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hruch trouble has been experienced w(th the present method of introducing air into the sewage. It seems impossible to prevent oil from passing over with the air to the lower side of the porous discs where it forms a film which increases greatly the resistance t o the passage of air through the material. The carborundum is very finely divided and is easily choked up, which detracts greatly from its other advantages as part of a permanent installation. As the oil collects the air pressure increases and less air escapes through the discs because of greater slippage in the blower. The increasing resistance to passage through the discs acts in the same manner as the closing of a valve does on a stream of water. This serious difficulty is being considered with care. A limit to the air supply limits the amount of raw sewage that may be passed through the tank, and, until the proper flow is determined, an elastic air system covering wide variation of volume is necessary. The air washer has been rearranged several times with negligible improvement. It may become necessary t o replace the carborundum discs with more porous ones like filtros. This, of course, loses to the equipment the advantage of producing very small air particles, b u t the first and most important consideration is t o put the operation on a sound working basis, which cannot be done, seemingly, with the present discs. It was originally decided t o conduct experiments in another tank aerated by a propeller enclosed in a draft tube, b u t it was found impracticable to run both experiments a t the same time. Laboratory analyses and physical observations on the action of the experiments have developed to a great extent our knowledge of this form of sewage purification, but we feel, in order t h a t the final conclusions may have value, that no pains should be spared to know exactly and make a record of each step and change in everything pertaining t o the experiment. The temperature of inflowing and outflowing raw sewage, the temperature of the atmosphere and the compressed air, and rate of sewage flow are recorded every hour. The rate of air supply will be recorded after the air meter has been installed. We have already proved t h a t it is possible to purify sewage by an activated-sludge method and t h a t operation may be made continuous. It remains to determine the rate a t which sewage can be purified and the total cost of operation. DEPARTMGST OF PUBLIC I M P R O V E M E N T S BALTIMORE
COMPOSITION OF THE EFFLUENT AIR FROM AN ACTIVATED-SLUDGE TANK B y F. s. CRAWFORD .4ND EDWARD BARTOW Graduate Student, Chemistry Department, and Director of Illinois Water Survey
The remarkable purification effected by aeration in the presence of activated sludge indicates t h a t air plays a n important rBle in t h e process. As an increase in the content of carbon dioxide and a decrease in the content of oxygen of the effluent air were expected, determinations of those two constituents of the effluent air from tanks a t an experimental plant treating domestic sewage a t the University of Illinois were made from January to April, 1916. Carbon dioxide was determined according to the method of Hesse with an accuracy between one and two parts per ro,ooo. The air was measured over mater in an unjaclreted U'inkler burette, and its content of oxygen was determined by absorption in alkaline pyrogallol in a Hempel double absorption pipette for liquid reagents mith an accuracy between 0 . 2 and 0 . 3 per cent; no corrections were made for temperature or pressure. The influent air, obtained from the compressed-air supply of the University, was measured by a common gas meter at about j lbs. more than atmospheric pressure. The effluent air was collected in Erlenmeyer flasks of 500- to 800-cc. capacity by displacement of the medium through xhich the air passed, no
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chance for outside contamination thus being afforded. The carbon dioxide was determined in the flask in which it %-ascollected. The compressed air, according t o numerous analyses, has substantially the same composition as the atmosphere, containing 4 3 parts per IO,OOO of carbon dioxide and 20 j per cent of oxygen. Aeration was carried on according t o a regular schedule, each aeration period occupying j hrs. Hourly samples were generally taken, or 6 samples for each aeration period. An initial decline in the amount of carbon dioxide blown out usually occurs a t the beginning of the aeration period. A marked increase usually follows this decline though the final amounts are not always the highest. These variations are illustrated by the results of the first five series of tests in the accompanying table of representative data. Some analyscs showed a n increase from beginning as indicated in the sixth series. The arerage flow of air into 400 gal. of sewage was 187 cu. ft. an hour a t atmospheric pressure. CARBOh DIOXIDGI N EFFLCENT AIR EXPRESSED I N PARTS PER 10,000
Date (1916) Start Jan. 10 . . . . . . . . . . . . . Jan. 12 . . . . . . . . . 41.2 Jan. 13 . . . . . . . . . 32.2 . . . . . . . 37.1 Feb. 4.. Peb. 1 8 , . . . . . . . . 41.3 ................ 19.1 Average.. . . . . . . 39.67
1 hr 32.4 33.0 29.2 36.6 38.3 26,s 37.5
BY
VOLUME After Aeration 2 hrs. 3 hrs. 30.8 38.4 44.2 .... 38.0 37.2 37.1 33.9 38.3 37.0 31.0 33.5 38.02 36.41
o-f 4 hrs. 40.0 36.8
5 hrs. 45.6 50.6
38.8 43.5 42.0 38.94
37.6 42.2 66.8 39.42
....
