275. The Miles Acid Process.—The Miles Acid Process for the treatment of sewage was devised and patented by G. W. Miles. It was tried experimentally at the Calf Pasture sewage pumping station, Boston, Mass., 1911 to 1914. In 1916 it was tried experimentally at the Massachusetts Institute of Technology, and it has been tested subsequently at other places, notably at New Haven, Conn., in 1917 and 1918. It is one of the most recent developments in sewage treatment and no extensive experience has been had with it. The process consists in the acidification of sewage with sulphuric or sulphurous acid, as the result of which the suspended matter and grease are precipitated and bacteria are removed. The equipment required for the process consists of devices for the production of sulphur dioxide (SO₂), and for feeding niter cake or other forms of acid; subsiding basins; sludge-handling apparatus; sludge driers; grease extractors; grease stills; and tankage driers and grinders.

The first step is the acidification of the sewage. The period of contact with the acid is about 4 hours. Sulphurous acid seems to give better results than sulphuric because of the ease in which it can be manufactured on the spot. It seems also to be more virulent in attacking bacteria than an equal strength of sulphuric acid. In experimental plants the acidulation has been accomplished in different ways such as: by the addition of compressed sulphur dioxide from tanks; by the addition of sulphur dioxide made from burning sulphur; or by the roasting of iron pyrite (FeS₂). The acidulation precipitates most of the grease as well as the suspended matter and results in a sludge which gives some promise of commercial value. In referring to the process R. S. Weston states:^[188]

(1) It disinfects the sewage by reducing the numbers of bacteria from millions to hundreds per c.c.

(2) If the drying of the sludge and the extraction of the grease can be accomplished economically, it is possible that a large part, if not all, of the cost of the acid treatment may be met by the sale of the grease and fertilizer recovered from the sewage.

(3) The use of so strong a deodorizer and disinfectant as sulphur dioxide would prevent the usual nuisances of treatment works.

(4) The addition of sulphur dioxide to the sewage also avoids any fly nuisance, which is a handicap to the operation of Imhoff tanks and trickling filters.

The amount of acid used varies with the quality of the sewage and the desired character of the effluent. At Bradford, England,^[189] 5,500 pounds of sulphuric acid are used per million gallons, producing about 2,340 pounds of grease or 0.43 pound of grease per pound of sulphuric acid. At Boston only 0.215 pound of grease were produced per pound of sulphuric acid. The difference is probably due to the great difference in the amount of grease in the raw sewage. In the East Street sewer at New Haven, Conn.,^[190] only 700 pounds of acid are used per million gallons of sewage as the alkalinity is only 50 p.p.m. This amount of acid secures an acidity of 50 p.p.m. whereas in the Boulevard sewer 1,130 pounds of acid had to be added to produce the same result. The results obtained by the experiments conducted by the Massachusetts State Board of Health in 1917 are shown in Table 97. The character of the sludge from the same tests is shown in Table 98. After acidification^[191] the sewage contains bisulphites and some free sulphurous acid, with some lime and magnesium soaps which are attacked by the acid liberating the free fatty acids. Part of the bisulphites and sulphurous acid are oxidized to bisulphates and sulphuric acid. It was found as a result of the New Haven^[191] experiments that the presence of sulphur dioxide in the effluent caused an abnormal oxygen demand from the diluting water and that this difficulty could be partly overcome by the aËration of the effluent after acidulation and sedimentation, without prohibitory expense. The effluent and sludge are both stable for appreciable periods of time and are suitable for disposal by dilution. The character of the sludge as determined by the New Haven tests^[192] is shown in Table 99.

The success of the Miles Acid Process in comparison with other processes is dependent on the commercial value of the sludge produced. The New Haven experiments indicate that 16 to 21 per cent of the grease in the sludge is unsaponifiable and seriously impairs the value of the process.

The conclusions reached as a result of the New Haven experiments are:^[193]

Our experience with New Haven sewage lends no color to the hope that a net financial profit can be obtained by the use of the Miles Acid Process, except with sewage of exceptionally high grease content and low alkalinity. They do, however, suggest that for communities where clarification and disinfection are desirable—where screening would be insufficient and nitrification unnecessary—the process of acid treatment comes fairly into competition with the other processes of tank treatment, and that it is particularly suited to dealing with sewages that contain industrial wastes, and to use in localities where local nuisances must be avoided at all costs and where sludge disposal could be provided for only with difficulty.

