The Canadian Engineer, Volume 30. April 27, 1916. pg. 473
A resume of results of experiments carried on during the year 1915 by the Milwaukee Sewerage Commission
By R.O. Wynne-Roberts, M.Can.Soc.C.E.
The second annual report of the Milwaukee Sewerage Commission (see The Canadian Engineer, October 18, 191 5) has just been published. It is a volume of over 200 pages and has many drawings, diagrams, photos and tables which afford the reader abundant matter to ponder over.
The activated sludge process has been investigated in various scales of magnitude ranging from the laboratory to a large working installation, and also by the intermittent and continuous methods of charging. The writer well remembers visiting Exeter and Yeovil (England) in 1897 to study the septic tanks and filters, and has watched the vicissitudes of that system up to the present with interest, because great and excellent results were anticipated by enthusiasts who had not fully considered the problem. The septic tank is capable of doing good work when carefully managed, and the biological filters have proved satisfactory when operated properly. The septic tank was boomed and the predictions were great, but its originators were not entirely to blame for the undue boosting it received. The public was then seeking a solution of the sewage problem as it is doing to-day, although in the meantime much progress has been made.
The activated sludge process is capable, under scientific control, of producing such wonderful results, that the public is apt to enthuse and raise too high expectations to be realized in practice. Hence the fortunate fact that the process is receiving careful and scientific investigations in its early stages before errors are made on any serious scale. On the other hand, the Milwaukee Sewerage Commission is to be congratulated on its enterprise in undertaking such investigations in a comprehensive manner.
A resume of the experiments tried out during the year was as follows : Fine and coarse screening ; grit chambers ; sedimentation and sludge digestion in Imhoff tank ; colloidal treatment by slate tanks ; chemical precipitation using lime and iron ; electrolytic treatment by Lautzenheiser process ; percolating filters and final sedimentation ; sterilization by liquid chlorine ; activated sludge process by fill and draw method ; activated sludge process by continuous flow method ; dehydrating sludge by pressing, gravity and by draining on beds.
In the present review experiments other than those on the activated sludge process will not be discussed. These experiments were started on March 1st, 191 5, by means of two 1 1/4 -inch glass tubes 6 ft long, and next in a tank 32 ft. 0 in. x 10 ft. 6 in. and 9 ft. average depth, on the fill and draw method which was put into operation on March 26th, 1915. The continuous flow tank was put into commission on June 28th, and had the capacity of 22,600 U.S. gallons (18,830 Imperial gallons).
The foregoing experiments produced such promising results that the commission decided to install a plant capable of treating two million (U.S.) gallons per day, with the view to ascertaining the results under normal working conditions. This plant consists of eleven reinforced circular concrete tanks 30 ft. internal diameter by 13 ft. deep, eight of which are used as aerating tanks, one as a final sedimentation tank and two as sludge aerating tanks. The layout is shown in Figs. 1 and 2. The total capacity of the eight aerating tanks is 360,000 U.S. gallons, two sludge tanks 88,200 U.S. gallons, and the sedimentation tank 33,000 U.S. gallons. The daily working capacity of the plant with 25 per cent activated sludge in the aerating tanks and four-hour tank passage is 1,620,000 U.S. gallons, and by reducing the tank passage period to three hours the working capacity is increased to 2,160,000 U.S. gallons.
Each of the aerating and sludge tanks is divided by a baffle wall which makes a spiral running through chamber about 6 ft. wide and 114 ft. long. Each chamber has a sloping bottom in the apex of which 12-in. x 12-in. filtros plates are set in castings built in units containing from five to seven plates. These castings have an air duct cast in them which discharges the air through a brass orifice to the under side of the plate. This orifice is designed to pass 2 cu. ft. of air per minute under 5 pounds pressure per square inch. This capacity being based upon our experiments showing maximum air required to be .25 cu. ft. per minute per square foot of tank surface.
The baffle walls separating the running through chambers of the eight aerating tanks are 2 ins. thick, built of Hyrib plastered with cement mortar. Those in the two sludge tanks are 6 ins. thick, built of the same material. The extra thickness is here necessary because each of the sludge tanks is divided into two distinct compartments permitting one compartment to be emptied while the other
is being filled and aerated.
The sedimentation tank is built with a hopper bottom terminating in a 4-ft. diameter cast iron pipe 24 ft. below bottom of tank, from near the bottom of which a 12-in. pipe extends to the top of the tank. Inside of this 12-in. pipe is a 1-in. pipe for delivering air to the sludge by which
it is lifted from the bottom of the 4-ft. pipe and delivered
to the sludge tanks, or to the sludge presses.
