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A Typical City Sewer System: Washington D.C.Modern urban sewage treatment can best be described by reference to a specific city.The Washington D.C.system has many aspects typical of any large modern city, though its early history is not representative of many others.The town’s first bathtubs were installed in the White House and the Capitol, for the members of Congre in the 1840s;in 1850 the U.S.Congre authorized the Corps of the Engineers of the U.S.Army to develop a city-wide water supply from the Potomac River.At this point Washington caught up with New York, London, and Paris, which were also encountering the problem of disposing of used water along with wastes.Washington’s solution was the same as that of other cities;the existing system of culverts and drains, built for street drainage only, was extended and developed into a sewer system for the disposal of domestic waste water from residence , government offices, and businees.The system followed the drainage pattern of the city street network and in general made a system of pipes with a sewer available to each private property.At the same time, again in common with other cities, streets drains wee built to empty into the nearest surface watercourse without any thought of degradation of the water quality.This was in spite of the fact that an engineering study and report(1890)recommended that all extensions of the sewer system separate storm runoff from domestic waste water.With continued growth of the city, the District of Columbia constructed in the first decade of the 20thcentury a series of intercepting sewers and a pumping station to lift the domestic waste water into an outfall line for discharge into the Potomac River south of the city.At the same time, pumping facilities were installed for the lifting of storm water drainage directly into the nearly Anacostia River.It was impoible to keep domestic and storm flow completely separate, but practical separation was attempted.With he accelerated growth of the 1920s , concern over pollution of the Potomac increased.The Potomac estuary had a remarkable ability to aimilate pollution because of the large “flats” on both sides of the river that were kept in a state of constant circulation by tidal variations, but a study made by the Public Health Service in 1932 revealed that the river was in such a condition that low flow would bring about serious pollution effects.As a result, Congre decided to proceed with the construcation of facilities for the treatment of waste water.This again was in line with decisions being made in many U.S.and European cities at the same period.Treatment plan at Blue Plains.During 1934-38 a plant was constructed on the left bank of the Potomac in the part of the city known as Blue Plains to accommodate a flow of 130,000,000 gallons per day and serve a population of 650,000.Initially, with the help of the Federal Emergency Administration of Public Works, money was allowed for construcation of a plant that would remove 90 precent of the organic matter from the waste-water flow.That level of treatment was in accordance with the Public Health Service recommendation contained in the 1932 report.Instead of constructing the plant in accordance with that recommendation, the District of Columbia decided to eliminate the second step in the treatment and construct a sedimentation plant, generally known as primary treatment.The plant was able to remove about 36 percent of the organic matter when in went into operation in August 1938, but, as the population load increased, accelerated by World War II, the plant was unable to maintain this level, and year by year efficiency dropped until it was regularly under 30 percent.During World War II, initial plans were made for the relief of the treatment burden, and by

1950 the District of Columbia had begun major construction to increase the capacity of the plant and make further plans for inclusion of secondary treatment.Activated-sludge plant.The activated-sludge proce pioneered in Britain had by now been widely tested.Washington constructed a high-rate activated-sludge treatment plant in anticipation of 70 percent removal of organic matter.While the new plant brought a major improvement in the river, there was no real poibility of keeping up with the pollution burden, even though the plant grew to a capacity of 290,000,000 gallons daily.In the early 1970s the District began planning to extend treatment to a much higher level-once more, a decision that was forced on many cities of the United States, Europe and Asia.Coordination with surrounding areas.One of the awkward problems confronting city engineers of the 20 century in nearly all countries has been the impoibility of isolating a metropolitan area from neighboring regions.Rivers carry pollution from city to city, even country to country.In Washington the problem was encountered in a relatively mild form: much of the Maryland suburban area drains into Rock Creek and the Anacostia River, which flow through the District of Columbia;to try to keep the two streams as clean as poible the District of Columbia and the Washington Suburban Sanitary Commiion(of Maryland)entered into an agreement to handle each other’s flow at a reasonable cost.All the domestic waste water of the suburban areas is now connected into District sewers, with payments made to handle the waste waters.As part of the agreement, the Maryland Commiion helps to finance both the construction and the operation of the District of Columbia Water Pollution Control Plant.Other developments.With continued growth and rising pollution control standards of the 1960s and 1970s, Washington like most other major cities has been turning toward additional treatment, including chemical treatment.One proposal calls for achieving so high a level of treatment that the Potomac estuary into which the effluent floes could be used as an emergency water source.Another direction in which Washington had headed in company with many other modern cities is toward separation of systems.This is a tedious and expensive proce, requiring piping changes on private property.Its longrange wisdom, however, is irrefutable.The redevelopment of certain major areas, such as southwest Washington, has given favorable opportunities for large-scale separation.An important advance in financing improvments has been adopted by Washington: the sewer-service charge on all those served by the drainage system.This system has been followed more and more by drainage systems serving both municipalities and industry.Since about 1959 the D.C.sewer system has been interconnected with the areas in Maryland that naturally drain through the District via the Potomac River and major areas in Virginia related to the intercepting sewer serving Dulles International Airport near Herndon, Virginia.As a result of this connection, the area served increased by 436 square miles(1,129 square kilometers)in Maryland and 228 square miles(590 square kilometers)in Virginia.The Metropolitan area in Arlington County and much of the Virginia suburban area adjacent to Arlington County are served by other treatment plants.Present treatment facilities.At the District of Columbia Water Pollution Control Plant(see Figure 1)the raw waste water enters the plant pumping station and is treated in the following succeive steps: girt removal, preliminary sedimentation, aeration., and final sedimentation.In addition, chlorine treatment may be given the flow prior to preliminary sedimentation or it may be th

given to the final effluent.With the first application, the effect of chlorine is to minimize odours from the sedimentation tanks.When fed to the final effluent, chlorine has a disinfectant effect.The purpose of the sedimentation tanks, both preliminary, is to separate solids from the wastewater flow;the solids removed must be given further treatment.At the D.C.palnt these solids are exposed to anaerobic digestion and dewatering on vacuum filters.The final product is a moist cake with approximately 70 precent water, suitable for land application as a soil conditioner.During the digestion of sludge, a gas consisting of approximately two-thirds methane is produced that is burned for heat for the plant building and to provide some power generation.The sludge gas has a heat value of about 600 BTU(British thermal units)per cubic foot and the quantity produced is about one cubic foot per person.Per day.A sludge gas engine of 1, 200 horsepower drives an 800-kilowatt generator for production of electric power.Initially, the power produced supplied about 90 percent of the needs of the plant, but with the growth of the plant, power requirements have increased rapidly and now the gas engine supplies only a minor proportion of the electric power.It supplies, however, through its jacket water-cooling system, a large amount of the heat neceary to maintain active biological digestion in the sludge digestion tanks.The total cost of the plant exceeds $ 25,000,000 and the annual operating costs in the late 1960s approximated $ 2,500,000.More than 250 persons are employed in operation and maintenance.The full extent of the undertaking may be appreciated when the vast waste-water collection system serving property throughout the area in visualized.In the District of Columbia alone, more than 1,700 miles(2,720 kilometers)of sewers serve this purpose, while 2,700 miles(4,320 kilometers)in the Maryland area give similar service to the properties of that jurisdiction.The maintenance of the system is a major activity, as 200 men are engaged in regular maintenance and minor construction related to the sewer system in the District of Columbia alone.Proper maintenance involves regular inspection of the lines and periodic cleaning to avoid difficulties that could cause great inconvenience and poibly properly damage to those served by the drainage system.