Scientists reveal key insights into emerging water purification technology — @ColoradoStateU

Left represents an omniphobic membrane, and right represents a conventional hydrophobic membrane with increased water-air interfacial areas (green lines). Credit: Kota lab

Here’s the release from Colorado State University (Anne Manning):

With water scarcity a critical challenge across the globe, scientists and engineers are pursuing new ways to harvest purified water from unconventional sources, like seawater or even wastewater.

One of those researchers is Tiezheng Tong, an assistant professor in the Department of Civil and Environmental Engineering, whose lab is studying an emerging technology called membrane distillation.

Membrane distillation involves a thin, water-repellant membrane that exploits vapor pressure differences between hotter impure liquid, called “feedwater,” and colder purified water, called “permeate.” During the process, water vapor passes through the membrane and is separated from the salty or dirty feedwater. According to Tong, membrane distillation works better than other technologies like reverse osmosis, which can’t treat extremely salty water such as desalination brines or produced water from hydraulic fracturing.

While it holds promise, membrane distillation doesn’t work perfectly. A key challenge is designing membranes to purify water efficiently while ensuring zero contamination of the clean water.

Tong and materials scientist Arun Kota in the Department of Mechanical Engineering joined forces to get at the fundamental science behind designing that perfect membrane. In new experiments they describe in Nature Communications, the CSU engineers offer new information into why certain membrane designs used in membrane distillation work better than others.

“The fundamental knowledge from our paper improves mechanistic understanding on the water-vapor transport within microporous substrates and has the potential to guide the future design of membranes used in membrane distillation,” Tong said.

How it works

In membrane distillation, the feedwater is heated, separating the pure and impure components by differences in volatility. The micro-porous membrane is a key component to the setup because it allows water vapor through, but not the entire impure liquid. Typically, the membrane is made of a “hydrophobic,” or water-repellant, material in order to let only the water vapor pass through but maintain a barrier for the feedwater.

However, these hydrophobic membranes can fail, because the feedwater, such as shale oil-produced water, can have low surface tension. This low surface tension allows the feedwater to leak through the membrane pores, contaminating the pure water on the other side – a phenomenon called membrane wetting.

Previous research had unveiled that using “omniphobic” membranes – membranes that repel all liquids, including water and low surface tension liquids – keep the vapor/water separation intact. But, omniphobic membranes typically slow down the rate and amount of water vapor passing through the membrane, dramatically reducing the efficiency of the entire process.

The CSU researchers set out to discover why this tradeoff between hydrophobic vs. omniphobic membranes exists. Through systematic experiments in the lab led by postdoctoral researchers Wei Wang in Kota’s lab, and Tong’s graduate student Xuewei Du, they found that conventional hydrophobic membranes create a larger liquid-vapor interfacial area. This increases the amount of evaporation taking place. With the omniphobic membranes, they saw a much smaller liquid-vapor interface. This explains the difference between the membranes’ performances.

The omniphobic membranes used in the experiments were made without depositing extra particles. Thus the researchers were able to determine that their observations weren’t the result of structural changes to the membranes.

Cross-sectional view of a conventional hydrophobic membrane used in membrane distillation. The blue represents water. Credit: Tong and Kota labs

Solving the tradeoff problem

While they didn’t offer a solution to the tradeoff, their insights reveal the core challenge around making membrane distillation a successful technology. “If you understand the problem thoroughly, then there is scope for solving it,” Kota said. “We have identified the mechanism; now we have to solve the tradeoff problem.”

For example, smart membranes with exceptional omniphobicity and simultaneously large liquid-vapor interfacial area can render membrane distillation a robust and cost-effective process for water purification. More collaborative research has been initiated by the team to design such smart membranes, with the goal of increasing efficiency of membrane distillation.

Tong added that the research happened at the interface of two disciplines: surface science and membrane technology.

“Arun and I utilized our complementary expertise to systematically conduct this work,” Tong said. “It is an example of good interdisciplinary collaboration across campus.”

Graduate students Hamed Vahabi in mechanical engineering and Yiming Yin in civil and environmental engineering also contributed to this work.

@ColoradoClimate: Weekly #Climate, #Water and #Drought Assessment of the Intermountain West


Click here to read the current assessment. Click here to go to the NIDIS website hosted by the Colorado Climate Center.

After a wildfire, a Colorado town’s residents reluctantly sue a historic railway — The Los Angeles Times

The 416 Fire near Durango, Colorado, ignited on June 1, 2018. By June 21, the wildfire covered more than 34,000 acres and was 37 percent contained. Photo credit USFS via The High Country News

Here’s a report about the lawsuit against the Durango & Silverton Narrow Gauge Railroad from David Kelly that’s running in The Los Angeles Times. Click through and read the whole thing. Here’s an excerpt:

…the federal government and others are pointing the finger at a local icon — the Durango & Silverton Narrow Gauge Railroad, which carries hundreds of thousands of passengers a year through the San Juan Mountains.

Claiming cinders from the coal-fired, steam locomotive ignited brush along the tracks during a time of heightened fire restrictions, the U.S. attorney’s office filed a suit against the train’s owners last month seeking $25 million to cover the cost of putting out the fire. Another two dozen or so citizens and businesses are also suing for damage to their properties.

Nobody wants the train to go out of business, but many fear the suits could eventually drive the railroad into bankruptcy, destroying a historic landmark and badly damaging the local economy…

“The responsible decision of train management would have been to not run the train in those super-dry conditions,” said Thomas Henderson, a Denver lawyer whose firm is representing individuals and businesses suing the train. “The train has started fires for years that the feds have had to put out. They should not get a free pass simply because they are big player in town. That’s not how democracy works.”

[…]

Few businesses are as tied to the railroad as the historic Strater Hotel, built in 1887. Roderick Barker’s family has owned it for 93 years, and he figures at least 50 to 60% of his guests ride the train.

“The train is the lifeblood of this whole town,” he said. “If it were to fail it would certainly be one of the most significant things to happen in the history of Durango.”

He believes the train caused the fire and needs to change its operations. But given its contribution to the economy, he questions why any local business would sue the railroad…

Bobby Duthie, an attorney, grew up on 33rd Street in Durango. The train whistle woke him each morning. He’s ridden it more than 50 times. Now he’s working with Henderson in representing those suing the train.

“I was initially reluctant to get involved because I love the train. But I also know that their decision to run it that day was reckless,” he said, sitting in his downtown office. “They had started fires on the tracks the month before and it was just a matter of time until it got out of control.”

According to the federal lawsuit, the wildfire, dubbed the 416 fire, began on Shalona Hill where the grade is steep. As the train climbed, it cast off sparks and cinders. A metal screen on the smokestack caught many but not all.

“I talked to eyewitnesses,” Duthie said. “I know the train started the fire. I’m sad they chose to run it on June 1, 2018.”

Debris flow from 416 Fire. Photo credit: Twitter #416Fire hash tag

Kristi Nelson’s home escaped the fire but suffered major damage in the mudslides.

“They took 23 dump-truck loads of mud from my property,” she said. “It was devastating. I still have a mortgage on top of $116,000 worth of damages. Let’s say I don’t want to do this work. Can I sell it?”

She said people have urged her not to ruin the train. That stings for the former vice president of sales and marketing for the railroad.

“It is with a heavy heart that I entered into this lawsuit because I love the train,” she said. “But if I crashed my car into the train depot they would expect my insurance to pay. The train’s insurance should do the same.”