The conclusions reached as a result of the Chicago experiments are:^[194]

The results on hand indicate that treatment of this sewage with acid results in a somewhat greater retention of fat. An apparent reduction in the oxygen demand over that resulting from plain sedimentation, while remarkable, is probably not real, being simply due to a retardation of decomposition by the sterilization of the bacteria present, the organic matter being left in solution.... However, there appears the added cost of acid treatment and the cost of recovery of the grease, as well as the uncertainty of the price to be received for the grease recovered.

The cost of the treatment is estimated by Dorr to be $18 per million gallons, and the value of the sludge obtained from the Boston sewage as $24 per million gallons, giving a net margin of profit of $6 per million gallons. At New Haven, the total return is estimated at $7.09 per million gallons. Based on the production of sulphur dioxide by burning sulphur (assumed to cost $36 per long ton) and on drying from 85 per cent to 10 per cent moisture with coal assumed to cost $7.50 per ton, it appears that the acid treatment of sewage should be materially cheaper than either the Imhoff treatment or fine screening under the local conditions. A comparison of the cost of the treatment of the East Street and the Boulevard sewage at New Haven and the Calf Pasture sewage in Boston is given in Table 100. The cost of construction was estimated by Dorr and Weston in 1919 as greater than $15,000 per million gallons of sewage per day capacity.

Electrolytic Treatment

276. The Process.—This process has been generally unsuccessful in the treatment of sewage and has grown into disrepute. In the words of the editor of the Engineering News-Record:^[195]

Thirty years of experiments and demonstrations with only a few small working plants built and most of them abandoned—such in epitome is the record of the electrolytic process of sewage treatment.

It is probably true that the process has never received a thorough and exhaustive test on a large scale, but the small-scale tests have not been promising of good results. Among the most extensive tests have been those at Elmhurst, Long Island,^[196] Decatur, Ill.,^[197] and Easton, Pa.^[198]

Whatever degree of popularity the method has possessed has been due possibly to the mystery and romance of “electricity” and to the personality of its promoters. The process should, nevertheless, be understood by the engineer in order that it may be explained satisfactorily to the layman interested in its adoption.

In this process, sometimes called the direct-oxidation process, all grit is removed and the sewage is passed through fine screens before entering the electrolytic tank. In the electrolytic tank the sewage passes in thin sheets between electrodes and an electric current is discharged through it. A recent development has been the addition of lime to the sewage at some point in its passage through the electrolytic tank. From the electrolytic tank the sewage flows to a sedimentation tank, where sludge is accumulated, and from which the liquid effluent is finally disposed of.

It is claimed that the action of the electricity electrolyzes the sewage, releasing chlorine, which acts as a powerful disinfectant. The constituents of the sewage are oxidized so that the dissolved oxygen, nitrates, and relative stability are increased and the sludge is rendered non-putrescible. It is said that the addition of lime increases the efficiency of sedimentation and enhances the effect of the electric current. The results obtained by tests at Easton, Pa., are shown in Table 101. It will be observed from this table that the combination of lime and electricity does not have a more beneficial effect than either one of them alone. The amount of sludge produced by the combination is about the same as by chemical precipitation alone, but the character of the sludge produced with electricity is less putrescible. The cost of the treatment as estimated at Elmhurst is shown in Table 102.

As a result of the tests at Decatur, comparing lime alone with lime and electricity together, Dr. Ed. Bartow stated:

The purification by treatment with lime alone was greater than that obtained in several of the individual samples treated with lime and electricity.

Disinfection

277. Disinfection of Sewage.—Sewage is disinfected in order to protect public water supplies, shell fish, and bathing beaches; to prevent the spread of disease; to keep down odors, and to delay putrefaction. Disinfection is the treatment of sewage by which the number of bacteria is greatly reduced. Sterilization is the destruction of all bacterial life, including spores. Ordinarily even the most destructive agents do not accomplish complete sterilization. Chlorine and its compounds are practically the only substances used for the disinfection of sewage. The lime used in chemical precipitation, the acid used in the Miles Acid Process, the aËration in the activated sludge process, all serve to disinfect sewage, but are not used primarily for that purpose. Copper sulphate has been used as an algaecide but never on a large scale as a bactericide.^[199] Heat has been suggested, but its high cost has prevented its practical application to the disinfection of sewage.

