Scientists from Rice University have produced a new filter that can remove more than 90 percent of hydrocarbons, bacteria and particulates from contaminated water that was produced by hydraulic fracturing (fracking) operations at shale oil and gas wells.
The work was led by Rice chemist Andrew Barron and his team. The work turns a ceramic membrane with microscale pores into a super-hydrophilic filter that eliminates most of a common problem of fouling.
The researchers determined one pass through the membrane should clean the contaminated water enough for reuse at a well. This significantly cut the amount that has to be stored or transported.
The filters keep emulsified hydrocarbons from passing through the material’s iconically charged pores which are around one-fifth of a micron wide. It is small enough that other contaminants can’t pass through. The charge attracts a thin layer of water that adheres to the entire surface of the filter to repel globules of oil and other hydrocarbons to keep it from clogging.
A hydraulically fractured well uses more than 5 million gallons of water on average, and only 10 to 15 percent of that water is actually recovered. “"This makes it very important to be able to re-use this water," Barron said.
Not every type of filter actually reliably removes every type of contaminant.
Solubilized hydrocarbon molecules slip through microfilters designed to remove bacteria. Natural organic matter, like sugar from guar gum, is used to make fracking fluids more viscous, and require ultra or Nanofiltration. But those foul easily, especially from hydrocarbons that emulsify into globules. A multistage filter could remove all the contaminants aren’t practical due to the cost and energy it would require.
"Frac water and produced waters represent a significant challenge on a technical level," Barron said. "If you use a membrane with pores small enough to separate, they foul, and this renders the membrane useless. In our case, the superhydrophilic treatment results in an increased flux (flow) of water through the membrane and inhibits any hydrophobic material – such as oil – from passing through. The difference in solubility of the contaminants thus works to allow for separation of molecules that should, in theory, pass through the membrane."
Barron and his team used cysteic acid to modify the surface of an alumina-based ceramic membrane, which makes it superhydrophilic or extremely attracted to water. The superhydrophilic surface has a contact angle of 5 degrees.
The acid covered the surface and the inside of the pores, which kept particulates from sticking to them and fouling the filter.
In tests with fracking flowback or produced water that contained guar gum, the alumna membrane showed a slow initial decrease in flux, but it was stabilized for the duration of lab tests. Untreated membranes showed a dramatic decrease within 18 hours.
Researchers believe that the initial decrease in flow through the ceramics was due to a purging of air from the pores, after the superhydrophilic pores trapped the thin layer of water that prevented fouling.
"This membrane doesn't foul, so it lasts," Barron said. "It requires lower operating pressures, so you need a smaller pump that consumes less electricity. And that's all better for the environment."
A paper on this research was published in Scientific Reports.