WATER injected at high pressure into rock deep underground during the process of hydraulic fracturing, or “fracking”, often returns to the surface as brine, having picked up a lot of salt on its journey. It is also contaminated with chemicals from the fracking process itself. So a cheap and effective way of separating the salt and other chemicals from the water would be welcome. General Electric (GE), an American engineering conglomerate, is now putting one through its paces.
The system in question, developed by a firm called Memsys Clearwater, which is based in Germany and Singapore, is called vacuum multi-effect membrane distillation. It combines the two established ways of desalinating water: distillation and membrane separation. Already used to produce drinking water from seawater, it has not previously been applied to cleaning up water used in fracking. But recent trials of the system at a gas-fracking plant in Texas have been encouraging.
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The usual way of dealing with exhaust water from fracking is to transport it by road to a processing facility where it is put into giant steel evaporators. The water is boiled and clean water is condensed from the vapour; the residual brine is then reduced to salt by passive evaporation. But all that boiling requires a lot of energy. Membrane-based systems, which work by forcing water through what is, in effect, a molecular sieve, are less greedy. The holes in the sieve will allow water molecules to pass, but not sodium and chloride ions (the components of salt), because these electrically charged species are surrounded by retinues of water molecules that make them too big. If sufficient pressure is applied, in a process called reverse osmosis, clean water emerges on the other side of the membrane.
But reverse osmosis, though less energy-intensive than thermal evaporation, still requires a lot of energy, and can deal only with relatively low salinities, like that of seawater, which has a salt content of around 3.5%. Higher concentrations require more pressure, and the exhaust from fracking often has a salinity of 8-12%—too much for reverse osmosis to cope with.
Vacuum multi-effect membrane distillation tries to have the best of both worlds, by making the process of evaporation take place through a membrane. It also, as the name suggests, operates in a partial vacuum. This reduces the boiling point of water to 50-80°C which means less energy is needed.
The apparatus itself consists of a series of plastic cells, each of which has a condensing foil at the front and a membrane at the rear. Incoming liquid is heated and injected into the first cell. Some of it evaporates through the membrane at the rear and into a chamber, where it meets the condensing foil on the face of the second cell. This causes the water vapour to turn back into liquid, which is drained out and collected. The remaining brine in the first cell, now cooler and more concentrated, continues into the second cell, where it is warmed by the heat gathered by the condensing foil on its face. As before, some of this brine evaporates through the rear membrane of the second cell and into the next chamber where it condenses on meeting the foil on the third cell.
The process is repeated, cell after cell. Eventually, only highly concentrated brine remains, which is cooled and taken away for disposal. The clean water drained from the chambers between the cells, meanwhile, is pure enough to be used for irrigation, or even as drinking water, according to Florian Bollen, one of Memsys’s directors. Alternatively, it can be reinjected into the ground during the next frack.