7
....
The rate of flow of air has little relation t o the amount of carbon dioxide in the effluent air; with an average flow o€ 240 cu. f t . an hour the content of carbon dioxide was 4 j . 6 and 6 6 . 8 parts per 10,000, u-hereas a t another time, with an inflow of only 89 cu. ft. an hour, the content was as much as j o . 6 parts per IO,OOO. During a n aeration period, while the tank was filled with sewage and no sludge, the content of carbon dioxide was highest a t the beginning, dropping from 14,2 t o 8 . 6 parts per 10,000. During another, on the second day after sludge had begun t o accumulate the contents of carbon dioxide a t the beginning and the end were substantially alike, 19.7 and 2 0 . 8 parts per 10,ooo. When the content of carbon dioxide increased, the content of oxygen of the effluent air was I ,o t o I ,2 per cent less than that of the influent air. The effluent air contained 19.3 t o 1 9 ,j per cent of oxygen; that is, about j per cent of the oxygen in the air was used in the process. The increase o€ carbon dioxide probably could not take place by simple oxidation of carbonaceous matter without bacterial action. I t was believed that some of the carbon dioxide might be dissolved in the sewage; that the initial decrease in content of carbon dioxide of the effluent air was due t o loss of dissolved carbon dioxide was proved by aerating t a p water under conditions similar to those under which sewage was aerated. \\'hen tap n-ater in a tank was aerated the temperature remained constant throughout the aeration period, but the content of dissolved carbon dioxide rapidly decreased as shown by tests of half-hour samples. The general average content a t the start in three experiments was 2 2 . 8 , a t the end of I hr. 14.1, a t the end of z hrs. 6 . j , and a t the end of z1/2 to 3 hrs. it became 0 . 0 . The disappearance of dissolved carbon dioxide was quickly followed by an alkalinity in presence of phenolphthalein, which slowly increased t o 26 to 32 parts per million a t the end of the aeration periods. The total alkalinity, a.bout 3j o parts per million, remained practically constant in the experiments. The arerage content of carbon dioxide of the effluent air at the beginning of aeration was.30 parts per 10,000, and it decreased steadily throughout aeration though it decreased to the amount normally present in the atmosphere in only one experiment. The average content of oxygen of the effluent air a t the start was 1 9 . 6 per cent, but before the end of aeration the amount normally in the atmosphere. 2 0 . 5
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T H E JOURNAL OF I N D U S T R I A L A N D ENGINEERIXG CHEMISTRY
per cent, was attained. The low oxygen a t the start was probably due to the high content of iron of the water. The weight of carbon dioxide in the effluent air, calculated from the average content of carbon dioxide of the effluent air and the volume of air used, was 46 to j I g. per run. The weight of carbon dioxide originally in the water, calculated from the dissolved carbon dioxide and the capacity of the tank, was 43 to 56 g.; that is, the carbon dioxide content of the effluent agreed reasonably well with the content of dissolved carbon dioxide in the original tap water. The alkalinity in presence of phenolphthalein resulted from the breaking down of bicarbonates naturally in the water. According to the titrations in the experiments the amounts of carbon dioxide set free varied from j z to 64 g. per run. This gas, in accord with the law of Henry, was not liberated because the inflowing air contained carbon dioxide. When sewage was aerated in the presence of sludge the net gain of the effluent air in content of carbon dioxide was IZj g. If 46 to j~ g. are deducted for the carbon dioxide in the effluent air obtained by blowing air through tap water, the 7 j to 80 g. of carbon dioxide that remains may be considered as having been produced by bacterial action during the process of aerating sewage. That this deduction is warranted is shown by the fact that when fresh sewage was aerated without sludge the dissolved carbon dioxide disappeared and the phenolphthalein alkalinity appeared a t nearly the same stage of aeration that it did when tap water was aerated. Sewage allowed to stand after the aeration period loses its alkalinity to phenolphthalein. The sewage was not alkaline in presence of phenolphthalein 17 hrs. after aeration had ceased. When tap water was used, however, the alkalinity in presence of phenolphthalein remained several days, and the total alkalinity remained unchanged. The disappearance of the alkalinity to phenolphthalein of the raw sewage was evidently due to putrefaction of the sewage because of lack of air Strong bacterial action does not take place for the first few hours.
