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Tiny bubbles clean oil from water

November 12, 2010 Leave a comment

Tiny bubbles clean oil from water

Microbubbles in a chemical reactor like the one shown here are crucial to a new method for cleaning up oil sheen and other pollutants released into water from oil drilling, refineries, mining of tar sands and oil, leaking underground gasoline tanks and other sources. Civil and environmental engineering Professor Andy Hong developed the method, which puts a new twist on two conventional techniques: bubbling ozone gas through polluted water and then filtering the water through sand. Hong’s method uses repeated cycles of pressurizing and depressurizing ozone to create microscopic bubbles that are much more effective than larger bubbles at converting and removing oil. Credit: College of Engineering, University of Utah.

Small amounts of oil leave a fluorescent sheen on polluted water. Oil sheen is hard to remove, even when the water is aerated with ozone or filtered through sand. Now, a University of Utah engineer has developed an inexpensive new method to remove oil sheen by repeatedly pressurizing and depressurizing ozone gas, creating microscopic bubbles that attack the oil so it can be removed by sand filters.

“We are not trying to treat the entire hydrocarbon [oil] content in the water – to turn it into carbon dioxide and water – but we are converting it into a form that can be retained by sand filtration, which is a conventional and economical process,” says Andy Hong, a professor of civil and environmental engineering.

In laboratory experiments reported online this week in the journal Chemosphere, Hong demonstrated that “pressure-assisted ozonation and sand filtration” effectively removes oil droplets dispersed in water, indicating it could be used to prevent oil sheen from wastewater discharged into coastal waters.

Hong says the method – for which patents are pending – also could be used to clean a variety of pollutants in water and even soil, including:

  • So-called “produced water” from oil and gas drilling sites on land. Such oily water normally is re-injected underground. “If we have technology to clean it, it could be put into beneficial uses, such as irrigation, especially in arid regions where oil and gas tend to be produced,” says Hong.
  • Water from mining of tar sands and oil shale.
  • Groundwater contaminated by MTBE, a gasoline additive that reduces harmful vehicle emissions but pollutes water due to leaking underground gasoline storage tanks.
  • “Emerging contaminants,” such as wastewater polluted with medications and personal care products.
  • Soil contaminated with polychlorinated biphenyls (PCBs, from electrical transformers) or polycyclic aromatic hydrocarbons (PAHs, from fuel burning). Water and contaminated soil would be mixed into slurry, and then treated with the new method.
  • Heavy metals in soil. Instead of ozone, air and metal-grabbing chelating agents would be pressurized with a slurry of the contaminated material.
  • Refinery wastewater and oil spills at refineries or on waterways. The spill could be vacuumed, and then treated with the new method on-site or on a barge.

Hong conducted the study with two University of Utah doctoral students – Zhixiong Cha, who has earned his Ph.D., and Chia-Jung Cheng – and with Cheng-Fang Lin, an environmental engineering professor at National Taiwan University.