It was 6:30 a.m. - the beginning of the morning peak demand for water when people wake and use their bathrooms, kitchen faucets and showers.

Tommy Campbell, a short, slim plant operator for the Washington Suburban Sanitary Commission, walked about 500 feet from the main office of the Potomac Water Filtration Plant on River Road in Montgomery County to the plant's finished water pumping station to turn on an additional pump - one of five used to send water out to the Bethesda-Wheaton area. All he had to do was turn a valve to allow the water to flow at full capacity.

But, according to what Campbell later told his supervisor, the valve would not open. Campbell asked an electrician on duty if he could turn off the M-I pump altogether and put the M-2 pump into operation instead, according to Campbell's supervisor, Ernest W. Bond.

When the M-2 pump started, things appeared to be back to normal. But, seconds later the pump began slowing down and smoke shot up from the pump followed by fire. Campbell hit the emergency button which would shut off the pump but it did not work.

When Campbell turned to call the fire department, he saw smoke coming out of the tail retangular gray boxes that severed as the pumping station's control board. Campbell shut off the power at the station.

It was 6:59 a.m., according to all the cooks at the plant which late yesterday remained stopped at that moment, and the damage to the plant was already done, fire investigators said later.

On a day of jungle heat and humidity, a 21-minute fire caused the total breakdown of suburban Maryland's main water plant. From a normal output of 170 million gallons per day, the plant could not even send out a trickle. Within six hours trucks carrying an emergency supply of water were rolling into parts of Gaithersburg as water pressure there and in Olney dropped steadily.

At first WSSC officials believed that damage was limited to a third of the station's pumping control board, the M2 water pump and the two circuit breakers and one transformer that supply the electrical power that operates the water pump. Later in the day Potomac Power Co. workers cooperating with WSSC technicians discovered that two of the plant's transformers were damaged beyond repair and would have to be removed.

On routine days the plant, which sits on a 45-acre lot in a rural section of Potomac, is hardly noticeable to passers-by along River Road, except for a stone sign identifying the facility.

Yesterday the facility looked like it was under siege as pale blue WSSC trucks and blue and white Pepco vans coverged around the plant's six box-shaped concrete and granite buildings.

A WSSC helicopter landed and took off periodically bringing equipment and supplies to the engineers trying to repair the crcuit breakers and control panels.

In mid-afternoon two yellow cranes rolled onto the site to help pull out the transformers - steaming hot under the 100-degree sun - that Pepco and WSSC electricians had dismantled. A rig pulled up to cart the two transformers away.

Shirtless Pepco and WSSC employees wearing yellow, orange, and green construction hats scaled the 15-foot high metal maze of electrical transformers and circuit breakers like children scaling monkey bars. Inside the plant other electricians worked on the intricate wiring and on the gauge of the nine-foot long metal control panel.

One Pepco worker had ripped the sleeves from his sweat-soaked work shirt. "I've had to alter this shit for the occasion," he remarked.

Inside the plant's six-story main building, it was as hot as it was outdoors. An underground electrical cable that had remained undamaged by the fire powered a few lights. Employees made an effort not to flush toilets or run water in the bathrooms.

"The way we're all sweating, we won't need toilets today - you've got to keep your sense of humor in a situation like this," said supervisor Bond, who had taken off his work shirt.

He said he had "hopped out of bed" when he got Campbell's call about the fire in the morning pulled on his pants and drove to the Potomac station from his home in Annapolis.

As several of the plant's employees sat in the shade of a tree eating what they said was their breakfast - hamburgers and vanilla milkshakes - at 12:30 p.m., Bond proudly recounted how his mother in Annapolis woke up yesterday, turned on her television set and saw him explaining the plant's breakdown.

Plant workers, like Bill Snoots, senior plant operator, who was still working on overtime late in the afternoon after starting at 7:30 a.m., said yesterday's crisis made chopping the ice from the river last winter in order to let the water flow into the system seem insignificant by comparison.

The process of supplying Montgomery County with fresh water each day begins with the river water flowing through a bar screen that removes any large debris. The water then pours into a treatment basin that can hold three million gallons of water where chloride is added for disinfection and alum, another chemical, is added to cause dirt particles in the water to come together.

Then water then goes through a flocculation chamber where the particles of dirt are gently agitated to cause them to become larger. At this point they are heavy enough to settle to the bottom and be removed.

The water is again filtered to remove any small remaining particles. Lime is added to make the water less acidic. Fluoride also is added to retard tooth decay.

The water is then transmitted to reservoirs (water holding sites), standpipes (tall basins resembling Popsicles on a stick) and elevated tanks (large metal balls) set on top of long metal legs that are often seen in open field on roadsides. Eventually, the water makes its way to fire hydrants, buildings and homes.