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Washington, D. C., the Potomac River widens gradually and becomes a tidal stream as i t flows into Chesapeake Bay. Its narrow, deep channel is bordered on both sides by spacious tidal flats which are covered with vaned aquatic plants. This dense vegetation provides a shelter for small fish and a breeding ground for microscopic plant and animal organisms. At low tide these flats rmemble meadows, but when the tide rise’s, polluted water from upstream flows across them and deposits suspended impurities. Since the water here is not disturbed by winds or river currenta, clarification is rapid.
oxygen values of 103% saturation on sunny days; on cloudy days 75% saturation was recorded. Values of 128% saturation were recorded on particularly favorable sunny days. The water leaving the flat on the morning ebb tide had been on the flat during the night, so it did not receive the sunlight. Nevertheless, i t showed an average of 83% oxygen saturation. On the other hand, water which had been on the flat in the daytime showed 93% oxygen saturation. In October and November when th6 biological activity was lower, the average oxygen value of all water leaving the fiat was 88% saturation as compared to the average oxygen value of 75% for all water entering the flat. It should be understood that these waters had also purified the raw Eiewage from Alexandria. Purdy computed the results per acre of tidal flats. The production of oxygen over that required for the raw sewage pollution was 17.7 pounds per acre per day. He concluded as follows: “This Huntington Flat provides sufficient oxygen for the first 24 hours natural purification of nearly 3,000,000 gallons of raw sewage from Washington. This is in addition to the first 24 hour natural purification of 6,000,000 gallons per day of raw sewage fvom Alexandria.” Nature has built an efficient waste disposal plant in the Potomac River. All of the factors involved appear to be a t their optimum effectiveness in carrying out the cycle of growth and decay, with sunlight and enzymes as the energizing forces. The absence of any one of these factors would seriously impair the balance. The sewage is the fertilizing agent for luxuriant growth of plants and organkms. The flats serve as reaction chambers for the biological process. The tides function as automatic pumps to move the water in and out of the chemical reactor-the flats. The plants and microscopic organisms serve a8 catalysts in transforming carbon dioxide from decayed organic substances into pla+t-building chemicals. Coproduct of this reaction is, the oxygen which transforms unstable raw sewage into stable, simpler, and nontoxic organic chemicals. these chemicals can break down partially to carbon diodde. Sunlight is the powerhouse for the cycle of biological and chemical reactions which occur.
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The sunlight penetrates this clear water readily and stimulates the growth of the chlorophyll-bearing plants and microscopic plant organisms. Such green vegetation producw, by this process of photosynthesis, two and one half times as much oxygen as the weight of carbon added to the plant tissue. These flats are, therefore, efficient chemical factories which produce oxygen to counteract the oxygen-consuming sewage burden placed upon the Potomac River. The tides exert a vital influence upon these flats. They increase many fold the effective length of the river and provide a long retention period for the water and the activity of organisms. They also mix the channel water, the water on the flats, and the sewage, which promotes the biological processes. These tides average about thirty inches. Without them these large shallow basins would become semistagnant bayous, and the virility of plant growth on the flats would be lost. The tides automatically lift huge amounts of contaminated channel water into the flats and 6 hours later they drop the now oxygenated water back into the river channel. The Huntington flat, just below Alexandria, Va., was particularly interesting to W. C. Purdy in 1916, because the entire aewage of that city passed over this flat on its way downstream. Purdy made extensive chemical and biological tests on the waters and muds of this flat for almost a year. This particular body of water was 1.3 square miles i n area and fronted the river for 1.25 miles. A marsh of about 0.18 square mile constituted the extreme upper portion most remote from the river. Three small streams entered the upper end of the marsh, two of which were heavily polluted by raw sewage from Alexandria. The water in these small streams was putrid where they entered the marsh, and when the bottom mud was stirred, the anaerobic decompositiotl gave rise to foul gases. The water leaving this marsh showed little evidence of pollution. Tests made near the middle of the fiat showed dissolved
The trickling filter and activated sludge processes used in industrial sewage disposal plants are based upon the same biological processes which take place in the Potomac River. Such processes have been employed by industry in some instances, but careful attention should be given to the elimination of toxic materials in waste before this biological principle is applied more extensively. (Continued on page 98) 97
The trickling filter is essentially a bed of crushed stone, 5 to 10 feet deep. The stone is uniform in size and varies from
0.5 to 3 inches in diameter, depending upon the preference of the engineer. After the raw sewage has been screened to remove the coarse suspended solids, it is sprinkled over the stones so that it will pass through the filter in 15 to 30 minutes. A copious deposit of slime is built up on the surface of the stones by the circulation of a suspension of sewage and activating bacteria through the filter. When the slime rewhes the desired effectiveness, 75 to 80% of the biological oxygen demand is usually removed. The activated sludge process is another method which simulates the biological actiop of self-purifying streams. Slime and bacteria do not coat the stones, as in the trickling filter, but are kept suspended in the sewage by vigorous agitation with air. I n this process the removal of B.O.D. can be 90 to 95% of the raw sewage value. These methods are not utilization processes; they merely dispose of the waste material. More attention should be given to actual utilization of the organic chemicals which result from the biological conversion of sewage. The tremendous volume of sewage available creates a challenge to technical men to find profitable products from the effluent of the trickling filter or activated sludge operations. New absorption chemicals and developments in organic chemistry during the war period should be combed carefully with this purpose in mind. The field is almost unexplored.
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Several versatile industrial companies have gone to the sewage disposal plant in search of water. A certain steel company had overpumped its underground water supply as did other near-by war-expanded industries. Many wells became contaminated with salt or highly acid waters, and the situation was critical. Wartime demands necessitated the expansion of the steel plant. Extensive surveys for water were made. Finally a chemist suggested using the sewage disposal effluent of a large near-by city. After careful study and correction of the sewage by control of industrial wmtes which entered the system, i t was found feasible to build a plant which could process 50,000,000 gallons per day of trickling filter effluent. The activated sludge effluent was satisfactory to the steel mill without further treatment. A simiiar use of sewage effluent from an activated sludge plant has been made by a midwestern railroad. An oil refinery is reported to be using trickling filter effluent. Studies have been completed which indicate that trickling filter emuent can be utilized as a substitute for underground waters in the wood pulping industry. In Europe sewage plant effluent is employed for fish culture as the chemicals in the water are considered to be suitable foods €or small fish. Such uses for the effluent of sewage disposal plants will expand, particularly where underground waters are a t critically low levels. But the real opportunity is for utilization of the chemicals present in processed sewage effluents. Obviously this suggestion brings up difficult problems, but difficulties encountered are problems to solve, not obstacles to defeat the objective. 98
