The stringent and sometimes criticized sewage treatment controls adopted nine years ago to protect Northern Virginia's Occoquan Reservoir should be maintained, a state-hired consultant has concluded.

The controls "are, with a few exceptions, necessary to protect public health and water quality," according to a report from the Boston consulting firm of Camp Dresser & McKee.

Under one of the few changes proposed, nitrogen would be removed from effluent on a seasonal, rather than year-round, basis. The net result would be no overall increase in nitrogen concentrations, but a savings of $500,000 a year in operating and maintenance costs.

The report was presented this week to the seven-member State Water Control Board, which accepted the recommendations with no objections. Before taking final action, the board will hold a public hearing. No date has been set for the hearing.

The report is a victory for those who have opposed major changes in the Occoquan Policy. The water board adopted the policy in 1971 when pollution from outmoded and overtaxed sewage plants threatened to turn the reservoir into an unusable sinkhole. Water in the reservoir, which serves 650,000 Northern Virginians, was choked with algae caused by a heavy concentration of nutrients (primarily phosphorus and nitrogen) in sewage.

To head off a drinking-water disaster, an $82 million advanced treatment sewage plant was built. Prince William County, which uses the plant, has questioned the necessity of the elaborate treatment measures, which were the focus of the Camp Dresser & McKee study.

Under pressure from Prince William, which has been searching for ways to cut its high sewage costs, the water control board last year ordered a reevaluation of the Occoquan policy and hired Camp Dresser to do the job.

In its report, Camp Dresser defended "for the present" the duplicate and standby systems built to ensure that no mechanical failures or excessive volume of sewage would knock out the highly sophisticated plant.

The report said the duplicate systems could be reduced -- thereby increasing the plant's treatment capacity from 10.9 million gallons daily to 15 million gallons -- when excessive volume of storm- and groundwater flowing into the system is corrected.

Plant officials contend the heavy storm-related flows at the plant have resulted from problems with poor construction of sewer pipes in Prince William County. When it rains, officials say, water seeps into the pipes. The water mixes with the sewage, and both have to be treated. A heavy storm can swamp the pipes with 10 times as much water as sewage -- and all of it has to be treated.

Camp Dresser said cutbacks in duplicate systems should be made in stages as problems with excess storm and groundwater is eliminated.