From The Editor | June 8, 2017

How Natural Light Can Separate Oil From Wastewater

Peter Chawaga - editor

By Peter Chawaga, Associate Editor, Water Online

How Natural Light Can Separate Oil From Wastewater

Oil and water are two components that famously should not mix. Unfortunately, many industries around the world depend on processes that inevitably bring the disparate elements together. While a range of solutions to separate the resulting produced water exist, some of the nation’s greatest minds have been working on a way to improve the process.

“Separation of oil-water mixture is crucial in wastewater treatment performed in various fields, including the oil and gas industry, pharmaceutical, and metals processing,” said Divya Panchanathan, a graduate student at the Massachusetts Institute of Technology (MIT), who was involved in a recent project to develop a new method to separate oil and water.

The research project yielded a new system that utilizes light to control the way brine water moves over a surface, providing a new tool to separate oil and wastewater. While the system has extensive industrial potential, it could be particularly game-changing for the oil and gas industry.

“The oil and gas industry in the U.S. generates about 21 billion barrels of wastewater annually,” Panchanathan said. “This produced water must be treated before discharge in order to meet environmental regulation. However, current produced water treatment techniques are either energy-intensive or require special environmental conditions. Therefore, there is a need to develop a new solution for produced water treatment.”

Traditional separation techniques include gravity separation, air flotation, absorptive materials, electrocoagulation, and the use of membranes. Gravity separation does not separate smaller oil droplets, air flotation typically requires the use of chemicals, absorptive materials often absorb water as well as oil, electrocoagulation requires lots of energy, and membranes are subject to fouling and degradation. MIT’s new system is thought to be free of these drawbacks.

The research first fabricated a thin film of titanium dioxide, coated with an organic dye that can absorb visible light. When this surface is illuminated with visible light, a droplet of brine (such as water containing 10 percent sodium chloride) will spread on the surface, while oil will not. This allows brine to coalesce when light is shined on the surface and would allow treatment operations to separate the briny wastewater from the oil.

“When visible light is illuminated, brine droplets spread on our dye-sensitized titanium dioxide surface,” said Panchanathan. “This allows for phase separation of emulsion into brine-rich and oil-rich phases within a few minutes. We found that can remove more than 99.9 volume percent of brine from the oil phase. We believe that our new technology can substitute the current techniques used in the oil and gas industry when it is scaled up and improved in separation efficiency.”

As part of the effort to improve this technology and get it into the hands of industrial users, the researchers are focused on two design parameters. Firstly, they want to develop a better light-absorbing dye to coat the titanium dioxide film.

“A dye molecule plays a key role in incident light absorption, electron transfer, and the regeneration process… Effective electron transfer and dye regeneration process is important to facilitate spreading of brine droplets on our surface,” Panchanathan said. “Therefore, a new class of dye molecules that can absorb a broad range of wavelength of light and facilitate wetting needs to be developed.”

Secondly, the researchers are looking for ways to deliver light to the surface more efficiently, without it getting blocked by the produced water.

“We can avoid this light interference by illuminating light from the opposite side of emulsion, which eliminates interference,” said Panchanathan. “Therefore, a separation apparatus must be carefully designed.”

If and when the system is perfected and accessed by industrial operations, the MIT team expects them to reap economic and environmental benefits. The materials used to fabricate the titanium dioxide surface are commercially available and affordable and the dye used is a chlorophyll derivative, making it relatively environmental. The light necessary for the system can be accessed naturally and the separation is achieved using gravity, both without external energy requirements.

“We anticipate that our technology is energy efficient and economically feasible,” Panchanathan said.

Image credit: "Shaft of light," gaynorhenry © 2014, used under an Attribution 2.0 Generic license: https://creativecommons.org/licenses/by/2.0/