Of all the sewage effluent that metropolitan Washington sends into the Potomac River, only a relative thimbleful comes from the town of Herndon in northwestern Fairfax County. More sewage, in fact, is funneled through the giant Blue Plains plant in five minutes than passes through the Herndon plant in 24 hours.

But the Herndon plant has suddenly assumed importance seemingly all out of proportion to its size.

That has happened because the facility has become entangled in metropolitan water and sewer politics, which involves such various things as drinking-water intakes, multi-million-dollar federal grants, the pace of development, and moratoria.

To one public official, Fairfax County Supervisor Audrey Moore (D-Annandale), the continued operation of the plant is a threat to the drinking water supply of most of metropolitan Washington.

To another supervior, John P. Shacochis (R-Dranesville), the immediate closing of the plant - favored by Moore - could trigger a moratorium on sewer taps, and thus, he says, discourage industry and lead to still-higher taxes for country homeowners.

Between the two widely differing views may, or may not, lie the truth. But unfortunately in that vast middle ground between Moore and Shacochis is a lot of confusion.

An exchange at a recent meeting of the board of supervisors pointed up the confusion over the Herndon plant. Supervisor Joseph Alexander (D-Lee) called for a study by county staff before the board took a position on whether the plant - operated by the town of Herndon - should be closed. Alexander said he wanted a study of "alternatives."

"What alternatives?" asked Moore. "For what purpose?"

"To solve problem?" Alexander replied.

"What problem?" said Moore.

"Don't ask me," said Alexander.

Alexander was only half-joking.

Problem solvers would probably have more luck untangling the Gordian knot than picking apart all the elements of the Herndon sewage plant controversy.

The original problem was a simple one: The Herndon plant was badly polluting Folly Lick, a creek that runs through Fairfax and Loudoun County. The creek is aquatically dead, says T. M. Schwarberg, regional director of the State Water Control Board. Folly Lick merges with Sugarland Run, which flows into the Potomac 7.2 miles above the drinking-water intake for the District, suburban Maryland and parts of Fairfax Conty. No effluent is supposed to enter the Potomac closer than 15 miles upstream from the intake.

There is even a simple solution: The plant could be closed anytime by the simple expedient, as Moore said, of "turning a valve" and switching the sewage flow to the big, sewer pipe - the Dulles Interceptor - that connects to Blue Plains.

But nothing is simple in sewer politics, especially when Fairfax currenly is trying to win a $12.4 million grant from the Environmental Protection Agency.

Fairfax wants the $12.4 million to help build what has come to be known as the "pumpdown," a $17.5 million total project, a pipe traversing the county, which would divert future sewage from the regional Blue Plains plant to the county's Lower Potomac plant, which will have ample treatment capacity for treatment after a current expansion is completed.

The pumpdown was conceived as an answer to sewer problems in the northern Fairfax-Herndon area. It was the answer to a court decree ordering Fairfax to provide sewer capacity for Herndon and developers in the Dulles-Reston area of the county.

During planning of the pumpdown, county officials, faced with escalating costs of constructing the pumpdown, decided to seek federal funds.

An EPA grant would underwrite 90 per cent of the cost. But there was an important proviso: The pumpdown would have to be justified as a water-cleanup project. It's thrust could not be to accommodate growth.

Critics of the pumpdown called it a growth project. Supporters replied that the pumpdown would preserve the environment by heading off septic-tank development that requires large lots to accommodate sewage into the ground. And, the supporters said, the pumpdown would permit the closing of the Herndon plant.

As relatively small a problem as it was, the Herndon plant occupied an important, perhaps crucial, part in the pumpdown grant strategy.

As the water control board's Schwarberg said in a letter to Herndon's town manager in September, "The abandonment of the Herndon . . . plant is the only component of the pumpdown project that directly protects and improves the quality of state waters."

When the Fairfax supervisors decided to give the pumpdown No. 1 priority in their requests for federal sewer money for this fiscal year, it would be closed when its permit expired Dec. 31.

But, as a result of informal negotiations between Herndon and Fairfax sewer officials, the town decided to seek renewal of the permit.

The negotiations were a surprise to some of the supervisors, including Moore and Marie B. Travesky (R-Springfield).

According to one official familiar with the negotiations, "Glen Ehrich (Fairfax's director of public works) didn't want the plant closed until it got the grant award from EPA. The earliest we could get the grant award was March or April."

Ehrich, in a memo on Oct. 22 to County Executive Leonard Whorton, has spelled out the danger of closing the plant before the grant money was in hand: "Without new capacity (pumpdown) to charge future building permits to, this will require reimposition of the (building) moratorium in the Blue Plains service area as early as April 1978."

How could the tiny Herndon plant trigger a moratorium in an area where already 70,000 people live?

The answer is that Fairfax has an allocation of about 5.9 million gallons daily in Blue Plains. It has already used about 3.5 million gallons. If Fairfax had to accept Herndon's 300,000 gallons daily, that would amount to about one-eighth of the remaining Blue Plains capacity.

The Fairfax business community blamed an earlier moratorium in the Blue Plains service area with discouraging industry from coming to the county. A recent report reflecting business interests said industry's share as a taxpayer has been shrinking.

just as Fairfax officials wanted assurance that the EPA grant would be made, so did Herndon, which has undergone rapid expansion in recent years.

As Town Manager Robert S. Noe Jr. said, "If the grant failed, we'd give up something we have and get nothing in return."

While Schwargerg wanted the plant closed, he said he agreed to extend the expiring permit until March 31 as a "courtesy" to Fairfax County.

For that courtesy, he drew criticism from Moore, who wanted the plant closed immediately.

She cited Schwarberg's own judgement that the Herdon facility had caused "serious degradation to Folly Lick and Sugarland Run." She noted that Sugarland Run carried the effluent into the Potomac above the water intake. And she contended that a mere turning of a valve could close the plant and send the sewage to Blue Plains, below the water intake.

But the other supervisors turned that effort aside, agreeing to hold up a decision on immediate closing until county staff can look at what Alexander called "alternatives" - the alternatives to solve the problem, which he wasn't sure it was.