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When oil comes to the surface it is often mixed with water. Oil produced at the Nimr field in Oman, for example, contains up to 90% water, as well as other substances like salt. 

In the past, this water had been cleaned and then pumped back underground, which uses lots of energy.

Now a different approach purifies this water for natural disposal using reeds, gravity, sunlight...and hungry microbes.

The oil and water are first piped into a bathtub-like separator.

The water spirals out through a plughole taking traces of oil, leaving thicker crude to be sucked off the top. 

The water is then fed into a series of sloping fields of grassy reeds, in all the size of 450 soccer pitches.

Oil-eating microbes

Oil produced at Nimr in Oman reaches the surface mixed with 90% water, which traditionally must be cleaned before being pumped back beneath the ground

Oil produced at Nimr in Oman reaches the surface mixed with 90% water, which traditionally must be cleaned before being pumped back beneath the ground

As gravity pulls the water downhill, the reeds act as a filter. Oil sticks to them, and is eaten by microbes that naturally feed on hydrocarbons in the ground. Finally, the water is pumped into pools to evaporate cleanly under the sun, leaving only a thick crust of salt.

German company Bauer Resources developed this approach on behalf of Petroleum Development Oman, in which Shell has a 34% share. After a successful trial, Bauer is planting more reeds to double the filtering capacity. There are plans to turn used reeds into biofuel, and research is under way to find industrial uses for the salt.