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the beds were unable to treat adequately the sewage, some of which is I j hrs. old when it gets to the beds. A few years ago the area of the beds was increased by I O acres and three years ago a 0.j acre trickling filter was constructed. The results from the trickling filter have been disappointing; very little nitrification (only 4.55 parts of nitrates per million on an average in 1915) has been secured. The effluent was stable for only about two days. Furthermore, the effluent from the secondary or humus tank is always putrid and the trickling filter unloads not intermittently but continuously throughout the year. Considerable relief has been obtained by resting the filter for two hours daily, but even a t rates as low as j4j,ooo gal. per acre per day the results were unsatisfactory. During passage through the humus tank the nitrates in the trickler effluent were reduced from 4 . 5j t o 3.32 parts per million and the oxygen from 2 ,2 to I , 6 parts per million. This is due to vigorous decomposition of the sludge, the so-called humus, which a t Brockton has all the characteristics of a badly digested septic-tank sludge. When the trickler effluent was applied to sand beds a t a Igo,ooo-gal rate, free ammonia was decreased from 30.6 (which is high for a trickling-filter effluent) to 1 6 . 3 parts per million, and nitrite from 0,732 to 0.069; while, strange to say, nitrate was decreased in the intermittent sand filter from 4 . j j to 2 , 6 2 parts per million. The remedy for a sewage so stale and one which cries for oxygen so loudly is obviously more air. I n 1915, aeration experiments were made with a cascade aerator 20 ft. long having a slope of I f t . in IO. The devices were so arranged that the sewage could be passed over this aerator ten times, but this was not enough to show the presence of the slightest trace of dissolved oxygen in the effluent. After each aeration the sewage was passed through a bed of cobbles 6 in. in diameter in order to make use of the contact action employed so successfully by Mr. Clark a t Lawrence in his aerated tank but all without sufficient benefit. The effluent from this aerated contact tank passed through an Imhoff tank with little if any change, and after the effluent from the Imhoff tank had been UNIVERSITY OF ILLINOIS, URBANA applied to a trickling filter the results were not enough better than those obtained by the municipal filter to warrant further continuance of the experiments. The colloidal matter was not SEWAGE DISPOSAL EXPERIMENTS AT BROCKTON, appreciably decreased in amount but was carried through the MASSACHUSETTS whole row of devices. By ROBERT SPCRRWESTOS Along with the other 1915 experiments a small activaConsulting Sanitary Engineer, Boston ted-sludge tank was operated on the fill-and-draw plan. This The city of Brockton, which has a present population of 62,000, tank was aerated for 4-hr. periods and contained Z j per cent of is located in southeastern Massachusetts, and is sewered on the sludge. The effluent was discharged on an intermittent sand separate system. The sewage drains to a low point in the city, filter 5 ft. thick a t a rate of 500,ooo gal. per acre per day. T h e where it collects in a subsiding basin. It is then fine-screened results were excellent and in striking contrast with the results and pumped to the disposal area about three miles away. obtained by the municipal sand beds receiving the trickling The disposal works consist of 37 acres of sand beds and a trickling filter having an area of acre. The average daily filter effluent. The effluent from the activated sludge tank conflow of sewage is 2,100,ooo gal. Approximately two-thirds tained 254 parts of suspended matter, 3 7 , 6 parts of free ammonia, of the screened sewage is applied directly to 30 acres of sand no nitrite, only a trace of nitrate, and 3 . 2 8 parts per million beds a t an average rate of 52,000 gal. per acre per day; the re- of dissolved oxygen. Nevertheless, when this effluent was apmainder is first applied to the o . j acre of trickling filter and then, plied to the experimental sand filter filled with sand from one of the beds which had been receiving trickling filter effluent, after passing through a secondary subsiding basin, to 7 acres of excellent results were obtained; i. e., free ammonia was decreased sand beds. The rate of application on the sand beds is about to 6.62 parts and nitrate was increased to 13.89 parts per mil130,000 gal. per acre per day. According to the last report of the Sewerage Commission lion on the average, while the effluent was brilliant in appearance and stable even a t the high rate of j o 0 , O o ~gal. per acre the sewage has the average composition shown in Table I: per day. The activated sludge experiments were so promising TABLEI-AVERAGE COMPOSITION(P. P. M.) OF SCREENED BROCKTON that they have been continued during 1916 on the continuous S E W A G E DURING 1914 Total Suspended Free ALBUMINOIDAMMONIA CHLORINE plan with a large aerating tank I O it. square and 8 ft. deep Solids Solids Ammonia T o t a l Dissolved Suspended below the flow line, discharging into a settling tank and from it 778 204 55.8 12.6 6.9 5.7 138.6 on a sand bed I O ft. square and 2.5 f t . thick that operates a t a half-million rate. The original plant, built in 1894, consisted of 27 acres of sand beds. These gave excellent results for many years after 4 s the object of these experiments was to devise a process their installation, but with increase in the amount of sewage t o relieve the present plant as much as possible, no attempt and addition of an increasing amount of shoe-factory waste, was made to obtain complete nitrification but only a clarified