The compressed air for the two-million-gallon plant is furnished by means of a Connorsville blower having a capacity of 2,400 cu. ft. of free air per minute to five pounds per square inch pressure. This is operated by a 75 h.p., a.c, variable speed motor. The air, after passing through an Excelsior filter, is measured by General Electric Company air meters.
The sewage is taken from an intercepting sewer which discharges about 12,000,000 gallons of sewage per day and some of this originates seven miles above the outlet and is quite septic on reaching the works.
As this plant was not completed until the end of 1915, not much information is available as to the results obtained.
Without in any way minimizing the excellent work done at Milwaukee, and before discussing some of the results obtained there it will be instructive to refer to a few of the engineering considerations, which require to be further and more fully developed. These are referred to in the report and go to prove that to obtain the maximum efficiency at the least cost it involves many studies and experiments.
“Activated sludge accomplishes four principal functions : The clarification of the liquor, removal of the putrescible organic matter, reduction of bacteria, and finally, if the process be continued a sufficient period, oxidizes the ammoniacal compounds into nitrates.”
William R. Copeland, the chief chemist, defines the process as follows : — “The sludge embodied in sewage, and consisting of suspended organic solids, including those of a colloidal nature, when agitated with air for a sufficient period, assumes a flocculent appearance very similar to small pieces of sponge. Aerobic and focultative aerobic bacteria gather in these flocculi in immense numbers, from 12 to 14 million per c.c. ; some having been strained from the sewage, and others developed by natural growth. Among the latter are species which possess the power to decompose organic matter, especially of an albuminoid or nitrogenous nature, setting the nitrogen free; and others, absorbing this nitrogen, convert it into nitrites and nitrates. These biological processes require time, air and favorable environments, such as suitable temperature, food supply and sufficient agitation to distribute them throughout all parts of the sewage.”
As the supply of air under suitable conditions is a primordial requirement, experiments were made with air jets, filtros plates, monel metal cloth, and Kisselghur, to ascertain the best practicable method of diffusing the air and the relative area of the diffusers to that of the tank was investigated. Whether superactivated sludge will lead to an economy in the supply of air in the tanks is a most interesting problem to study. The most effective depth of sewage under aeration to absorb the maximum of oxygen as the air passes through, as well as the effects of low temperature of the air on the efficiency of the process, are receiving attention, whilst the best method of applying the process according to the varying hourly flows of sewage and to the fluctuating strengths of the liquor will afford engineers and chemists ample scope for their ability and ingenuity. The advantages of continuous flow over intermittent flow methods do not appear to be very pronounced except as to the cost of construction and operation, where the continuous flow tanks are superior, although “with a wide variation in strength of sewage and rate of flow a more uniform standard of effluent can be obtained with the fill and draw method because it is susceptible of better control.”
The question of how best to reduce the sludge to a fertilizer and extracting the grease is a most important one, because it is anticipated that sludge, which is now often an abomination to be got rid of by any means, will in future be saleable at profit, and if this is accomplished, then the ancient slogan of “back to the land” will be realized with advantage to the municipal authorities and to the farmers.
If space is available, fuller references will later on be made to the various engineering problems referred to, because the success of a sewage treatment plant depends largely on the careful development of details. A fifty million-gallon plant is not constructed very often and, furthermore, what might answer admirably under scientific management which can be obtained for large works, may not be equally attainable at average installations.
Mr. Copeland’s report is a comprehensive one and provides statistics which show what results were obtained, and these will now be discussed.
The sewage was screened through a 3/4-in. bar screen to remove the coarse materials, passed through a grit chamber to settle out mineral substances and sampled every hour in a gallon measure. A representative portion of the sample — about 250 c.c. — was taken out each hour, chloroformed and put into a bottle which was packed in ice so as to suspend biological changes pending the time when a 24-hour collection of samples was available for analysis. Settleable solids were measured in tapering glass vessels ; portions of sewage were filtered through filter paper, and these, as well as unfiltered sewage, were evaporated to dryness to determine the weights of the total and soluble solids — the difference being recorded as suspended matter. Free ammonia was determined by direct Nesslerization. The tests for numbers of bacteria contained by the sewage were made upon agar incubated at 20° C. for 48 hours, the sample being diluted with sterile distilled water.
