The Martha's Vineyard Commission on Thursday closed a public hearing for a planned 35,000 square-foot YMCA building, sending the project into the home stretch with few visible potholes in the road.

As is the custom following the close of a public hearing, the commission's land use planning committee met last night to discuss the project and possibly begin drafting a list of conditions for possible approval.

The final public hearing before the full commission was again marked by a tangible sense of goodwill and collaboration. Several commissioners praised backers of the YMCA for their prompt and efficient response to questions and concerns.

"Your collaboration has been excellent," said commission member Katherine Newman of Aquinnah. "It's been great to see this project evolve."

YMCA backers want to build a 35,000-square-foot, green-designed facility on an undeveloped five-acre property leased from the high school behind the skate park in Oak Bluffs. The commission is reviewing the project as a development of regional impact (DRI). Building plans call for a full gymnasium, child care rooms, meeting facilities, an Olympic-sized swimming pool, hot tubs, saunas and steam room.

Proponents for the project on Thursday said they had worked out a solution with the Natural Heritage and Endangered Species Program to preserve the habitat of several endangered and threatened species - including several breeds of rare moths.

Judy Crawford, vice president of the YMCA board of directors, said state environmental officials had given tentative approval to a revised plan to preserve a moth-friendly swath of contiguous pitch pines and scrub oaks. To accommodate the moths, project planners moved a 16-slot parking lot to the northwest of the building and removed an outside basketball court.

Project proponents also shifted a large portion of their future expansion space to a different area.

"We created a flyway for the moths - they seem to like uninterrupted stretches of trees," Mrs. Crawford said.

She also updated the commission on the plan for the YMCA to join the high school in a wastewater disposal solution. Last week the Oak Bluffs wastewater commission unanimously approved separate plans for the high school and the YMCA to tie into the town treatment plant.

Wastewater superintendent Joseph Alosso said the high school plans to install a pump station on school grounds to send wastewater to the Oak Bluffs treatment facility. Another pump station will be installed at the treatment center capable of pumping a maximum of 20,000 gallons of treated wastewater a day back to the high school leaching field, located at the rear of the school property near the track.

Mrs. Crawford said the high school land use subcommittee approved the plan last month as an alternative to building a package treatment plant on high school grounds.

Mr. Alosso said a maximum of 5,000 gallons of untreated wastewater will be pumped from the high school and the YMCA to the treatment facility. A provision in the agreement between the wastewater commission and the high school will then allow the town to pump as much as 20,000 gallons of treated wastewater back to the leaching field behind the school.

The wastewater commission has also agreed to take on as much as 1,200 gallons of untreated wastewater a day from the YMCA.

In April, Oak Bluffs voters defeated an article at their annual town meeting to build a package treatment plant near the high school. The YMCA projects lies in the watershed for both the Lagoon and Sengekontacket Ponds, and because of its size the project was well above the commission's minimum threshold for releasing nitrogen into the watershed.

On Thursday commissioner John Breckenridge of Oak Bluffs questioned whether sending untreated wastewater away only to take back a larger quantity of treated wastewater was in the best interest of the watershed.

Mr. Alosso said the new plan to pump untreated wastewater to the plant and then pump treated wastewater back to the school will reduce the overall amount of nitrogen being discharged into the watershed, although at a lower rate than from a package treatment plant.

The one point of contention on Thursday centered on the commission's continued insistence that the YMCA consider the environmentally-friendly option of geothermal energy. Geothermal, or ground-source heat pumps use the natural heat storage capacity of the earth or ground water to provide energy efficient heating and cooling.

YMCA backers have maintained that the cost of such a system is unfeasible.

Kenneth MacLean, lead architect for the project, cited a study on Thursday by RFS Engineering indicating that installing the geothermal heat pump system would represent an annual energy cost savings of approximately $5,500 per year.

At that level of savings, it would take 45 years for the YMCA to recoup the cost of installing the system, he said.

"We were thinking the payback could take place in maybe four to six years, but now we're talking 45 years. It doesn't seem feasible," Mr. MacLean said.

Commissioners pressed YMCA proponents to try one last time to make a geothermal system work. Commissioner Peter Cabana of Tisbury suggested the YMCA could recoup its costs quicker if it increased the costs of fuel used to calculate the payback - $2.20 a gallon - to reflect rapidly increasing prices.

Mr. Cabana also noted that backers did not factor in potential savings from using geothermal energy to heat the building's pool and showers.

But Mr. MacLean politely disagreed.

"I am not personally confident that increasing the fuel costs and heating the water will get us where we need to be . . . we have to reduce the timetable for payback by a factor of nine," he said.

Commissioner Linda Sibley of West Tisbury urged project officials go back one more time and try to make the geothermal option work. While she said she understands the added costs, she said the scope and brand name of the YMCA put it in the ideal position to set a good example for the rest of the Island.

"This is an opportunity for [the YMCA] to step up and serve as a role model," she said.

When YMCA executive director John Clese asked if the commission could force project backers to use geothermal, commission chairman Douglas Sederholm of Chilmark replied: "Can we? Yes. But it would be a first."

In the end both sides agreed to compromise. Commissioners said they would submit any additional questions or concerns they had about geothermal energy to Mr. MacLean by this week. He said he would answer the questions promptly, and if need be would meet with the commission to discuss the geothermal option further.