A Danish study of alternative water supply systems suggests that adopting a mix of different technologies could optimise the potential for water self- sufficiency in urban areas.
Population growth and urbanisation are challenging current water supply systems. In many urban areas, attention has turned to a range of new technologies that increase self-sufficiency, enabling the use of water sourced from within cities.
Increased self-sufficiency is linked to the rapid development of three main solutions, namely wastewater reclamation, desalination and rainwater collection. A recent study by researchers at the Technical University of Denmark found that increases in self-sufficiency ratios of between 15 and 80% were possible by using one or more of these approaches.
Reclaimed water can be used directly, such as for irrigation, toilet flushing and industrial processes. In Costa Brava, Spain, for example, a plant based on constructed wetlands provides in excess of 600 000m3 of reclaimed water every year. It can also be used indirectly. In a plant in California, for example, wastewater is treated by microfiltration combined with reverse osmosis and ultraviolet and hydrogen peroxide disinfection before being used to recharge groundwater reservoirs.
Desalination capacity is growing rapidly worldwide, in line with a sharp decline in treatment costs. State-of-the-art reverse osmosis plants currently achieve the lowest costs but a range of new technologies are also being investigated, including improved membrane materials that lower pressure needs, and technologies such as forward osmosis and membrane distillation.
Rainwater collection is another important contributor to urban water self- sufficiency. In parts of Australia, for example, rooftop-collected rainwater is used for toilet flushing, washing of clothes and garden irrigation and typically supplies 25% of the domestic drinking water use.
However, the introduction of these alternative water resources presents several important challenges. Energy consumption, for example, varies considerably between the different systems and the potential environmental impacts can be a concern, especially in relation to desalination. Risks associated with the presence of trace levels of chemicals in reclaimed drinking water are also not well understood, and general public distrust of water that has been recycled or desalinated presents another challenge.
The best solution therefore may be to diversify and use several different techniques to overcome the various challenges. Wastewater reclamation and desalination are attractive because of their reliability and stability. Rainwater collection on the other is more intermittent, but is also more acceptable to the public and is less treatment intensive, and therefore less costly.
The desire for self-sufficiency will continue to be a major driver for new technologies and concepts in modern water supplies. These technologies will become increasingly common as water utilities seek more secure solutions by diversifying their approaches to water supply.
‘Increasing urban water self-sufficiency: New era, new challenges’; Martin Rygaard, Philip J. Binning and Hans-Jørgen Albrechtsen; Journal of Environmental Management: