404 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1939 



THE WATER SUPPLY AND FILTRATION PLANT AT DALECARLIA 



A typical modern city water supply system is that of the Dale- 

 carlia Water Plant at Washington, D. C. The water supply of 

 Washington, D. C, comes from the Potomac River, whose watershed 

 extends back to the Shenandoah River, receiving streams from West 

 Virginia and Pennsylvania as well as from Maryland and Virginia. 

 The intake is at a low dam at the head of Great Falls, 16 miles 

 above the center of the city. The average discharge at this point 

 is 7,690 million gallons per day. At the time of the record low 

 flow during the drought of the summer of 1930, the discharge dropped 

 to 525 million gallons per day. Since the present daily maximum 

 water consumption during the summer months is from 100 to 105 

 million gallons per day, or only one-fifth of the record low flow, it 

 is evident that this river supply will suffice many generations. 



The water flows by gravity through two conduits from an eleva- 

 tion of 151.6 feet above sea level at Great Falls for a distance of 9 

 miles to the inlet of the Dalecarlia Reservoir, which is at an eleva- 

 tion of 143.0 feet. This reservoir, which has a capacity of 210 mil- 

 lion gallons, holds the water for 1 day before it goes into the filtra- 

 tion plant. This retention period with its change in velocity allows 

 a heavy reduction in the turbidity of the water at periods of high 

 mud content when the weight of suspended matter is often 3,000 

 parts per million. Not all the mud, however, is settled out, nor does 

 much improvement result at times of low turbidity in the water 

 when the weight of the suspended matter is 30 parts per million 

 or less. 



About 80 to 100 million gallons flow from the reservoir, part being 

 drawn into the Dalecarlia filtration plant, part going to the Mac- 

 Millan Park filters. An additional 120 million gallons is diverted 

 daily to the hydroelectric plant. 



As the water is drawn from the large Dalecarlia Reservoir for a 

 distance of several hundred feet through a 6-foot conduit and Venturi 

 meter to the rapid sand-filtration plant at Dalecarlia, a stream of 

 aluminum sulfate is added. This coagulating agent mixes with the 

 water as it passes through the conduit to the mixing basins, which 

 are covered rectangular concrete basins about 10 feet deep, so built 

 that the water passes backw^ard and forward around a series of 

 wooden baffles, causing the intermingling of all the water with the 

 chemical. The amount of the aluminum sulfate or alum added 

 varies, according to the amount of mud and the composition of the 

 water as shown by the laboratory tests, from 145 to 450 pounds per 

 million gallons. The time of the flow of the water through the mix- 

 ing basins varies from 10 to 20 minutes, the time being governed by 



