February, 1927
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Practical Application of Hydrogen-Ion Control in the Digestion of Sewage Sludge' By Stuart E. Coburn METCALF AND
EDDY,CONSClLTINQENGINEERS, BOSTON, MASS.
N THE digestion of sewage sludge in Imhoff tanks or in separate sludge-digestion tanks, particularly the latter, trouble has frequently been experienced with an acid condition accompanied by offensive odors and the production of foul-smelling sludge not readily dried. Attempts have been made to correct this acid condition by the use of lime. In some cases these attempts have been successful and in others unsuccessful, owing perhaps to the failure to produce and maintain the proper reaction throughout the tank. Much experimental work on the relation of pII value to sludge digestion has been done by Willem Rudolfs, chief of the Sewage Substation of the New Jersey Agricultural Experiment Stations, with the cooperation of John R. Downes, superintendent of the Joint Sewage Disposal Works a t Plainfield, N. J. Dr. Rudolfs found that the most favorable digestion took place a t pH 7.3 to 7.6. H. G . Baity, Rockefeller Foundation, research scholar a t the Harvard School
I
1 Presented before the Division of Water, Sewage, and Sanitation at the 72nd Meeting of the American Chemical Society, Philadelphia, Pa., September 5 to 11, 1926.
of Engineering, reached the same conclusion by independent investigations. This paper deals with the practical application of hydrogenion control of the digestion of sludge in an Imhoff tank a t Plainfield, Conn., to correct troubles encountered with unfavorable acid digestion which had produced a small amount of offensive sludge and a very large amount of offensive scum, Description of Plant The treatment plant a t Plainfield, Conn., consists of a screen chamber, sewage pumping station, a covered Imhoff tank, a dosing tank, and two acres of intermittent sand filters. It handles the sewage from a community of about eighteen hundred persons, including a large cotton mill owned by the Lawton Mills Corporation. The plant was designed to care for a population of two thousand persons and an average daily per capita flow of 40 gallons, equivalent to a total flow of 80,000 gallons per day. The actual flow, however, is nearly 120,000 gallons per day, owing largely to surface water and ground water entering the sewers. _-I
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Sectional Elevation A-A
Sectional Elevation B-B The Imhoff tank consists of three compartments-two upper compartments or sedimentation chambers, with a scum and sludge compartment below the sedimentation chambers, which have the ordinary trapped slots. A plan and sections of the tank are shown in the accompanying figure. It will be seen that the tank affords a large gas vent area and a large space for scum, and that a scum draw-off pipe has been provided, connecting with the sludge pipe. Troubles with Imhoff Tank
Details of Imhoff Tank
Operation of the plant was started on November 18, 1921, a time of year rather unfavorable for bacterial action. During the first winter there was practically no gas ebullition from the sludge compartmenf, and the scum accumulation was very small. About the middle of April scum began to form and this gradually increased, reaching a depth of over 6 feet on August 1. This Rcum had a hard surface crust covered with a grayish green mold and underneath it had a yellowish green color and a very offensive odor. On October 4, 1922, eleven
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months after the plant was put in operation, the scum had reached a depth of over 8 feet and solids began to rise through the slots, interfering with the efficiency of sedimentation. The scum was then removed by first shoveling off the heavy portion on top, which was in a spadeable condition and then drawing the remainder to the sludge beds. At the time of drawing, the sludge was yellowish brown, had a marked offensive odor, and was acid to litmus. It dried very slowly and the dried sludge was similar to that on sludge beds receiving crude sewage sludge from plain sedimentation tanks. There was very little sludge accumulation up to this time and that which was produced was yellowish green and very offensive. During the following winter considerably more gas was given off. than during the previous winter, showing that digestion of some kind was taking place, but scum continued to form, reaching a depth of 9 feet in May, when solids again rose through the slots, causing a great increase in the amount of skimmings to be removed from the sedimentation chambers. The scum was again drawn to the sludge beds. This scum was similar to that drawn previously, foul-smelling and acid in reaction. It contained a large amount of grain mash, evidently from home-brew operations. While this large amount of scum was accumulating, very little sludge was produced. Occasionally a little of this was drawn for observation, and this was always greenish yellow or yellowish brown, very offensive in odor, and acid to litmus paper. Seeding
