The Prince William county Board of Supervisors, after long and bitter resistance to hooking up to a new regional sewage treatment plant, surrendered yesterday rather than face possible fines of $10,000 a day.

The Virginia State Water Control Board had voted Sunday to have the state Attorney General's Office take legal action against any jurisdiction causing unnecessary delay in hooking up to the Upper Occoquan Sewage Authority plant on Bull Rum.

The sewage agency has asked Prince William to agree to begin hooking up Monday, but the county had refused to do so. Fairfax County had agreed to do the Monday start-up and workmen there began rechanneling sewage flows to the highly advanced, $82 million plant.

The Prince William supervisors, after voting 4 to 3 yesterday to agree to the water control board demand, said they would have continued the fight against the sewage authority except for the June 30 expiration of the discharge permit at their Greater Manassas Sanitary District treatment plant.

The water control board had made it clear that it would not extend the permit to use the Greater Manassas plant and the discharge of treated sewage without a permit would have subjected the county to a possible fine of $10,000 a day.

County Attorney Terrence Emerson said the water control board "had all the guns" and advised the board that the risk of fines was too great to warrant further opposition.

The board agreed in executive session and then ratified the action by a margin of one vote with members Donald White, T. Clay Wood and Andrew J. Donnelly in opposition. One board member later said that had the vote gone against ther hook-up, "we would have had to vote again," indicating that the members were resigned to approval of the politically unpopular hook-up.

The start-up of the Upper Occoquan plant had led to a major residents' protest after county officials announced that water and sewer rates in the western county area previously served by the Greater Manassas plant would rise from $36 a quarter for the average user to $140 a quarter.

The rates since have been revised downward several times and a Greater Manassas sewer district official said yesterday that a rate of $60 a quarter now appear possible.