ORLANDO, Fla. -- It was supposed to be an answer to global warming but transportation companies say biodiesel fuel poses a serious problem and at least one of those companies says the fix could be costly.

"It's a cost factor, it's a safety factor," said Rusty Dwork, a mechanic with Associated Transportation.

Associated Transportation has a huge fleet of buses but after making the biodiesel switch, mechanics started noticing clogged filters after just a few hundred miles.

"We're finding that in our diesel buses, fuel filters are having a plugging situation," said Dwork.

A comparison of fuel filters, one using biodiesel and one without shows a much dirtier biodiesel filter. The supposedly cleaner fuel leaves it covered with thick black substance.

There is evidence that frequent filter changes actually increase the cost of using biodiesel. Companies that have switched can be stuck changing filters every few days. While they save about $.10/gallon on the fuel, they trade that for about $.30/gallon in filter changes. That is a total increase that turns off some bus riders, who fear the cost being passed along. That's an issue because the Lynx bus system plans to convert its entire fleet to biodiesel within the next year.

"If it's going to (affect my) pocket then no, I don't want it," said Gilbert Bryant, a Lynx rider.

The problem and potential for increased cost are both documented in a Department of Energy study of five biodiesel buses. It shows buses were unexpectedly sidelined by clogged fuel filters five times in a year. A spokesperson for the largest public transportation system in Central Florida said Lynx is aware of the issue and has a plan to beat it.

"If you take the necessary precautions there won't be a problem," said Matt Friedman

Friedman said the biodiesel cleans out old engines and if Lynx increases the biodiesel blend slowly, then overall cost to operate won't increase.

"Phasing it in is the way to go," said Friedman.

If mechanics are wrong, and the problem can't be corrected, it could cost an operation such as Lynx an extra $3-million a year.