sewage treatment works. However, unlike a conventional plant, where the breakdown products aggregate as small particles and finally settle out as a sludge, in the MBR the particles are effectively caught in an extremely fine sieve. “We found that the submerged MBR was the most technologically effective system for a range of circumstances, from a single household to a very large building,” says Dr Judd.

The researchers also made important discoveries relating to the recycling of so-called ‘grey water’. This is water that comes from showers, baths and washbasins. “Our research demonstrated first that there is a vast variation in the quality of grey water and secondly that it can degrade very quickly simply depending on how it is stored,” says Dr Judd. If the grey water is aerated relatively modestly during its storage, the waste matter within it breaks down substantially. These findings will be significant for anyone designing small, self-contained water treatment facilities for buildings.

Meanwhile Dr Paul Jeffrey has been investigating people’s attitudes towards ‘in-building’ recycling of water. “Social enquiry is an important part of the process because it can help to identify what is acceptable to people rather than simply concentrating on what is technologically feasible,” says Dr Jeffrey.

For example a survey on people’s attitudes to using recycled grey water showed that while most would be prepared to use it to flush the toilet, slightly fewer would wash their cars with it and fewer still would use it to water the garden.

Furthermore, people’s willingness to use recycled grey water was also affected by its origin. Nearly everyone surveyed would be prepared to use their own grey water if it were recycled, but fewer than half if it were taken from throughout the neighbourhood.


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Engineering and Physical Sciences Research Council (EPSRC)