A Fouling-Free Filter For Produced Water

While the debate over how harmful hydraulic fracturing still rages, oil and gas operations are undoubtedly on the lookout for a cheaper and more efficient way of cleaning the water produced by the process.

Hydraulic fracturing, or “fracking,” is a method that involves extracting oil from ground deposits by injecting highly-pressurized water to break up the rocks that block them. The wastewater produced in the process contains a range of contaminants and is far from fit for source bodies. Oil and gas operations are charged with cleaning this product before disposing of it.

Rice University scientists may have had a breakthrough that will help them. They claim to have developed a new water filter that can remove 90 percent of hydrocarbons, bacteria, and particulates from fracking operation’s produced water. And furthermore, this filter is said to eliminate fouling, the process by which nearly every other filter in the industry is dirtied by contaminants and made to require replacement, and therefore to be vastly more cost efficient.

The scientists were able to adopt a ceramic membrane with microscale pores into this “superhydrophilic” filter, based on work that they were doing for the military.

“We were working on a project for the U.S. Navy involving materials that would allow salt water to flow through but not bacteria, viruses, or hydrocarbons,” said Andrew Barron, a Rice University chemist and lead scientist on the project. “We had developed a nanoparticle-based treatment for fabrics and thought that the chemistry could be applied directly to ceramic substrates.”

The researchers have declared that a single pass of produced water through the membrane should render it clean enough for reuse at an oil and gas well, which would eliminate the common practice of storing and transporting the produced water for treatment elsewhere. The key is the superhydrophilic surface, one that is extremely attracted to water. This surface was modified from an alumina-based ceramic membrane by the Rice team with the use of cysteic acid.

“Hydrophilic surfaces on membranes have been shown to be superior for rejection of organic molecules as well as showing a reduction in membrane fouling,” Barron said. “This study has shown that the cysteic acid functionalization is not just on the exterior surface of the channels, but also on the internal surface of the pores as well and thus creates a highly-ordered [zwitterion-charged] superhydrophilic membrane surface, which repels the oil components when the quantities are sufficient to form globules.”

Because the cysteic acid covers both the surface and pores of the membrane, it keeps particulates from sticking to it and keeps them from fouling.

“The low fouling of the cysteic acid functionalized alumina membrane can be related to the zwitterionic nature of the surface, creating a superhydrophilic layer that is wetted with water,” said Barron. “When organic globules contact this wetted layer, they are repelled and do not have access to the membrane surface in order for deposition to occur.”

The modification also improved the flux of the membrane, the rate at which produced water can permeate the membrane. This is another reason that fouling has been reduced.

“The modified surface of the membrane significantly increases the membrane flux when compared to the original membrane,” Barron said. “The flux obtained at low pressure for the modified membrane is the same magnitude for the untreated membrane… As the flux is the same, there will be no impact on fouling.”

Without the use of chemicals and a hugely reduced need for energy, the new filter is a solution that many in the oil and gas industry will want to pursue. And Barron believes the industry is ready for the filter just as the filter is ready for it.

“There is a need to recycle and reuse frack and produced water since deep well injection of these waters is becoming unsustainable,” Barron said. “We have already scaled this for field trials and demonstrated its use over many months.”

The researchers are currently building a membrane unit for an oil company to utilize in field trials. Following that, wastewater treatment in the fracking industry may never be the same.