PORTLAND, Ore. (AP) - Oregon's largest consumer of diesel fuel, TriMet, is turning to biodiesel. Carson Oil Co. will deliver the first shipment of a 5 percent biodiesel blend on Monday for TriMet's 611 buses. "I think it's a huge step forward," said Jeff Rouse, alternative fuels manager for Carson Oil. "This is a pivotal point in TriMet's relationship with alternative fuels." TriMet will now be the state's largest consumer of biodiesel. The transit agency estimates it will use 327,000 gallons of biodiesel a year, more than the state's next three biggest biodiesel users combined. Biodiesel comes from plant oils, used cooking oils and waste animal fats. The B5 fuel will consist of 5 percent biodiesel and 95 percent petroleum diesel. Biodiesel helps reduce harmful emissions and the nation's reliance on petroleum sources. A handful of other transit agencies have been more aggressive in switching to biodiesel. Buses in St. Louis and Cincinnati burn a 20 percent blend, and this year, the Central Ohio Transit Authority began using a 90 percent blend in its buses. TriMet is sticking with the 5 percent blend for now because the agency's engine manufacturers will only warranty their engines for use with this blend, spokeswoman Mary Fetsch said. "The industry is moving toward that. We hope to see an increased level of allowable biodiesel in the next year," she said. Kevin Downing, clean diesel program coordinator for the state Department of Environmental Quality said TriMet's move will make a difference in the air quality in the Portland area. He said a 5 percent biodiesel blend cuts particulate emissions by about 1 percent. The higher the blend, the more it benefits air quality. "The real strength in biodiesel is not so much on the air quality side," he said "It is in renewability, the global warming benefits, and the fact that you're not going to the Middle East, you're going to the Midwest." During the past year, TriMet tested the biodiesel in its fleet of buses that serve people with disabilities and the elderly. Agency officials had concerns about the fuel gelling in cold weather but the tests showed no problems. TriMet's one of several efforts within the Portland area to tackle the environmental effects of diesel trucks. The Portland City Council recently adopted an ordinance that will require gas stations to sell the 5 percent biodiesel blend next year. And last month, the city's Water Bureau vehicles switched to a 99 percent biodiesel blend. The TriMet contract makes Carson the state's largest biodiesel distributor and will help build demand for biodiesel. Rouse said the city's other moves will send encourage motorists to consider biodiesel. "And it's a matter of civic pride," he said. "All of a sudden you realize you're replacing 5 percent of the fuel you use with a renewable resource, and you're not contributing to foreign oil issues. It's a great American story."