The Loudoun County Board of Supervisors last week sold $6.9 million in general obligation bonds to pay for the first phase of a $6.5 million expansion of the county library system and half of the $6.4 million landfill expansion project.

Merrill Lynch & Associates, the lowest of seven bidders, gave the county an interest rate of 8.4 percent. The net interest cost to the county over the next 15 years will be more than $4.5 million. County voters approved the bond sale on Nov. 6.

County Administrator Phil Bolen, Board Chairman Frank Raflo and Finance Director Kirby Bowers said they were pleased with the interest rate and cited the county's low unemployment rate of 2.4 percent and high personal income -- Loudoun's median family income is $32,839 -- for the relatively low interest rate from Merrill Lynch. Bowers noted later that Fairfax County sold its general issue bonds the day after Loudoun's sale at an interest rate of 8.6 percent over a 20-year period.

The rating of the bonds themselves was not as high as county officials had hoped, however. After a trip to New York last month with members of the county staff, Bolen said that he "would be personally very disappointed" if the bond rating agencies of Standard & Poor's and Moody's Investors Service did not raise Loudoun's rating, which has not changed in 20 years. But at sale time last week, Moody's rating remained at A1 and S&P's at A.

According to Bowers, the reason S&P gave for the relatively low rating was that while the county's industrial and commercial tax base picture is fairly rosy, "We don't have it in the ground yet."

The board authorized Bolen to take option on 50 acres near the Leesburg Airport to be used for county fire and rescue squad training and other county purposes. Supervisor James Brownell abstained from voting because the property is owned by his son, Bruce.

The board has put a $10 hold on the land until April, by which time, the board said, it hopes to determine a price and buy the parcel. Other possible uses for the site are a garage and warehouse, a data processing center and a youth home. As part of the option, developer Brownell will provide an easement for public water and sewer lines and electrical service.

In other business, the board sent a $53 million school budget back to the School Board asking that it revise its request of $4 million in new local funds by taking into account an expected increase in state money.

Raflo originally had asked the board to approve a $400,000 reduction in the school budget, but a majority of the supervisors declined to support that proposal.

Bowen told the board that, in addition to $6.7 million in carryover funds from this year's budget, the county estimates that it will have an increase of more than $6.5 million in local revenues in fiscal 1986. Despite the healthy figure, Bowen projected a $1 million deficit. He suggested an increase in the real estate tax rate from $1.10 to $1.14 per $100 assessed valuation to make up the difference. Bowen will present his proposed budget to the supervisors by April 1.