A partial ban on the use of phosphates in soaps and detergents sold in the District would reduce the city's wastewater treatment costs by about $1.5 million to $2 million annually, a city official said yesterday.

The estimates were presented at a City Council Committee on Public Works hearing on a bill that would limit the sale and use of all cleaning agents containing phosphorus compounds. The proposal would permit small quantities of phosphate in products used in dishwashers and for cleaning equipment used for health care and food.

Phosphate is added to detergents to soften water and ease dirt removal. But environmentalists maintain that the presence of phosphorus in wastewater contributes to pollution of the Chesapeake Bay. Seven states, including Maryland, have adopted some form of phosphate ban. Maryland's limited ban will go into effect in December.

Yesterday, Jacqueline Davison, a representative for the city's Department of Consumer and Regulatory Affairs, told the council panel that the proposed ban on phosphate would reduce one of the major activities of the Blue Plains Wastewater Treatment Plant, removing phosphorus before wastewater is discharged into the Potomac River.

"There is no doubt that this a ban will reduce the operations and maintenance costs related to phosphorus removal and disposal of the related quantity of sludge," said Davison, who estimated the savings would range from $1.5 million to $2 million.

Charles Fox, director of the Chesapeake Bay Project for the Environmental Policy Institute, said phosphorus creates problems by stimulating the growth of algae. He noted that a 1983 U.S. Environmental Protection Agency study concluded that excessive nutrient enrichment led to a 62 percent decline in submerged aquatic vegetation since 1971 and a decline in finned fish and shellfish.

Despite such findings, Theodore Brenner, president of the Soap and Detergent Association, testified that major improvements in water quality would be unlikely.

"There can be no environmental benefit in the District," Brenner said. "There could be some marginal cost savings at the Blue Plains treatment plant, but more probably, any savings could be difficult to identify."

Brenner also said that consumers using phosphate-free products would pay more money for detergents and have difficulty getting their clothes clean. He predicted that some clothes would not last as long.

City Council member Hilda Mason (Statehood-At Large), who introduced the phosphate legislation, said she has used a phosphate-free detergent for years. She said that her clothes come out clean and that they are "outlasting me."

The public works committee will determine whether the legislation should be sent to the full council for a vote.