A sophisticated Northern Virginia sewage treatment plant that serves portions of Fairfax and Prince William counties has taken the first step toward selling its effluent as drinking water.

Confident that its discharge is safe enough to drink, the Upper Occoquan Sewage Authority has asked for a $750,000 grant from the Environmental Protection Agency to prove its point with a year and a half of laboratory testing.

"I don't have any qualms about drinking it," said Millard H. Robbins Jr., the authority's executive director said yesterday.

"It's kind of flat," he said, "but it's drinkable." The effluent is flat, he said, because the treatment plant removes most of the minerals that can give water a special taste.

While Robbins is confident his product will pass all tests, he said it might be years before the public's psychological barriers to drinking purified waste water are eliminated.

But there has been some movement in Virginia toward reclaiming effluent for industrial - rather than drinking - purposes. The Greater Manassas Sanitary District, a utility that serves part of Prince William, wants to buy the plant's effluent for industrial purposes.

Robbins said the authority has offered to sell up to 3.6 million gallons a day for a price that would be set somewhere between 5 cents and 25 cents a thousand gallons.

Drinking water could be sold at a much higher price than industrial water. At the present time, the Fairfax County Water Authority charges a minimum of 70 cents per thousand gallons, more for high levels of usage. If the Upper Occoquan authority got 70 cents per thousand gallons, it would gross about $2 million annually and cover about 80 percent of its current operating expenses of $2,600,000.

The 8 million gallons of effluent that its plant - one of the most advanced in the world - procudes daily could serve a population of 80,000, according to some estimates.

With the Washington region's population growing rapidly in a period when no major new sources of water supply are planned, reclaimed water could become an important factor in meeting water demand.

No other sewage treatment plant in the country markets potable water. However, a plant in Orange County, Calif., pumps its effluent into the area's depleted groundwater supplies. That plant, along with some other advanced facilities around the country, is seeking to demonstrate the potability of its product.