| Water desalination research: worth its salt


Water desalination plant

Thermal

The two main types of water desalination technology are thermal and membrane. Thermal desalination is mostly used in the Middle East – where the costs of the energy needed for the process are still affordable – or close to locations where waste heat can be recovered as an energy source. In other places it is difficult for thermal desalination to compete with membrane technologies unless it is combined with renewable energies, such as solar thermal collectors.

A number of projects funded under the EU’s FP5 explored issues related to thermal desalination and solar energy, such as AQUASOL (Zero Discharge Seawater Desalination Using Hybrid Solar Technology).

Membrane

Current membrane desalination plants are based on reverse osmosis (RO) processes. These are benefiting from scientific and technological advances – notably in terms of materials development and energy needs – and are combined with energy recovery devices. Projects under FP6 addressing membrane desalination include MEDINA and MEDESOL. The latter for instance is working on seawater desalination by an innovative solar-powered membrane-distillation system.

The costs per unit of treated water in membrane systems depend to a large extent on the operational and management costs, with the lifespan of the membranes being a particularly important factor. For decades scientists have been expanding their knowledge of phenomena like membrane fouling and scaling as well as membrane transfer and fluxes. R&D efforts are tackling these and other issues like pre-treatment, cleaning and disposal strategies.

In only 15 years the cost of membrane-desalted water has been halved to close to 0.40€/m³ (worth comparing with the costs of the bottled water consumed in Europe!). And thanks to scientific and technical efforts the costs of RO water are expected to drop still further in the near future.

Respecting the seas

All human activities have an environmental impact of some sort. The uncontrolled disposal of waste into the sea for one may have a detrimental effect. The effluents from RO plants are usually called concentrates or brines because they contain the rejected salt at a concentration higher than the waters receiving the effluents. The higher concentration of such brines may affect marine ecosystems (Posidonia seagrass beds in the Mediterranean, for example) locally at the point of disposal. Scientists are looking into this potential problem and are proposing solutions based on disposal facilities (taking into account diffusion, dilution, distance from the shore and immersion of the disposal devices) that are designed to neutralise potential harmful effects.

It is also important for decision-makers and stakeholders to have at their disposal tools that address environmental aspects in an objective way. For example, the MEDINA project is developing a sound methodology for environmental impact assessments of membrane-based desalination plants as well as indicators for lifecycle analysis.