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This week we ask: Is recycling, not desalination, the most sustainable way of providing fresh water in areas where supply is short?

Debate Recycling water

Water is going to be the most precious resource of this century, and engineers are split over the best way to make the most of what the planet can provide.

The facts

Desalination does not create any 'new' water, but merely converts that which exists to human use/consumption.

Projects in the Middle East involve constructing a power and water system in Abu Dhabi, with a water production capacity of 450,000 m 3/day and a reverse osmosis system of 55,000 m 3/day, using seawater as the saline water source.

Ten Environment Agency staff agreed to have greywater recycling systems retrofitted to their houses. Water was taken from the bathroom sinks, showers and baths, treated and reused to flush the toilets.

Water from hand basins in the Millennium Dome was collected and treated to potable standards before being reused as toilet flush water. The Dome is the UK's largest example of a greywater system.

Linacre College in Oxford takes water from dormitory showers and baths for reuse +as toilet flush water.

Reclaimed water passes through a bioreactor, then on to a sand filter and finally through a granular activated carbon filter, before being reused. (CIWEM, the journal, November 2001).

Yes

Chris Binnie, independent water consultant

The world has, on one hand, finite resources of fresh water while on the other, increasing population and the need for increased water supplies for use in the home, industry and agriculture to feed the world. It is most important that the world has a sustainable environment.

In certain dry areas, desalination is used. The source of water is generally seawater. To produce water for drinking or agriculture by desalination requires 'In Windhoek in Namibia, for the last 20 years, the treated sewage effluent is sent directly into the inlet of the municipal water treatment works. No adverse effects have been found.'

huge amounts of energy. In those areas, this comes from the burning of fossil fuels, resulting in significantly increased greenhouse gas emissions and exacerbating climate change. If the need is inland from the coast then the water has to be pumped there using even more energy.

Recycling makes greater use of existing sources. For industry, used water can either be utilised where a lower grade of water is appropriate or else treated and reused. In the home, grey water from baths and basins can be stored, provided limited treatment, and then used for flushing the WC or watering the garden. On the municipal scale, wastewater is treated at sewage treatment works and then discharged to the river or sea.

That to the sea is lost. That in the river is already often abstracted by the town downstream and treated as part of its municipal water supply.

In Europe and elsewhere standards of effluent treatment have been raised and have become much more reliable, while polluting industrial discharges have considerably reduced. In some parts of the world, treated effluent is used for irrigating agricultural crops or recharging groundwater aquifers.

In Windhoek in Namibia, for the last 20 years, the treated sewage effluent is sent directly into the inlet of the municipal water treatment works. No adverse effects have been found.

To be sustainable in the long term, we must consider greater recycling of water.

No

Robert Querns, desalination consultant

Desalination involves the extraction of fresh water from seawater and, in some cases, from brackish waters, high salinity ground waters and municipal waste waters. This process renders otherwise unusable waters fit for human consumption, irrigation, industrial applications and various other purposes.

Modern desalination plants utilise three basic processes - distillation, reverse osmosis and electrodialysis.

Distillation is the dominant process involving the generation of vapour from a saline solution, with subsequent condensation.

The resulting condensate is fresh or potable water. Reverse osmosis is a membrane based process in which the natural phenomenon of osmosis is reversed by the application of pressure to a saline solution in contact with an appropriate membrane. Electrodialysis is another membrane process in which the driving force for separation is an electric field across alternating anion and cation exchange membranes.

Of the three process options, electrodialysis is economically effective for saline waters with a total dissolved solids (TDS) content up to 10,000 parts per million (ppm). The other processes can economically handle TDS levels up to 45,000 ppm, although distillation can operate at higher TDS levels.

Distillers with unit capacities up to 55,000 m 3/day are in operation. Combined power and water systems are in operation in the Middle East with overall capacities around 350,000m 3/day.

Reverse osmosis plant with capacities of 45,000m 3/day and over are in operation.

Experience of water supply in arid regions has shown that, where there is no alternative to exploitation of saline water sources, desalination offers a reliable solution for the production of potable water. The choice of desalination process is largely an economic issue, with distillation showing advantage where there is a combined power and water demand.

For the single purpose, water only, situation reverse osmosis can show economic advantage, although the more sophisticated pre-treatment requirements of reverse osmosis plant can affect economics adversely, particularly where there is significant biological activity.

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