The action which takes place on the addition to sewage of chlorine or its compounds is not well understood. The idea that the bacteria are burned up with “nascent” or freshly born oxygen, has been exploded.^[200] Likewise the idea that the toxic properties of chlorine have no effect has not been borne out by experiments. It has been demonstrated, particularly by tests on strong tannery wastes, that the action of chlorine gas is more effective than the application of the same amount of chlorine in the form of hypochlorite. All that we are certain of at present is that the greater the amount of chlorine added under the same conditions, the greater the bactericidal effect.

Chlorine is applied either in the form of a bleaching powder or a gas. In ordinary commercial bleach (calcium hypochlorite) the available chlorine is about 35 to 40 per cent by weight. In order to add one part per million of available chlorine to sewage it is necessary to add about 25 pounds of bleaching powder or 8½ pounds of liquid chlorine per million gallons of sewage. This can be computed as follows:

The molecular weight of calcium hypochlorite is 127.0. This reacts to produce two atoms of available chlorine with a molecular weight of 70.9. If the bleaching powder were pure the available chlorine would therefore represent 70.9 ÷ 127, or 56 per cent of its weight. Then to obtain one pound of chlorine it would be necessary to have 1.79 pounds of pure bleaching powder. Since 1,000,000 gallons of water weigh approximately 8,300,000 pounds, in order to apply one part per million of chlorine to 1,000,000 gallons of sewage it is necessary to apply 1.79 × 8.3 or 14.9 pounds of pure bleaching powder. Commercial bleaching powder is only about 60 per cent calcium hypochlorite. It is therefore necessary to add 14.9 ÷ 0.60 or about 25 pounds of commercial bleach.

Since liquid chlorine is very nearly pure, approximately 8½ pounds of it applied to 1,000,000 gallons of sewage are equivalent to a dose of one part per million.

Commercial bleaching powder is a dry white powder which absorbs moisture slowly, and which loses its strength rapidly when exposed to the air. It is packed in air-tight sheet iron containers, which should be opened under water, or emptied into water immediately on being opened. The strength of the solution should be from ½ to 1 per cent. The rate of the application of the solution to the sewage may be controlled by automatic feed devices, or by hand-controlled devices.

Commercial liquid chlorine is sold in heavy cast steel containers, which hold 100 to 140 pounds of liquid chlorine under a pressure of 54 pounds per square inch at zero degrees C. or 121 pounds per square inch at 20 degrees.

The amount of chlorine used is dependent on the character of the sewage to be treated, the stage of decomposition of the organic matter, the desired degree of disinfection, the period of contact, and the temperature. The amount of chlorine is expressed in parts per million of available chlorine, regardless of the form in which the chlorine is applied. In general about 15 to 20 parts per million of available chlorine with 30 minutes’ contact at a temperature of about 15° C. will effect an apparent removal of 99 per cent of the bacteria from the raw sewage. The effect is only apparent because many of the bacteria encased in the solid matter of the sewage escape the effect of the chlorine, or detection in the bacterial analysis. Stronger and older sewages, higher temperatures, and shorter periods of contact will demand more chlorine to produce the same results. A septic effluent will require more chlorine than a raw sewage because of the greater oxygen demand by the septic sewage. The results of experiments on disinfection made at different testing stations have shown such wide variations in the amount of chlorine necessary, as to demonstrate the necessity for independent studies of any particular sewage which is to be chlorinated. For instance, at Milwaukee approximately 13 p.p.m. of available chlorine applied to an Imhoff tank effluent effected a 99 per cent removal of bacteria, whereas the same result was obtained at Lawrence, Mass., on crude sewage with only 6.6 p.p.m. and at Marion, Ohio, only 9 per cent removal of bacteria was obtained by the addition of 4,815 p.p.m. to crude sewage. The Ohio and Massachusetts reports show irrational variations among themselves. For instance, 6.2 p.p.m. applied to a septic effluent effected 88 per cent removal whereas in another case 7.6 p.p.m. effected only 36 per cent removal. At Lawrence in one case it took 8.6 p.p.m. to remove 99 per cent from a sand filter effluent, but only 6.3 p.p.m. to effect the same result in the effluent from a septic tank. The most consistent results are those found at Milwaukee which show a steadily increasing percentage removal with increasing amounts of chlorine.

