The water that most suburban Maryland residents drink and wash with each day is actually muddy, smelly Potomac River water that the Potomac water treatment plant in Montgomery County makes almost crystal clear by the time it flows out of home faucets.

The river water is sucked into the treatment plant from a secluded spot of Potomac where the river is split by Watkins Island.

There the water flows into a concrete structure 18 feet high covered with metal grates where, with the help of gravity and pumps, it is pulled in a well on the plant site.

Yesterday, officials said the water level had dropped to a dangerously low level because of the continuing drought. The water level was barely one foot above the point where the water intake pipe begins sucking in air instead of water, they said.

The water sucked up from the river is then chlorinated before being subjected to further treatment. Gaseous chlorine, stored in one-ton tanks in a building called the chlorine house, is piped through regulators and mixed with the river water.

The water is then pumped into 100-foot-long settling basins. There alum is added which forces any sediment in the water to coagulate and settle out as the water is churned by long paddles and permitted to seep through a series of wooden boards called baffle boards. The water becomes noticeably clearer as it passed through each baffle board section.

The last treatment step before water becomes purified enough for drinking is the filtering process. The water moves from the sediment basin into another basin equipped at the bottom with filters made of layers of sand, gravel, granite and other materials resembling a towel. At this point, too, fluoride is added to prevent tooth decay. The water no longer smells like fish and resembles the highly chlorinated water found in swimming pools.

The water is then pumped into ltwo 11-million-gallon reservoirs on the plant site. Before it is pumped further, it is subjected to a series of tests to insure that it is free of harmful bacteria and iron, and that it has the right amount of chlorine and fluoride.

The pumping station that pumps the water from the reservoirs into the main water lines leading to distribution storage facilities in Wheaton and Gathersburg is the facility that was damaged by fire Wednesday morning.

From Wheaton and Gaithersburg the water is then distributed through pipes to homes and businesses in all other parts of Montgomery and Prince George's County.

The 4,000 volts of electricity needed to operate the water plant pumps is fed into two circuit breakers along side the main pumping station. The electrical current passes from the circuit breakers through transformers to a control board filled with gauges and switches inside the pumping station. To start a pump all a plant operator has to do is flip a switch on the control panel.

One of the reasons why the plant cannot operate at its full capacity is because two of its four transformers were destroyed as a result of Wednesday's fire.

The control panel is about 90 feet long and eight feet high and resembles a series of gray metal lockers. On the outside are several gauges and buttons and on the inside intricate wiring.

The main contral room for entire plant, however, is not at the pumping station but in a separate building about 500 feet away. In that building, is a room where one full wall is lined with a gray metal panel board consisting of graphs, buttons and a series of red, green, blue and yellow lights.

The graphs indicate how much water each pump at the facility is sending out into the distribution system or drawing into the system from the Potomac River. When there is a malfunction an alarm sounds lights on the panel come on to indicate which pump is not working.

The Potomac water treatment plant is located on the Potomac near Watts Branch, a stream near Montgomery Country. It initially went into operation in 1961 and had a capacity at the time of 30 million gallons of water a day.

Since then the plant has been expanded to provide about 170 million gallons of water o day for normal usage, and it has a peaked design capacity of 240 million gallons a day.

The plant sits on a 48-acredf site on River Potomac surrounded by horse farms and private residences. The plant is set back from the road and elevated, but some of its structures can be seen from the road, such as its 25-foot-high granite and concrete settling basin and its six-story white frame main office building, where the control room and testing laboratory are located.

Currently, several new laboratories are being installed in the building and when they are finished they will permit plant employees to make even more stringent tests of the water's cleanliness before it leaves the plant.