Sometime this summer, a United Airlines flight will take off from Los Angeles International Airport bound for San Francisco using fuel generated from farm waste and oils derived from animal fats.

For passengers, little will be different -the engines will still roar, the seats in economy will still be cramped -but for the airlines and the biofuels industry , the flight will represent a long-awaited milestone: the first time a domestic airline oper ates regular passenger flights using an alternative jet fuel.

For years, biofuels have been seen as an important part of the solution to reducing greenhouse gas emissions. And airlines, with their concentration around airports and use of the same kind of fuel, have been seen as a prom ising customer in a biofuels industry that has struggled to gain traction. Now that relationship is showing signs of taking off.

United plans to announce a $30 million investment in one of the largest producers of aviation biofuels, Fulcrum BioEnergy , the biggest investment so far by a domestic airline in the small but growing field of alternative fuels.(Cathay Pacific, based in Hong Kong, last year announced a smaller investment in Fulcrum.) The quantities that United is plan ning to buy from Ful crum constitute a small drop in its vo luminous fuel con sumption. Last year, United’s fleet con sumed 3.9 billion gal lons of fuel, at a cost of $11.6 billion.

But airlines are increasingly under pressure to reduce carbon emissions.

The Obama adminis tration proposed this month that new limits on aviation emissions be developed, and the International Civil Aviation Organization, a United Nations agency , is expected to complete its own negotiations on limiting carbon pollution by February 2016.

For the first two weeks, four to five flights a day will carry a fuel mixture that is 30% biofuel and 70%traditional jet fuel; after that, the fuel will be blended into the overall supply , United said.