Some time after sewage has received its dose of chlorine the number of bacteria may be greater than in the raw sewage. Such bacteria are called aftergrowths. Certain forms of bacteria, particularly the pathogenic or body temperature types, are most susceptible to disinfecting agents. These are killed off and leave the sewage in a condition more favorable to the growth of more resistant forms of bacteria. As the latter are non-pathogenic and are generally aËrobic their presence is usually more beneficial than detrimental, as they hasten the action of self-purification.

REFERENCES

The following abbreviations will be used: E.C. for Engineering and Contracting, E.N. for Engineering News, E.R. for Engineering Record, E.N.R. for Engineering News-Record, M.J. for Municipal Journal, p. for page, and V. for volume.

No.

1.

Grease and Fertilizer Base for Boston Sewage, by Weston, E.N. V. 75, 1916, p. 913 and Journal American Public Health Association, April, 1916.

2.

Getting Grease and Fertilizer from City Sewage, by Allen. E.N. V. 75, 1916, p. 1005.

3.

New Haven Tests Five Processes of Sewage Treatment. E.N.R. V. 79, 1917, p. 829.

4.

Recovery of Grease and Fertilizer from Sewage Comes to the Front. E.N.R. V. 80, 1916, p. 319.

5.

Miles Acid Process may Require AËration of Effluent, by Mohlman. E.N.R. V. 81, 1918, p. 235.

6.

Promising Results with Miles Acid Process in New Haven Tests. E.N.R. V. 81, 1918, p. 1034.

7.

Baltimore Experiments on Grease from Sewage. E.N. V. 75, 1916, p. 1155.

8.

Report on Industrial Wastes from the Stock Yards and Packingtown in Chicago to the Trustees of the Sanitary District of Chicago, 1914, pp. 187–195.

9.

The Separation of Grease from Sewage, by Daniels and Rosenfeld. Cornell Civil Engineer. V. 24, p. 13.

10.

The Separation of Grease from Sewage Sludge with Special Reference to Plants and Methods Employed at Bradford and Oldham, England, by Allen. E.C. V. 40, 1913, p. 611.

11.

Acid Treatment of Sewage, by Dorr and Weston. Journal Boston Society of Civil Engineers, April, 1919. E.C. V. 51, 1919, p. 510. M.J. V. 46, 1919, p. 365.

12.

The Miles Acid Process for Sewage Disposal. Metallurgical and Chemical Engineering, V. 18, p. 591.

13.

Miles Acid Treatment of Sewage, by Winslow and Mohlman. Journal American Society Municipal Improvements, Oct., 1918. M.J. V. 45, 1918, pp. 280, 297, and 321.

14.

New Electrolytic Sewage Treatment. M.J. V. 37, 1914, p. 556.

15.

Electrolytic Sewage Treatment. M.J. V. 47, 1919, p. 131.

16.

Electrolytic Treatment of Sewage at Durant, Oklahoma, by Benham. E.N. V. 76, 1916, p. 547. Municipal Engineering, V. 49, 1916, p. 141.

17.

Electrolytic Treatment of Sewage at Elmhurst, Long Island, by Travis. Report to the President of the Borough of Queens, Aug. 31, 1914. E.R. V. 70, 1914, pp. 292, 315, and 429. M.J. V. 39, p. 551. Municipal Engineering, V. 47, p. 281.

18.

Tests of the Electrolysis of Sewage at Toronto, by Nevitt. E.N. V. 71, 1914, p. 1076.

19.

Electrolytic Treatment of Sewage Little Better than Lime Alone, by Bartow. E.R. V. 74, 1916, p. 596.

20.

Electrolytic Sewage Treatment Not Yet an Established Process. E.N.R. V. 83, 1919, p. 541.

21.

Tests of Electrolytic Sewage Treatment Process at Easton, Pa. Journal of the Franklin Institute, Aug., 1919. E.N.R. V. 83, 1919, p. 569.

22.

The Disinfection of Sewage. U. S. Geological Survey, Water Supply Paper, No. 229.

23.

Sewage Disinfection in Actual Practice, by Orchard. E.R. V. 70, 1914, p. 164.

24.

Water and Sewage Purification in Ohio. Report of the Ohio State Board of Health, 1908, pp. 738–762.

25.

Water Purification, by Ellms. Published in 1917 by McGraw-Hill Book Co.

26.

Electrolytic Sewage Treatment, A Half Century of Invention and Promotion. E.N.R. V. 86, 1921, p. 25.