The Prince William County Service Authority, grappling to provide water and sewer service to residents as the county's population soars, is set to embark on a dramatic expansion plan that includes more than $120 million in new construction over the next five years.

The authority's board of directors is expected tonight to approve a capital improvements program that engineers say is essential if Prince William is to accommodate the thousands of new residents arriving annually.

Raymond Spittle, general manager of the five-year-old service authority, said the vast majority of the money for the proposed expansion would come from fees on developers, not rate increases for current customers.

Hardly a glamorous subject, water and sewer projects typically receive far less public attention than do roads, schools and parks -- public facility proposals that have led to heated debates in the county in recent years.

Nonetheless, development experts say that sewers are one of the most important factors influencing the pace and location of suburban growth.

In Montgomery County in the 1970s, for example, officials placed a moratorium on construction, citing insufficient sewers.

However appealing that notion might sound to those in Prince William who wish to close the door on growth, the service authority is required under Virginia law to provide sewer service to developments, and the code has established repeatedly that local governing bodies cannot reject proposed developments on the basis of insufficient sewer capacity.

"We're like a volleyball team, trying not to let the ball hit the ground," said Spittle of the service authority's task in accommodating growth.

Spittle said the breakneck pace of Prince William's growth -- more than 50,000 new residents since 1980 -- has caught even the planners by surprise.

"We've grown so fast that many things have happened in five years that I did not count on happening for 10 to 15 years," he said.

Of the nearly $120.4 million in new construction called for in the capital plan, about $110 million is for new projects, while almost $9 million will be used to replace or repair old water and sewer facilities.

By 2010, Prince William will need to triple its current daily capacity of about 20 million gallons of water and 20 million gallons of sewage, according to service authority engineers.

Among the leading items called for during the next five years in the capital plan are an expansion of Prince William's participation in two regional programs: the treatment plant in Fairfax County run by the Upper Occoquan Sewage Authority and the water plant run by the Fairfax County Water Authority.

In addition, the plan calls for expanding the service authority's H.L. Mooney treatment facility along Neabsco Creek in eastern Prince William at a cost of $40.1 million, and constructing additional water storage tanks around the county, at Cherry Hill, Gainesville, Haymarket, Hoadly and south of Manassas and other sites.

If the plan is approved, funding for specific items will be worked out in the authority's annual budget process.

The service authority was formed five years ago from a collection of sanitary districts serving the eastern and western ends of Prince William. Unlike the sanitary districts, the authority does not have taxing powers, but it does have the power to set customer rates and to sell bonds without going to referendum.

Service authority customers are charged $1.37 per 1,000 gallons of water, and $2.70 per 1,000 gallons of sewage. The average Prince William homeowner used 80,000 gallons of water last year.

Although the capital program attempts to predict where growth will require expansion, service authority officials emphasized that the plan is subject to change as circumstances dictate. A sewer expansion in western Prince William to accommodate the Robert Trent Jones golf resort complex, for example, was not expected and never appeared on the service authority's capital plan.

"You have to adjust to conditions," Spittle said. "You really don't have control over what these conditions are."