President Reagan's controversial effort to clean up the nation's scandal-ridden sewer programs and reduce the federal government's involvement in them got its first test in a Senate committee yesterday, with criticism loudest from Sun Belt legislators who are also mostly Reaganites.

The Environment and Public Works Committee agreed to reduce the ceiling on federal funding for municipal sewer plants from the current 75 percent to 65 percent next year through 1984, and to 55 percent thereafter. It is part of Reagan's overall effort to make the states shoulder more of the burden.

At least $2.4 billion is at stake in the committee talks, the amount that Reagan promised to provide for new sewer construction next year if his proposed reforms are passed. If they are not, he has said, there will be no money at all.

Any abrupt halt in the nation's largest public works program would leave dozens of towns and cities in at least 40 states with torn-up streets, open ditches and half-built sewage treatment plants, said Sen. John H. Chafee (R-R.I.), in presenting the package. Members of Congress would hate to try to defend that in next year's elections.

Part of Reagan's reforms are in Chafee's bill, which authorizes $9.6 billion over four years. The plan would focus more federal money in urban areas while spending $23 billion less overall over the next decade.

That is less than one-third of the $90 billion a recent Environmental Protection Agency study said the government would have to spend to meet current needs, although there is debate over that figure.

Sun Belt legislators, including Sen. Pete V. Domenici (R-N.Mex.), oppose aspects requiring growing towns to pay more of the costs of new sewers. Urbanites oppose the end of funding for sewer repairs and for storm flood tanks. Wrangling could delay the measure past any chance of being included in the budget reconciliation package due out by the end of the month.

The measure would then have a dubious future in a possible supplemental appropriation bill later on, where Reagan budget cutters are reported to be seeking another $10 billion in cuts. The actors in the debate are feeling the pressure.

Debate, which continues next week, involves these areas:

Provision for growing communities. The bill would stop aid for reserve sewage treatment plant capacity for future suburb expansion, and for "collector" networks that extend the reach of large central treatment plants to suburban and rural areas.

In the past, many towns built systems beyond their current needs to take advantage of the 75 percent federal funding, and then were unable to pay operating costs.

Provision for storm runoff in the cities. The administration wants to eliminate sewage system overflow catchment tanks from the list of programs eligible for federal aid. Cities argue that without the "combined sewer overflow" provision, raw sewage will continue to flow into nearby waterways after major storms.

Repair work. The administration wants to eliminate funding for repairs to leaky systems on grounds only 5 to 10 percent of previous years' funds have gone for this and it can easily be shouldered by the states.

The Clean Water Action Project argues that at least 40 percent of the sewage going to treatment plants in older cities is really clean groundwater that has seeped into the system. Repairing those pipes would save millions of dollars, said the group's Larry Silverman, since "Leaks just get bigger."

Innovative technology support. Nearly 20 percent of fiscal 1981's $2.3 billion went to set up new approaches to sewage treatment, including sludge composting centers and crop applications. The committee agreed to provide up to 85 percent of the funding for such projects.

Industrial cost exclusion. The House last year voted to end the requirement that industries pay local sewer systems for treating their processing wastes, and although it is nowhere in the current bills, everyone expects an effort to revive the idea.