From the results obtained at Manchester by the originator of the activated sludge process, Messrs. Adern and Lockett, it would appear that the time required for the maturing of sludge was in the first instance about six months, but this period has been reduced considerably. As already stated, the first experiments upon this process at Milwaukee were started in the laboratory about March 1st, 1915. The apparatus used consisted of two glass tubes 6 ft. long by 1 1/4 ins. diameter. At the bottom of one tube a filtros plate was placed to diffuse the air through the mixture. A small glass tube was placed inside the other tube to carry the air to and discharge it near the bottom of the tube in an open jet.
On March 6th these tubes were filled with raw, coarse screened sewage and some sludge from the final sedimentation tank of the 8-ft. sprinkling filter, air was turned on, of unknown quantity, but sufficient to keep the mixture in violent agitation. On March l0th, or at the end of four days’ continuous aeration, the sewage was clarified, the sludge had a brown color and settled readily. The clear liquor was decanted off, raw sewage substituted and the mixture continuously aerated for another 24 hours, when the air was shut off.
Within one-half hour the sludge had settled, leaving a clear supernatant liquor, containing 20 parts per million of nitrates. Fresh sewage was again placed in the tubes ; within 12 hours after aeration began nitrites and nitrates were present and at the end of 24 hours’ continuous aeration the liquor contained but a trace of free ammonia, one half part of nitrite and 20 parts of nitrates.
Dr. Edward Bartow, of Urbana, was also able to produce activated sludge in a few days.
The laboratory test showed that an open air jet gave about equal results as the filtros plate diffuser. The results gave such promise that a tank which had been used for chemical precipitation experiments was converted into a fill and draw tank for trying out the activated sludge process. This tank has a capacity of 22,200 (U.S.) gallons or 18,500 Imperial gallons. There are 50 filtros plates, each I sq. ft. in area. The area of the tank is 336 sq. ft. and filtros plates 50 sq. ft. or a ratio of 1 to 6.7. The air was measured by a venturi meter and controlled by a hand valve. The cycle of operation was, roughly, as follows : — filling, I hour; aeration, 3 1/2 hours; settling, 1/2 hour; decanting, 1 hour. Samples were collected before and after treatment and the average monthly results of the analyses were as follows : —
| Crude Sewage | Effluent from Tank | Per centage Removed | |
|---|---|---|---|
| Settleable solids in two hours, cu. yds per million gallons | 17.5 | 0.8 | 95.4 |
| Suspended matter, in p.p.m. | 250 | 12 | 95.3 |
| Total solids in p.p.m. | 1,067 | 777 | 27.1 |
| Nitrogen as free ammonia, p.p.m. | 14.61 | 9.70 | 32.4 |
| Nitrogen as alb. ammonia, p.p.m. | 30.3 | 13.5 | 55.5 |
| Nitrogen as nitrite, p.p.m. | 0.24 | 0.81 | |
| Nitrogen as nitrate, p.p.m. | 0.42 | 2.51 | |
| Oxygen consumed, p.p.m. | 116 | 23 | |
| Alkalinity, p.p.m. | 255 | 240 | |
| Chlorine, p.p.m. | 185 | 183 | |
| Dissolved oxygen, p.p.m. | 1.1 | 5.2 | |
| Temperature, degrees Fahr. | 62 | 63 | |
| No. bacteria per c.c. at 20 C. in millions | 2.100 | 0.115 | 95.3 |
This tank treated sewage at the following average daily rates : —
June: 52,911 gallons, 1.76 cu. ft. of air per gallon
July: 56,308 gallons, 1.91 cu. ft. of air per gallon
August: 72,539 gallons, 2.05 cu. ft. of air per gallon
September: 76,950 gallons, 2.00 cu. ft. of air per gallon
October: 57,097 gallons, 1.36 cu. ft. of air per gallon
November: 60,824 gallons, 2.14 cu. ft. of air per gallon
Average amount of sewage treated daily, 62,771 gallons.
Average amount of air used per gallon, 1.87 cubic feet.
The figures for May and December are not given.
It is interesting to note that whereas Messrs. .Adern and Lockett, in their experiments in 1914, found the amount of free air used did not exceed 15 cu. ft. per square foot of tank area per hour with a sewage depth of about 5 ft. , whereas the average at Milwaukee for sewage 9 ft. deep was about 25 cu. ft., Mr. Adern thought 12 cu. ft. would suffice, and Dr. Bartow is reported to have succeeded in using still less quantity of air.
