A House-Senate conference committee approved $2.6 billion for sewer construction yesterday, hoping to shoehorn it into the 1982 budget process in time to keep federal money flowing to the states.

The agreement, expected to get supersonic final approval in both houses, gives President Reagan most of the program reforms he said he had to have to keep the grants going.

The question is whether he will decide to quibble over the things he didn't get in order to avoid spending the $2.6 billion this year.

An Environmental Protection Agency spokesman said the administration would wait to see the actual language of the measure on Monday before deciding its position.

If no funding is forthcoming, 30 states will have to halt ongoing projects early next year.

The U.S. Conference of Mayors wrote the president immediately urging him to approve the measure and fund the program.

The administration sought, and got, elimination of its responsibility to fund reserve sewage-treatment capacity in areas expecting population growth.

But the rules do not change until Oct. 1, 1984, the beginning of fiscal 1985, and all sides expect a mad scramble in booming Sun Belt areas to launch projects before then.

After Oct. 1, 1984, new projects would be able to bill the federal government only for their needs to 1990 or earlier.

Now states may get help for projects aimed 20 years ahead, and this provision has led many communities to build plants larger than they could realistically run.

Beginning in fiscal 1985, federal money would only be available for building treatment plants and the main lines in a sewer system, and for repairing leaky pipes.

It would also pay only 55 percent of a project instead of the current 75 percent.

However, states could spend up to 20 percent of their federal allotment for projects the new measure makes ineligible for federal aid: "collector" networks that extend the reach of large plants, so-called "combined sewage overflow" projects that keep raw sewage out of waterways after rain floods the system, or large-scale rehabilitation projects.

EPA could agree to fund sewage overflow projects that a state designates as a major priority if the governor asks it.

The measure authorizes $2.4 billion for each of the next four years, plus $200 million for programs protecting bays and estuaries.

It retains the existing fund-distribution formula until fiscal 1983, and no state would get less than 90 percent of its current allocation after that.

At least 4 percent of the grant must be spent on innovative treatment projects, including field testing and overflow controls.

Larry Silverman of the Clean Water Action Project, an environmentalist group, called the measure "a good bill" and said the only question now is funding.

Senate Budget Committee Chairman Pete Domenici (R-N.M.) said he would try to keep his promise to fund the program as soon as it passed, but noted that he now has no eligible appropriation vehicle to which he can attach it.

This could mean delay until supplemental appropriations bills begin in January, he said.