Ordinary air bubbles, which occur naturally in water, can be a source of concern in drinking water treatment. Paolo Scardina, a post-doctoral associate in the Via Department of Civil and Environmental Engineering (CEE), has learned why. He began his research as an undergraduate and has now earned a highly competitive $150,000 grant from the American Water Works Association Research Foundation (AWWARF).

Scardina’s research is already being used by engineers with the California Department of Health Services to identify problems at two facilities that have experienced eruptions of air bubbles.

Why are air bubbles a problem? “When you open a can of soda, bubbles form and rise to the surface,” explains Marc Edwards of the CEE faculty. “The same thing can happen in water from lakes and rivers. When air bubbles are released in a ‘burp’ during treatment, pathogens and other particles can escape removal.”

The last treatment barrier in most drinking water treatment plants is filtration, Edwards says, and a burp of bubbles can punch holes in filters—tiny holes, but large enough to let particles and pathogens escape into the water that goes out to customers.

“The field of drinking water treatment is about 3,000 years old,” Edwards notes, “but in all that time, air bubbles have not been studied in terms of their ability to affect treatment processes.”

Scardina, who began studying air bubbles at Edwards’ suggestion during his senior year at Virginia Tech, identified the causes of bubble formation while he was working on his master’s degree. “Before Paolo’s findings, we knew that bubbles could cause problems,” Edwards says, “but we didn’t know how they formed or the range of the impacts.”

In searching the literature about water treatment, Scardina noticed that serious problems often occur at plants that experienced air bubble eruptions. In addition to studying why bubbles form and how they punch holes in treatment plant filters, Scardina has made some discoveries. Air bubbles can interfere with the first drinking water treatment process (settling) where solid particles from incoming surface water drop to the bottom of treatment tanks. “If bubbles are present,” Scardina notes, “pathogens and other particles can attach to them and float through the treatment plant.”

Another important discovery by the Virginia Tech student is that bubbles can cause a dilemma for treatment plant operators at the end of the process. “When bubbles form after filtration, water quality tests may wrongly identify the bubbles as dirt particles or pathogens, even though the bubbles themselves are harmless,” Edwards says. “This decreases the validity of and confidence in water quality tests.”

Scardina has published papers on his findings and made a presentation at an international water treatment conference. In September, the Mendocino District Office of the California Department of Health Services flew Scardina to the West Coast to help engineers there identify the source of air bubble eruptions at two water treatment plants. “Paolo’s research has given us a good understanding of the sources of our problems,” notes Guy Schott, associate sanitary engineer for the Mendocino District. “He’s the only person I’ve found in the United States who does work in the field of dissolved gases and their impact on treatment.”

In the plants Schott and Scardina have investigated, air bubble eruptions have carried solid particles into the filtration process, which leaves the treatment systems open to pathogen contamination. Schott is concerned about the potential for outbreaks of viruses, as well as Giardia and Cryptosporidium, microscopic parasites that can cause severe gastro-intestinal illness. These parasites are impervious to chlorine disinfection, so they must be removed through settling or filtration. A major water-borne outbreak of Cryptosporidium in Milwaukee in 1993 caused illness in an estimated 403,000 people.

Scardina will use the AWWARF grant to “get a handle on the magnitude of the problem,” he says. Working with treatment plant engineers, he is investigating air bubbles in Boulder and Denver, Colorado; Bay City, Michigan; San Diego, California; Myrtle Beach, South Carolina; and Buffalo Pound in Canada. “I’m studying parameters, such as treatment plant design and operations, that can affect formation of bubbles.”

For more information, contact Paolo Scardina at paolo@vt.edu or Dr. Edwards at edwardsm@vt.edu

Scardina received his Ph.D. from Virginia Tech in 2004. His Ph.D. dissertation can be viwed online.

— Written by Elizabeth Crumbley