Fairfax City, unable to treat enough water to meet demands, yesterday extended through September its mandatory ban on outdoor water usage on weekends.

City resident's who ignore the ban - which prohibits watering lawns and washing cars - could be fined up to $100.

The town of Herndon, which is supplied by Fairfax City, indicated to city officials that it also would extend it's mandatory ban.

But the city is powerless to impose a mandatory ban on its customers in Fairfax and Loudoun counties. Fairfax has asked for voluntary conservation among residents served by the Fairfax County Water Authority, and Loudoun has no ordinance that would permit officials of that county to impose any kind of water restrictions.

Although Fairfax City has more than enough water at its Goose Creek reservoir in Loudon to supply its 60,000 customers, it has limited treatment facilities.

The city's treatment plant, which is being expanded, can process only 6 million gallons daily on the average. Demand has sometimes outrun even the peak load capacity of 10 million gallons daily, forcing the city to buy up to 7 1/2 million gallons monthly from the Fairfax County authority.

The county authority is having problems of its won. Its Occoquan Reservoir is down to almost one-third of its capacity, and the forecast for September calls for below-normal rainfall.

Fairfax City officials think most of the abnormal demand on their water system is coming from those jurisdictions that don't have mandatory restrictions.

"We've asked the Loudon County Sanitation Authority to request the Loudon Supervisors to take some action, but nothing has happened, Richard R. Fruehauf, Fairfax City's water and sewer services director, said.

"We're penalizing people in Fairfax City and Herndon, while others are not conserving," Fruehauf said.

The possibility of mandatroy restrictions in Fairfax County has become more real as the Occoquan Reservoir drops toward the critical 102.5 foot level. That point should be reached Tuesday, according to water authority engineer-director James J. Corbalis Jr.

The Fairfax County Board of Supervisors, which had been scheduled to convene at 7:30 p.m. Tuesday after a month's recess, has moved its session back to 6 p.m. to deal with the worsening water situation.