On account of the failure of the tank to produce welldigested sludge of the character usually obtained from this type of tank, it was decided to seed the tank with well-digested Imhoff tank sludge. Therefore, on October 10, 1923, about 60 gallons of Imhoff tank sludge were transported from Fitchburg, Mass., for seeding. Previous to seeding as much sludge was drawn from the tank as practicable, reducing the sludge content to about 6000 gallons. The well-digested sludge was added by pouring it into the sludge compartment through holes made in the scum. Throughout the winter a large amount of gas was given off, indicating that digestion was taking place. But examination of the sludge, made during the following spring and early summer, showed it to be yellowish brown, offensive in odor, and acid to litmus, and it was evident that not enough of the good sludge had been added to correct the acid conditions. pH Control
As the seeding of the tank had not produced the desired results, it was decided to add hydrated lime to neutralize the acidity and determine whether favorable bacterial action could be obtained by controlling the pH. This treatment was started during the summer of 1923 and was first carried out by adding the dry lime at the rate of 50 pounds per week to the sewage in the sedimentation compartments, introducing the lime near the slots with a view to its passing through into the sludge compartment. A short time after beginning this treatment the filter beds became covered with a crust of carbonate of lime, which interfered with filtration. The method was then changed by sprinkling the lime over the surface of the scum and working it in with a long-handled shovel. This procedure was kept up until May, 1924, when for the first time since the plant was started the scum had the same tarry odor characteristic of Imhoff tank sludge, but i t still retained an underlying sour smell. A portion of this scum was drawn to the sludge beds and dried quite rapidly, being ready for removal within two weeks, whereas the scum
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drawn previously required a t least two months to dry, and even then was not in as good condition. Only a small amount of sludge had accumulated up to this time, but this had a faint tarry odor and was much better in appearance than before. On account of the favorable digestion which appeared to have been established, the addition of hydrated lime was stopped. With the approach of summer weather, both sludge and scum continued to improve without the aid of lime. The scum depth remained practically constant and sludge was formed in the normal manner. The sludge became grayish black with a marked tarry odor, and was alkaline to litmus. Sludge subsequently drawn to the sludge beds dried very readily and produced no offensive odor while drying. The favorable digestion continued throughout the winter, and sludge drawn the following spring and summer was excellent in all respects. Records of pH
The physical changes in the sludge and scum were accompanied by changes in the hydrogen-ion concentration, and some of the pH results are recorded in Table I. Table I DATE Sovember 6, 1923 February 11, 1924 March 21, 1924 M a y 16, 1924
SAMPLE
Scum Sludge Scum Sludge Scum Sludge Scum Sludge
June 28, 1924 Seutember 11. 1924 h-ovember 22, 1924
Scum Sludge Scum Sludge Scum Sludge
PH
NOTES
6.5 6.3
Yellowish green; offensive odor Yellowish green' offensive odor Yellowish green: objectionable odor 6 . 8 Yellowish brown; faint t a r r y odor; less objectionable 8 . 2 Yellowish brown; objectionable odor No sludge present 7:s Objectionabe odor with very faint t a r r y smell 6 . 8 Yellowish brown; slight tarry odor with underlying sour smell 7.6 ... 6.8 7 . 3 Grayish black; tarry odor 7 . 2 Grayish black; tarry odor 7 . 3 Grayish black; tarry odor 7 . 3 Grayish black; tarry odor 8.0
Recent Experience
On Kovember 1, 1925, by accident, nearly all of the sludge in the tank was drawn off, leaving little or no actively digesting sludge for seeding the freshly forming sludge. Thereafter, acid digestion began again with rapid accumulation of offensive scum, which had to be removed early this summer. Treatment of the accumulating scum with hydrated lime was then resumed in the same way as before; and a t the time of the last inspection the scum had changed to a dark, spongy mass, indicating that favorable digestion had become established once more. The pH value of the scum was 7.4. The unfortunate accident in drawing off all of the good sludge has thus led to confirmed demonstration of the value of pH control in sludge digestion. Conclusions
The experience in operating this Imhoff tank leads to the following conclusions: 1-It is undesirable to start a new Imhoff tank plant in the late fall when bacterial action is inhibited by low temperatures. 2-Considerable amounts of grain mash tend to promote acid digestion of the sewage solids. 3-Seeding is of no value unless a sufficiently large amount of good sludge is used. 4-Even under unfavorable conditions it is practicable to control the H-ion concentration of the sludge in an Imhoff tank by the judicious application of hydrated lime and obtain a pH value of 7.3 to 7.6, which is favorable to satisfactory sludge digestion. &-Having established favorable digestion, it may not be necessary to continue the application of lime.
