Federal authorities said that they are seeking more indictments in the wake of Baltimore City Council President Walter S. Orlinsky's guilty plea in an alleged kickback scheme involving a lucrative, city sludge-hauling contract here.

"We should be at the indictment stage by the end of the calendar year," said Dana Caro, special agent in charge of the FBI office in Baltimore.

Neither Caro nor federal prosecutors would say whether the indictments are being sought against city officials or employes of the sludge-hauling firm in the case.

Last Tuesday Orlinsky, City Council president for 11 years and a veteran of Baltimore politics, pleaded guilty to accepting a $2,532 bribe in exchange for using his influence to help Modern-Earthline Companies, a Philadelphia-based firm, obtain a five-year contract valued at up to $47 million to remove sludge from a Baltimore sewage-treatment plant to Garrett County in western Maryland.

The plea was to a single count of extortion in a 12-count indictment in which Orlinsky had been charged with accepting more than $10,000 in kickbacks from Modern-Earthline lobbyist Edward J. Russell.

The other counts were dismissed as part of Orlinsky's plea agreement with the government.

Modern-Earthline obtained a two-stage contract in November 1980, but the contract was canceled in its first, demonstration, stage when Garrett County officials objected.

The federal prosecutors said Russell told them that if the second, long-term, stage had gone into effect, Modern-Earthline Companies planned to pay Orlinsky and unnamed "others" 2 percent of the $47 million contract. This would have amounted to $940,000.

Investigators would not say what portion of the $940,000 Orlinsky was to have received.

The government would not name the other suspects in the scheme.

A statement of facts read by Assistant U.S. Attorney Richard E. Dunne III at Orlinsky's plea hearing indicated that investigators have questioned numerous Baltimore City officials, as well as employes of Modern-Earthline ranging from office secretaries to the firm's former chief executive, Douglas T. Murray.

Two former aides to ex-governor Marvin Mandel, Maurice R. Wyatt, Mandel's patronage chief, and Frank A. DeFilippo, his press secretary, had dealings with Modern-Earthline.

Wyatt was Modern-Earthline's resident agent in Maryland from 1979 to April 1981. In June 1980, Wyatt was convicted of bribery in connection with an attempt to influence state issuance of sewer permits for three Baltimore-area developers in 1974.

DeFilippo was retained by Modern-Earthline to do public relations work and said he was instrumental in introducing Orlinsky to company officials, although he said he knew nothing of any kickback plan.

Russell, the firm's lobbyist, began cooperating with the FBI during its investigation in 1981 and recorded several conversations with Orlinsky, using a tape recorder concealed on his body.

Russell has been described as not only a lobbyist, but a sometime foreign car salesman, cock-fighting enthusiast and promoter of Caribbean business ventures.