House-Senate conferees reached agreement yesterday on a compromise clean-water bill that would relax existing industrial anti-pollution standards and exempt federal dams and other major federal water projects from strict environmental controls.

The amendments to the 1972 water act were quickly hailed by industry as "a reasonable compromise" between environmental and economic considerations. But environmental groups attacked the conferees' work as "serious step backward" in cleaning up the nation's waterways.

The House and Senate have debated more than two years over the controversial bill, which affects the spending of billions of dollars in public funds for sewage treatment for industrial cleanup equipment.

The conference room crowded with lobbyists reflected the range of interested parties - environmentalists, cities and states, steel, oil and chemical companies, paper mills and timber interests, farmers and miners.

"There comes a time when you have to compromise or you don't bring out a bill," said Den Jennings Randolph, chairman of the Senate Environment and Public Works Committee. "But I think on balance it's a fair compromise. The basic principles of the 1972 act have been retained."

Under the new amendments, industry could be exempted from installing expensive new cleanup equipment characterized in the 1972 law as "the best available technology" if it shows that the cost of the equipment exceeds the benefits.

However, this broad exemption applies only to so-called "conventional" pollution, measured by such factors as suspended solids in the water, fecal bacteria and oxygen-demand of the discharge. The deadline for complying with conventional pollution standards would be moved from 1983 to 1984.

A second category of pollution, caused by toxic chemicals often invisible to the human eye and hardly dealt with in the 1972 act, would be more strictly controlled. For 129 chemicals listed to install the best available technology by 1984, with no exemptions.

For other potentially toxic chemicals - scientists estimate there may be hundreds more - industry would have until 1987 to install cleanup equipment, but would be permitted waivers under certain conditions.

Industry had been complaining bitterly ever since 1972 about the cost of meeting the 1983 deadline for the best available technology. More than 85 per cent of industrial plants - with notable exceptions such as the steel industry - have met the 1977 intermediate deadline for installing a lesser amount of cleanup equipment.

John E. Daniel of the American Paper Institute, which represents 700 pulp and paper mills - among the nation's most polluting factories - said the exemption for conventional pollutants would result in "considerable reduction in cost" for his industry. He estimated the present law would require $4.3 billion worth of equipment.

However, the relaxation of industrial standards was "a disappointment," according to Blake Early of Environmental Action, a Washington-based lobby group. The 1987 deadline for cleaning up potential toxics means "we're going to be dumping pollutants without knowing the impact," he said.

These potential toxic chemicals, not included in the bill's list of 129, include several substances suspected of causing cancer, nervous disorders and birth defects. For instance, pesticides such as lindane and mirex and heavy metals such as cadmium, which are now discharged in rivers, would not be regulated until 1987, according to a list supplied by the House Public Works and Transportation Committee.

Committee staffers said the Environmental Protection Agency does not have enough scientists, engineers and lawyers to test, set standards and issue permits for more than the first batch of 129 chemicals by 1984.

On another critical issue, environmentalists deplored the blanket exemption of any federally authorized project from the requirement to get a swampy areas near rivers which control floods, spawn fish and naturally permit for dredging wetlands, control pollution.

Private, state or local construction activities, from apartment buildings to highways, must now obtain a permit from the U.S. Army Corps of Engineers and the Environmental Protection Agency before dredging near streams or rivers. With a federal exemption, Corps of Engineers' dams and navigation projections, Bureau of Reclamation irrigation systems and Soil Conservation Service stream-channeling activities will not be required to get permits.

These projects are among the most ecologically disastrous in the nation, environmentalists assert, destroying wildfire, fisheries, and dredging up toxic chemicals from river bottoms.