hen Jim Farrell drives his pickup truck past the Red Bicycle, he says he has one thought: “All right, eat more French fries.”

Farrell’s true interest lies not with the gastronomic habits of the bistro’s patrons, but with fuel for his truck, which runs on biodiesel made from the Bike’s fryer grease.

Once the restaurant has cooked a few days’ worth of fries, fish and chips and mozzarella sticks, chef Jack Chambers sets aside the used grease — some 120 pounds each week —  for islander Terry Roth. Roth picks it up and takes it to his home on Maury Island, where he begins the alchemy of turning grease into fuel any diesel-powered vehicle can use.

A former college chemistry professor, Roth has been turning waste into renewable, clean-burning fuel for seven years, initially for financial reasons, when energy prices skyrocketed.

“It was an economy measure and trying to get out from under those guys in the Middle East,” he said one day last week, standing outside the small, open-air structure he calls “the refinery,” just steps from the home he shares with his wife. But biodiesel’s earth-friendly benefits figure in as well.

Biodiesel is lead-free and contains no sulfur or chemical aromatics, which can lead to illness. It also emits far less carbon dioxide than petrodiesel, the diesel fuel commonly available at gas stations.

“I feel like I am doing my part for the environment,” Roth said.

Another bonus, Roth added, is that biodiesel is better for vehicles than its petroleum-derived counterpart, as it cleans and lubricates engines better and increases their lifespan.

Given the fuel’s benefits, home brewing biodiesel has been gaining in popularity in recent years, and Roth is not the only one to do so on Vashon. Zombiez gives its used grease to another islander — who would prefer to fly under the biodiesel radar. And Scott Durkee, who used to make his own biodiesel, once drove a bus to New Orleans after Hurricane Katrina on straight vegetable oil. Durkee has become so widely known as “the biodiesel guy” that he fielded a call from a woman who had fried her turkey at Thanksgiving and wondered if he would like to claim the oil.

Roth is quick to note that other people could do what he does and that college professor credentials are not required.

“The chemistry is simple; the outfit is simple,” he said.

Still, it is a process that requires time and careful attention to safety, and not all the owners of diesel cars could set up their own backyard operations like Roth’s because of the limited amount of oil available.

“There is only a finite of amount of fryers on the island,” he said.

At his homespun refinery, where a faint smell of French fries wafts through the air, Roth recently explained to a guest how the fuel is made. Simplified, the process starts with filtering, when Roth pours jugs of the used oil into a large settling tank, where it might sit for days or weeks. Settling completed, the oil is pumped into an old water heater — or processor —  where it is heated  to 130 degrees for two hours, and then Roth adds lye and methanol. After it sits for 24 hours, the liquid is pumped into a washing tank, where a mist of water removes any methanol or glycerol — a byproduct of the process —  and finally the oil is pumped to a drying tank, where excess water is removed. From there, it can be pumped into a vehicle.

From start to finish, Roth says, the process, using 35 gallons of vegetable oil, takes about four hours of his time and leaves him with 33 gallons of fuel. With that amount, he said, he fills both of his diesel cars and sells the leftovers to Farrell, who recently purchased the truck from him.

At $1.50 a gallon, his homemade fuel has saved him thousands of dollars, he calculates, and these days he only fills up at a gas station when taking a trip.

“When we travel, we have to pay for fuel,” he said. “It’s a shock to pay $4.50 a gallon.”

At the Red Bike, Chambers said he is happy to have the restaurant involved in this waste-to-fuel cycle, and noted that the business used to send its grease off-island. A company recently approached the Bike about purchasing its old oil again, but instead it will continue supplying it to Roth for a small fee, which Roth said he is glad about, or he might have found himself back at the pumps.

“If I can no longer make fuel, then I am out of business,” he said.

Jim Hassel, Zombiez’ owner, says he, too, is pleased to have the restaurant’s grease go on to serve a useful purpose.

A longtime chef, Hassel said he has been around the grease for 25  years — and understands its value.

“Although it’s not good for cooking anymore, it is good for a whole lot of other things,” he said.

In fact, he has designs on the oil himself, noting that it is possible to run a vehicle off of waste vegetable oil without converting the oil to biodiesel.

“It’s fantastic,” he said. “One day we would like to have our own diesel van or truck that runs off the stuff. … It’s rough seeing it disappear every week and then go buy 50 gallons of fuel to replace it.”

When large companies buy waste oil from restaurants, Roth said, they sometimes make biodiesel, but it is also shipped overseas for use in animal feed or used to make soap or degreaser. In fact, the used oil is in such high demand that theft of fryer grease has grown to be an increasing problem. Last week, an article in The Seattle Times noted that truckloads of the oil can be worth thousands of dollars and that competition for it has become cutthroat.

On Vashon, Roth said he considers these large companies a threat, and nearly all of the people in the biodiesel cycle on the island expressed interest in keeping the resource here.

Farrell, who is aware of the many benefits of biodiesel, says he is more likely to drive his new truck on Roth’s biodiesel than his car, which cannot use the fuel.

“It is less expensive to run and less environmentally damaging,” he said of his truck, tipping his hat to the Red Bike for making his excursions possible.

“That is why I am able to drive around in a full-size truck, because people are eating French fries” he said.