Polymers in osmosis with sea water
Reverse osmosis puts the membranes to the test
lthough it covers only 1% of the world's water needs, water desalination is a dynamic market, currently estimated at around 85 billion Euros, in which two techniques are currently facing off.
The first technique, thermal desalination, consists of boiling sea-water and condensing the desalted water vapour. There is renewed interest for this ancient but highly energy intensive technique because of lower costs linked to the emergence of organic photovoltaics. It has nevertheless been supplanted by reverse osmosis.
This process is derived from the process of osmosis that governs the balance of concentrations of two solutions through a membrane that is permeable only to liquids. According to this principle, the fresh water contained in a compartment will naturally pass through the membrane and dilute the sea-water on the other side.
The principle of reverse osmosis consists of exerting pressure on the sea-water to force it to fill the fresh water compartment.
Although highly effective, this technology also has its drawbacks. It is highly energy intensive as it requires very high pressures to be exerted, it provides a 50% yield compared to traditional techniques, and it generates residues that corrode the membrane and that are difficult to treat. All of this justifies the interest in synthetic fibres which, as advances are made, are becoming less expensive to maintain and increasingly efficient.
NanoH2O increases water flow
The smaller the gaps in the screen, the longer the water takes to filter! This rule also applies to membranes whose flow is theoretically proportional to the material's permeability. And even more so in the case of reverse osmosis, where the pressure used to improve the flow consumes a large amount of energy and sorely tests the membranes.
With the advances made in synthetic fibres, their consumption has been almost reduced to a third, decreasing from 15 kWh to 3.5 kWh per m3 of desalinated water. The first generation polyamide membranes have since been replaced by composite or multi-layer polysulfone membranes. Once again, however, nanotechnologies came to the rescue and recently enabled the decisive steps to be taken.
With the development of the Quantum Flux membrane, the Californian NanoH2O start-up demonstrated the effectiveness of the dispersion of nanoparticles of zeolite, a microporous crystal of the silicate family, on the surface layer of its membranes.
Increasing the number of nanoscale pores improves the fibres' permeability and changes their surface properties, enabling them to more easily reject salt, and limit clogging. This enables manufacturers in the industry to reduce energy consumption by around 20%, increase the flow of water treated, or, at equal production, reduce the size of their facilities.
Fresh water, even in extreme conditions
The solar still is possibly the simplest system for distilling sea-water. Although they are able to produce small quantities of fresh water for basic needs, they are inefficient for large-scale production. This is why most commercial models are intended for families or individuals, in the context of humanitarian aid or development.
They generally consist of a rectangular or circular basin lined with a dark material to maximise the absorption of solar rays, and covered with a transparent lid. The greenhouse effect causes the temperature to rise, and the sea-water evaporates and condenses on the underside of the lid.
The precarious conditions for using a solar still have led manufacturers to use light, strong and heat-resistant plastics.
The Watercone, designed by Munich-based engineer Stephan Augustin, is a circular black PVC basin covered with a simple polycarbonate cone. The device enables 1.5 litres of water to be distilled every day. The Texas-based SolAqua company markets small polycarbonate-covered fibreglass greenhouses whose daily yield, in a favourable climate, reaches around 8 litres per m2.