Development expected to sprout around a planned 23-mile extension of Metrorail, and the scores of associated new toilets, could spur Fairfax County to invest in greater sewage-treatment capacity, according to the county’s public works director.

Jimmie Jenkins, who heads the Department of Public Works and Environmental Services, said the county is now well under its capacity, and any potential expansions would be years down the road. But looking ahead two decades from now, he said the department may have to beef up its facilities in the Blue Plains Sewer Shed, which serves the northern part of the county.