Did you know that some water treatment plants clean their filtration systems with chemicals so harsh that they eat through the pumps that carry them?

It’s true.

“Using those chemicals is a real hazard for the treatment plant workers,” says water expert Alex Gorzalski. “It’s nasty stuff. Think Drano, but more harsh.”

And you say, Alex, that they use these chemicals to clean the membranes that filter our drinking water?

“Yeah, but those cleaning solutions never get through the membranes,” he assures us. “They never pass through to the ‘drinking side.’ And there are many procedures to wash the chemicals out to ensure they don’t pose any risk to consumers.”

But those chemical cleaners pose other problems. They break down the filter membranes, which are made of the same sort of polymer used to make nylon and Kevlar. They’re not cheap.

Also, cleaning solutions create huge batches of wastewater that utility workers have to deal with. Gorzalski, who earned his master’s degree at Carolina this year, spent two years figuring out how to create a better, less abrasive cleaning solution. For his effort he earned an Impact Award from the UNC Graduate School.

The reason those filters get so dirty has to do with the geology of southeastern North Carolina, where aquifers—underground layers of rock that hold water—mix with ocean water and pollutants from the surface. For eons, before humans piped water from the aquifers, rainwater would drain through the ground and eventually into the ocean. But now, because we build large developments near the coast and tap those aquifers, the process is reversed. 

“We wind up pulling in water from the ocean, which is problematic,” Gorzalski says. Also, the aquifers are so close to the surface that salt water and metals such as calcium and magnesium mix with clays and organic matter from decaying plants and animals.

New water treatment plants use nanofiltration and reverse-osmosis membranes to purify even the nastiest briny stuff into potable water. Those membranes eventually foul or clog, reducing water production by as much as 80 percent, which wastes energy. Fouled membranes have to be cleaned every one to six months.

Click to read photo caption. Photo by Alex Gorzalski

When the Cape Fear nanofiltration plant was built in 2009, the best cleaning solution came in 50-gallon drums. Those drums, Gorzalski says, were too large to lift to the top of the 1,000-gallon water tanks used to prepare the cleaners. So workers had to pump the cleaners to the top. But the cleaning solution was so corrosive that it ate through parts of the pump. “There were leaks everywhere,” Gorzalski says. The cleaning solution, it turned out, caused a real mess.

By the time Gorzalski completed his research, though, the plant was able to purchase cleaning solution in smaller amounts, making the pumps obsolete. The cleaning solution remained just as toxic and workers still had to handle the stuff in order to clean the 900 membranes inside the plant. And the solution, in the long run, will wear out the membranes.

To address the problem, Gorzalski collected samples from the first and last membranes of a 12-membrane sequence—one of 75 sequences in the plant. Extracting those samples was tedious—it took 10 hours. A portion of the treatment plant had to be shut down, which Gorzalski says threw a wrench in the workers’ week. “They said, ‘We like you, Alex, but if you didn’t come back for a few weeks, that would be great.’”

Back at the lab and with pieces of membrane in hand, Gorzalski first wanted to know if filtering the water before it went through nanofiltration would improve water quality and limit fouling of the membranes. “The answer was no,” he says. “Prefiltering water is great if you’re dealing with things suspended in water, such as sand. But Cape Fear’s nanofiltration plant deals with dissolved foulants such as salt and metals. Prefiltering the water would add a lot of expense without much benefit.”

As a result, the treatment plant workers rethought their idea to prefilter groundwater and focused instead on the membranes—why they get filthy so fast and how best to clean them.

On that front, Gorzalski found that the first membrane in a line of 12 captured different sorts of foulants than did the last membrane. But he says one cleaner worked best on both. That solution is proprietary, Gorzalski says, which means he couldn’t just read the label to find out the ingredients; he had to tease them apart himself.

Click to read photo caption. Wikimedia Commons

Turned out that the common denominator in the best solutions was EDTA—a water-soluble acid that looks like white sugar. It wasn’t clear to utility workers what EDTA actually did to clear foulants from membranes until Gorzalski continued his lab work and dug deeper into the scientific literature.

He found that the membranes were clogging because negatively charged organic matter was binding with positively charged metals. The more they bound together, the worse the membranes worked. Gorzalski found that EDTA latches onto positively charged molecules and dislodges them from the organic matter. The remaining negatively charged molecules then repel each other, leaving the membrane clean.

Over time, though, the harsh proprietary cleaning agent wears out the membranes. With each membrane costing $600 a pop and the treatment plant using hundreds at a time, replacing them gets costly. So Gorzalski prepared his own cleaning agent that contains EDTA and sodium tripolyphoshate—a chemical commonly used in detergents. His research revealed that his agent worked just as well. “With our solution,” Gorzalski says, “we could return water production to 100 percent, and our cleaner didn’t damage the membrane the way other cleaners did.”

The filtration plant is looking into converting to Gorzalski’s cleaner. This, he says, would add life to the membranes, which would save the utility money. And the workers he befriended wouldn’t be mixed up with those sketchy cleaners anymore.

Alex Gorzalski earned his master’s degree from the Department of Environmental Sciences and Engineering in the UNC Gillings School of Global Public Health in May 2013. His education was funded through a Department of Defense SMART Scholarship. His work in the lab of UNC assistant professor Orlando Coronell was funded through a grant from the Water Resources Research Institute of the University of North Carolina System. Gorzalski, who earned an Impact Award from the UNC Graduate School, now works with the Army Corps of Engineers at a water treatment plant that serves more than one million residents in the Washington, DC area.