Turning now to the continuous flow method. Another of the former chemical precipitation tanks was remodeled for this purpose. This tank is 32 ft. 0 in. x 10 ft. 6 ins. X 10 ft. 0 in. deep, divided longitudinally into three compartments. The sewage is admitted into and flows through the left-hand into the right-hand and out by the central compartment, and in doing so it travels 81 ft. and is aerated. The sludge, being very flocculent, is carried forward and settles in the sedimentation chambers whence it is pumped back to the point of sewage inlet. The air is distributed and diffused in the same way as in the fill and draw tank.
The continuous flow tank was operated by filling it with sewage on June 28th, 1915, running in 1,200 gallons of sludge from the Imhoff tank and from the secondary sedimentation tanks of the sprinkling filters. More sludge was run in on July 13th and 16th, and by the 19th July the black sludge had by aeration turned brown. The tank was put into working commission on August 5th. Samples were taken and analyzed and the following figures show what were the average results during four months, August to November inclusive.
| | Crude Sewage | Effluent from Tank | Per centage Removed |
|---|---|---|---|
| Settleable solids in two hours, cu. yds per million gallons | 20.77 | 2.4 | 87.7 |
| Suspended matter, in p.p.m. | 310 | 15 | 95.2 |
| Total solids in p.p.m. | 1,165 | 834 | 35.0 |
| Nitrogen as free ammonia, p.p.m. | 9.53 | 4.77 | 50.0 |
| Nitrogen as alb. ammonia, p.p.m. | 34.6 | 11.1 | 67.7 |
| Nitrogen as nitrite, p.p.m. | 0.20 | 0.41 | |
| Nitrogen as nitrate, p.p.m. | 0.17 | 5.94 | |
| Oxygen consumed, p.p.m. | 123.0 | 22.0 | 82.3 |
| Alkalinity, p.p.m. | 192 | | |
| Chlorine, p.p.m. | 196 | | |
| Dissolved oxygen, p.p.m. | 1.09 | 4.3 | |
| Temperature, degrees Fahr. | 64 | 64 | |
| No. bacteria per c.c. at 20 C. in millions | 1.620 | 0.086 | 95.0 |
The continuous flow tank treated sewage at the following average daily rates : —
August: 19,745 gallons, 4.52 cu. ft. of air per gallon
September: 47,755 gallons, 2.01 cu. ft. of air per gallon
October: 59,137 gallons, 1.62 cu. ft. of air per gallon
November: 60,393 gallons, 2.09 cu. ft. of air per gallon
Average amount of sewage treated daily, 46,760 gallons. Average quantity of air used per gallon, 2.56 cubic feet.
Omitting August, the averages are: Amount of sewage treated, 55,762 gallons; quantity of air used, 1.91 cu. ft. per gallon.
It is instructive to compare the results obtained by the two methods.
| Fill and Draw Method | Continuous Method | |
|---|---|---|
| Reduction in settleable solids | 95.4 % | 87.7% |
| Reduction in suspended solids | 95.3% | 95.2% |
| Reduction in total solids | 27.1% | 28.3% |
| Reduction in free ammonia | 32.4% | 55.0% |
| Reduction in alb. ammonia | 72.3% | 50.0% |
| Reduction in organic nitrogen | 55.5% | 67.7% |
| Reduction in oxygen consumed | 80.0% | 82.3% |
| Reduction in bacteria | 95.3% | 95.0% |
| Average number of tank volumes treated daily | 2.8 | 2.5 |
| Average quantity of air per gallon in cubic feet | 1.87 | 1.91 |
There would appear to be but little difference in the results obtained by the fill and draw or the continuous flow methods. A number of experiments, however, were made apart from the general tests and the results point to possibilities of great interest.
The cost of furnishing compressed air at five pounds pressure is estimated by Mr. Halton at $2.50 per million cubic feet of free air and on this basis the cost of treating sewage on the fill and draw method, using 1.87 cu. ft. per gallon, would be about $4.67 per million U.S. gallons, and by the continuous method, using 1.91 cu. ft. per gallon, $4.78 per million U.S. gallons, but Mr. Copeland states that 1,000,000 (U..S.) gallons can be clarified, freed from 95 per cent, of its bacteria and rendered stable for five days by the application of 1.75 cu. ft. of air per gallon at a cost of $4.38, which includes overhead charges upon that portion of the plant devoted exclusively to treatment of sewage but excludes plant and engine room labor and cost of disposing of the sludge.
Source Link: Internet Archive