The Bloomberg administration’s wide-ranging trash plan could face a roadblock in the City Council, as lawmakers are expressing doubt that key aspects of the proposal are feasible.

Mayor Bloomberg’s 20-year plan, first offered in 2004, centers on the idea that the city’s trash management should be distributed across the five boroughs. But a number of legal and legislative obstacles to the three trash facilities proposed for Manhattan threaten approval of the entire project.

The three Manhattan sites were examined at a hearing yesterday before the council’s Sanitation Committee, whose members said they were reluctant to back a plan that seemed clouded by uncertainty.

“I think this is a make-believe plan,” a Democrat of the Upper East Side, Council Member Jessica Lappin, told city officials testifying before the committee.

The bulk of Manhattan’s waste now travels largely by truck to facilities in the other boroughs and New Jersey. The city’s plan calls for the creation of three stations on the island where trash would be transferred to rail or barges and shipped out to landfills. Officials say the proposal could achieve the double objective of making Manhattan manage its own trash as well as alleviating traffic by getting hefty garbage trucks off the road. Two of the sites the city is eyeing for Manhattan are located near Hudson River Park, an area protected both by a 1998 state law governing land usage and by the watchful eye of several community advocacy groups.

Under the plan, the city would convert a marine transfer station at 59th Street from a paper recycling facility into one capable of exporting commercial waste. To handle the paper, the city would build a recycling and environmental center on Pier 52 at Gansevoort Street.

Both possibilities face significant hurdles. Constructing a sanitation facility at Gansevoort Street would require an amendment to the 1998 law, a potentially lengthy and combative process. Converting the 59th Street site to a commercial waste station may necessitate expanding the current facility, which community groups strongly oppose on the grounds that it would violate provisions of the state law.

The other proposed facility in Manhattan, at 91st Street on the East River, has been challenged in a lawsuit filed by community groups in October.

Zoning changes for four of the facilities across the city were approved last June after the mayor vetoed the council’s rejection of his proposal.

The council has yet to sign off on the city’s overall proposal – known as the Solid Waste Management Plan – and it appears unlikely to do so anytime soon.

A citywide trash management program became necessary with the closing in 2001 of the Fresh Kills landfill in Staten Island, which had handled all of the city’s solid waste. The city’s current system has long been considered a temporary solution. City officials yesterday acknowledged that issues remain to be worked out among the Manhattan sites, but they urged the council to approve the trash plan anyway so they could move forward with facilities in the other boroughs that face little opposition.

That request garnered little support from the committee chairman, Council Member Michael McMahon of Staten Island, who called the sites near Hudson River Park the “linchpin” of the entire project. Mr. McMahon took issue particularly with the proposed Gansevoort Street facility, which he said “seems to be covered in a blanket of impossibility.”

In testifying before the committee, the city’s sanitation commissioner, John Doherty, defended the proposed Manhattan sites and expressed confidence that community groups would see the benefits of the plan. “Is there uncertainty? There’s always uncertainty,” Mr. Doherty told the committee.

After his testimony, Mr. Doherty denied that the plan was in jeopardy, saying the city stands on firm legal ground. “I think the plan is good,” he said. “It’s strong, and it’s very viable.”

Some community activists begged to differ. “I don’t they’re being realistic,” the president of Friends of Hudson River Park, Albert Butzel, said.