Fairfax County Executive Leonard L. Whorton yesterday proposed raising the county sewer service rate by 73 per cent - 24 per cent more than the new rate he requested two months ago.

If the Fairfax Board of Supervisors, which takes up the issue Monday, accepts Whorton's recommendation, the county resident will find his annual sewer bill rising from about $91 to a little more than $158. The higher rate would be in effect for three years.

Initial reaction indicated that the Whorton proposal will encounter significant opposition at Monday's meeting of the supervisors.

"A $1.65 rate (the one proposed by Whorton, per 1,000 gallons) would be outrageous," Board Chairman John F. Herrity (R) said. The present service rate - 95 cents per 1,000 gallons - should not be raised any higher than to $1.27, Herrity said.

If the $1.27 rate is not enough to cover the steeply rising costs of sewage treatment and new construction, Herrity said the present $1,000 connection fee for new houses should be raised to $1,500.

Supervisor Audrey Moore (D-Annandale). who has argued that new growth has not been paying its share of the sewer bill, wants a three-year rate of no higher then $1.18. On the other hand, she wants the hookup fee - a charge paid by developers and then passed along to home buyers - doubled to $2,000.

The Whorton proposal, she said, "is a red herring." The county executive is trying to make it politically palatable for the Board to accept his original proposal for a $1.41 rate.

In his statistic-packed memo to the supervisors, Whorton specifically rejected a combined increase in the service rate and the hookup fee, preferring, instead, to raise all new revenue with a big increase in the service rate, which is paid in quarterly installments.

Hookup fees, because they depend on new construction, are not a regular source of revenue, he said. Relying on them, he went on, "could jeopardize the entire (sewer) financing program."

Whorton's requested increases went up 24 per cent in two months because it appears Fairfax will not get any U.S. funding for the $21 million diversion project that would pump sewage from the growing-but sewer-short Herndon-Reston area to the lower Potomac treatment plant at the other end of the county.

While Fairfax could contest the decision by the Environmental Protection Agency not to fund the divesion project - called the pumpdown - the county is under intense court pressure to solve the Herndon-Reston area's sewer problems.

According to a consultant's report to the county, even a $1.65 service rate would prove inadequate after three years. By 1981, when the county is providing complete advanced treatment for 36 million gallons daily at lower Potomac, a rate of $2 per 1,000 gallons will be necessary, the report said.