William D. Ruckelshaus returned yesterday to the embattled Environmental Protection Agency, receiving an emotional hero's welcome from more than 1,000 agency employes who applauded him noisily, laughed at his jokes and hoisted a banner reading: "How do you spell relief? RUCKELSHAUS."

Ruckelshaus, the EPA's first administrator who was nominated Monday by President Reagan to return to his old post, pledged to restore public trust in the troubled agency, to make it "adhere to an iron integrity," to respect civil servants and to "administer and enforce the laws as they're written by Congress."

Those simple promises drew thunderous ovations from the employes, who crowded into Waterside Mall, where the agency is headquartered, to hear Ruckelshaus' first major address since his nomination. He did not mention his predecessor, Anne M. Burford, and made no specific commitments on policy or personnel shifts, but he appeared to be sending a message that the atmosphere of the demoralized agency will change dramatically.

The mood was reminiscent, many employes said, of the early 1970s, when the agency was founded amid a national clamor for environmental protection. Many of the EPA workers said they came to the agency as young college graduates with a sense of mission and they viewed Ruckelshaus as their comrade.

"The agency and the world were different then," said Hugh Kaufman, a self-styled "whistle blower" in the EPA toxic waste cleanup program. "In the last two years, we have been very much oppressed. We had poor management, outrageous management, potential criminal mismanagement, oppression of civil servants, a feeling that we as EPA regulators were being spat on by the White House. Seeing Bill Ruckelshaus come back is like opening the gates and tearing down the bars."

Ruckelshaus waded into the crowd after his speech, as employes mobbed him to shake his hand or clap him on the shoulders. Several called out their names and the year they joined the agency as the tall and beaming Ruckelshaus moved among them.

"Welcome back, boss," said a smiling Fred Talcott, an economic analyst who came to the EPA in 1973. A white-haired woman pushed to the front, grabbed Ruckelshaus' hand and said through tears: "I'm so happy you're back, I missed you."

Another woman put one hand on each of his shoulders and said with feeling: "The toxic waste program needs you badly." Ruckelshaus answered confidently: "Great, we're going to straighten her out."

Ruckelshaus told the employes he is convinced that Reagan "is committed to doing the job we have been assigned by Congress and to giving us adequate resources to do it," one of the few statements in the speech that was not followed by cheers. He also portrayed himself as a supporter of Reagan's positions.

Ruckelshaus indicated that he will support the administration's effort to revise the Clean Air Act and other landmark environmental laws, despite strong resistance from Congress in the last two years. White House-sponsored amendments to relax portions of the Clean Air Act have been defeated, with members of Congress denouncing them as an effort to favor industry over the public.

"We need to examine the means to achieving these goals of environmental protection and where we find or where we believe that better means can be used, we should ask the Congress for the authority necessary to adopt those means," he said. "In the meantime, we will administer and enforce the laws as they are written by Congress."

Interior Secretary James G. Watt said yesterday that he had talked with Ruckelshaus and was "greatly impressed with his commitment to the Reagan philosophy."

Ruckelshaus acknowledged that he had worked on behalf of many industries regulated by the EPA since leaving the Nixon administration in 1973, and said he expects close scrutiny for those ties during his Senate confirmation hearings.

As a Washington attorney he represented manufacturers of vinyl chloride and aluminum and other products. Since 1975 he has been senior vice president of the huge forest products concern, Weyerhaeuser Co. of Tacoma, Wash., named one of the nation's "Filthy Five" companies by Environmental Action, an environmentalist lobbying group.

Weyerhaeuser has contested EPA efforts to curb spraying of forests with herbicides containing toxic chemicals, including dioxin.

"My job as a lawyer was to represent my clients. My job at Weyerhaeuser was to represent all the stakeholders in that enterprise," Ruckelshaus said. "My job as EPA administrator is the same today as it was when I held that job before, and that is to represent the public interest to the best of my ability." That comment also drew loud applause.

Ruckelshaus also hinted at plans to bring in a new team of top EPA managers, but gave no specifics other than saying that Reagan "is committed to getting the best people we can find in this agency, the best people with iron integrity." Employes cheered those words as several top EPA political appointees, targets of congressional and Justice Department probes, watched quietly. They included acting EPA Administrator John W. Hernandez, Assistant Administrator John Todhunter and general counsel Robert M. Perry.

Todhunter and Hernandez came under new fire yesterday as a draft report by a House subcommittee accused them of allowing the formaldehyde industry to influence an agency decision not to regulate the suspected cancer-causing substance. Both men have denied showing favoritism toward industry.

White House officials yesterday told presidential aides to report to White House counsel Fred F. Fielding all contacts with EPA officials in the last two years, regardless of how insignificant they may seem.

The instructions came after reports that James Medas, special assistant in the White House office of intergovernmental affairs, had not reported a discussion with ousted EPA official Rita M. Lavelle on the political impact of toxic-waste cleanup decisions.