Too many times, fishermen cast a line into a river and reel in an old tire -- so Yuefeng Xie came up with an idea to make that old tire purify rather than pollute a river.

Today, about eight years after the idea struck him while driving down a highway, U.S. Patent No. 6,969,469 names Xie, associate professor of environmental engineering at Penn State Harrisburg, as the inventor of Method of Using Waste Tires as a Filter Media. Xie's method involves breaking down old tires to pieces barely bigger than sand and putting them in filters to create crumb rubber filters, Xie said. Compared to sand and anthracite filters, crumb rubber filters are smaller, cost less to build and cost less to operate.

Crumb rubber filters are suitable for treating wastewater and water from storm drains before it is sent back into the environment, Xie said. The filters purify water to about the same extent as sand and anthracite filters, although they lack the proven track record of those established filters, said Bryon Killion, a research assistant for Xie. Crumb rubber filters are also suitable for treating ship-ballast water.

When a ship drops cargo in one port it may take water into its ballast tanks for stabilizing weight on the ocean, Xie said. When the ship reaches its next port and takes on cargo it may dump the water it took on at it previous port because it no longer needs it for stability. This can lead to inadvertently carrying invasive species from one port to another, Xie said.

Crumb rubber filters could treat water before it is taken into ballast tanks so that foreign fish eggs, for example, could not stowaway in one port, be released in the next, hatch and wreak havoc on the native ecosystem.

Turning an old tire from something that turns underneath a car to something that turns toilet water environmentally friendly requires either chopping the tire to pieces, freezing it in liquid nitrogen and smashing it to pieces or grinding it to pieces in or out of water, Xie said.

When Xie approached Howard Sachs, who was the associate dean for research and graduate studies at Penn State Harrisburg, with his "crummy" idea years ago, Sachs was skeptical, thinking "yeah, so what," Sachs said. Sachs' skepticism soon subsided.

"It's surprisingly a very effective kind of filter," he said, adding that it "has got kind of a 'gee-whiz' factor to it," he added.

Crumb rubber particles naturally attract and hold on to impurities in water. "Some of that is pure physical chemistry," Sachs said.

In fact, a crumb rubber filter, treating the same water as a comparably sized sand or anthracite filter, will clog less quickly. This means that smaller crumb rubber filters can be used to treat the same amount of water as larger sand or anthracite filters, Killian said.