Sewer costs are about the only charges for public services that haven't been raised in recent years in Fairfax County. But it is an almost foregone conclusion that sewer rates will be increased soon - perhaps as early as Dec. 1 - and by as much as 48 per cent over three years.

If the present cost of sewer service is increased to the level recommended by County Executive Leonard Whorton, homeowners would find their average annual bill going from about $91 to $135, according to county figures. The new rate would be in effect during the next three years.

Whorton's proposed increase would raise the rate from the present 95 cents per 1,000 gallons to $1.27, as of Dec. 1. It would go to $1.33 July 1, 1978, and $1.57 as of July 1, 1979. If a single increase were imposed for the three-year period, Whorton said the rate would have to go up to $1.41 on Dec. 1.

Whorton's proposed increases are lower than those recommended in a report by a consultant to the county, even though the county executive forsees steeper deficits than the consultant if the present rate structure is maintained. The reason Whorton could propose a smaller increase is that he and his staff, in an "update" of the consultant's report, assume that a major construction project - the $21 million pipeline to carry sewage from the Dulles-Herndon-Reston area to the Lower Potomac plant - will be built with 75 per cent federal funding, instead of being paid for the county.

But Whorton's proposed increases are higher than those offered several weeks ago by Supervisor Audrey Moore (D-Annandale), who suggested that the rate go from 95 cents to $1.11. Moore said she fears that the higher rate proposed by Whorton and his staff provides insurance money for funding the "pumpdown," which she had initially opposed, saying its purpose was not to clean up water but promote development in northwestern Fairfax. The county, though, claimed the "pumpdown" was necessitated by a court decision ordering Fairfax to provide sewage treatment in the area.

Moore wants construction financed by an increase in sewer hookup fees, which presently are $1,000 per house, one of the lowest rates in metropolitan Washington. Neither the consultant's study nor the Whorton "update" fake up the issue of whether hookup fees should be increased.

The consultant's study, prepared by Alexander Potter Associates, recommended increases over the next four years that would raise the sewer service rate to $1.37, $1.59, $1.78 and $2.02. However, Potter Associates said those increases would not be as steep if the pipeline project got federal funding - a possibility the study did not assume.

The sewer rate issue has been surrounded by controversy - primarily because the county suppressed the Potter report for almost half a year. In the face of an unfavorable court decision, Whorton released the report on Sept. 19. In a memo accompanying the report, Whorton said its release was withheld because "the report was adjudged at that time (April) to be incomplete and still is so adjudged."

Whorton said the report was incomplete because of uncertainties surrounding the "pumpdown."

As Whorton and his staff conceded in a memo Oct. 17, "uncertainty still surrounds the matter of federal grant financing of the Difficult Run pumpdown (the pipeline project)." But their memo adds, "However, the need for an upward adjustment in operating revenues is now judged to be critical and early Board of Supervisors action to increase the charge is contemplated."

One of the main reasons the rate must go up, the memo said, is the sharp increases expected in the cost of advanced wastewater treatment.

Under federal and state mandates, Fairfax has enlarged and improved its treatment plant at lower Potomac. The limited advanced treatment underway this year will cost $1,110,800. But by 1981, when the plant is expected to give full advanced treatment (removing most phosphorus and nitrogen as well as other, oxygen-demanding pollutants) that cost will rise to $7,728,000.

The county will also have to pay the higher advanced treatment costs at plants in other jurisdictions - the District and Alexandria primarily - where it buys capacity.

There is probably no more complicated area of county government than sewer service and construction. The county has nine sewage treatment plants, ranging from one so small it serves only a prison camp, to a major facility at lower Potomac. By 1981, seven of those nine plants will be consolidated into two, the enlarged and upgraded lower Potomac and the new advanced-treatment plant, which is almost completed, that will serve the Occoquan watershed in western Fairfax.

While the number of plants in Fairfax will be reduced, other complications will remain. For example, Fairfax handles sewage, directly or indirectly, from Fairfax City, Vienna, Herndon, Falls Church and Arlington. On the other hand, the county sends sewage to Alexandria, the District and Arlington. The public hearing on water rate increases will be held in late November.