The federal Environmental Protection Agency said yesterday it will not help pay for a $19.2 million sewage diversion project that was planned by Fairfax County to comply with a state court order to provide sewage treatment facilities in the fast-growing Reston area.

The EPA's regional office in Philadelphia said the project - under which the country was planning to pipe sewage from the Herndon-Reston area to its treatment plant on Pohick Bay - "is not in conformance" with Virginia's priority system for sewage-treatment projects or the federal agency's regulations governing such systems. Virginia had originally given the projects its lowest priority rating on water clean-up - D - then moved it up to a B rating, the second highest.

Faced with paying for the entire project itself, Fairfax now may explore other proposals, which could include building a new treatment plant in the northwestern part of the county or working with suburban Maryland and the District of Columbia on a regional solution.

In a letter yesterday, EPA regional administrator Jack J. Schramm ordered Virginia to take the Fairfax pipeline off the state's priority list and reinstate two other projects, which he indicated were more directly related to clean-up of the Potomac River.

The EPA decision, scheduled to be announced today at a press conference, means that Fairfax, if it wants to pursue the pipeline project, would have to pay the entire cost. Federal funding would have covered $14.4 million of the cost.

"I don't think Fairfax County is going to take this lying down," John F. Herrity, chairman of the Fairfax Board of Supervisors, said of the EPA decision. "The federal government seems to have the habit of getting local jurisdictions involved in projects and then abandoning them."

The project was conceived by Fairfax officials as an answer to a 1974 Circuit Court decision ordering the county to provide a long-term solution to the sewage-treatment needs of the new town of Reston and the surrounding area. Under the project, sewage that couldn't be handled by the Blue Plains regional plant in the District of Columbia would be diverted southward the length of the county to the expanded and improved Lower Potomac plant on Pohick Bay between Mount Vernon and Mason Neck.

The EPA decision on the project is the second time in the last 15 months that a major local sewage-treatment project has failed to win approval for federal funding. In August, 1976, the EPA rejected Montgomery County's proposal to build a big treatment plant at Dickerson.

Maryland and EPA officials are trying to negotiate a compromise on Dickerson that could result in construction of a smaller plant. Fairfax line to its lower Potomac plant rather than buy treatment capacity in Dickerson because the Maryland Plant, as originally planned, was too expensive. Now Fairfax will have to decide whether the pipeline, with no federal funding, is still more costworthy.

The EPA decision on the pipeline is a major victory for Fairfax Supervisor Audrey Moore (D-Annandale), who, virtually single-handedly on the Board, opposed the project, Mrs. Moore said the project was aimed more at encouraging rapid population growth in the Reston are a rather than cleaning up the Potomac River.

But the rest of the supervisors endorsed the project for higher priority the state eventually gave it . That priority shift first ran into serious trouble last June when Virginia Assistant Attorney General Frederick S. Fisher told the State Water Control Board that the project, under the state's own rules, should remain in category D. The same conclusion was reached by EPA regional administrator Schramm.