(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.
| TABLE 97 | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Average Analysis of Sewage Entering Boston Harbor, before and after Treatment, July 17 to September 27, 1917 | |||||||||
| (Eng. News-Record, Vol. 80, p. 319) | |||||||||
| Sample | Parts per Million | Bacteria, Millions | |||||||
| Ammonia | Kjeldahl Nitrogen | Chlorine | Oxygen Consumed | ||||||
| Free | Albuminoid | ||||||||
| Total | Total | Diss. | Total | Diss. | 20° | 37° | |||
| Paddock’s Island | |||||||||
| Raw sewage | 14.0 | 3.3 | 1.8 | 6.8 | 3.6 | 134 | 23.1 | 1.86 | 4.15 |
| Settled Sewage | 12.2 | 1.6 | 1.1 | 3.5 | 2.2 | 15.4 | |||
| Acidified and settled sewage | 20.9 | 5.2 | 3.9 | 10.0 | 7.5 | units 94 | units 91 | ||
| Deer Island | |||||||||
| Raw sewage | 23.3 | 8.2 | 4.8 | 16.8 | 8.9 | 3100 | 87.3 | 2.63 | 1.50 |
| Settled sewage | 21.1 | 5.6 | 3.9 | 10.7 | 7.3 | 62.2 | |||
| Acidified and settled sewage | 20.9 | 5.2 | 3.9 | 10.0 | 7.5 | units 147 | units 85 | ||
| Calf Pasture | |||||||||
| Raw sewage | 18.0 | 4.5 | 2.0 | 9.7 | 4.1 | 3254 | 41.2 | 1.89 | 0.98 |
| Settled sewage | 19.1 | 2.3 | 1.4 | 4.9 | 3.3 | 25.8 | |||
| Acidified and settled sewage | 17.8 | 2.4 | 1.6 | 4.9 | 3.3 | units 277 | units 149 | ||
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.
| TABLE 98 | ||||||
|---|---|---|---|---|---|---|
| Average Amount of Sludge and Fats Obtained from Sewage Entering Boston Harbor after Eighteen Hours Sedimentation With and Without Acidification | ||||||
| (Eng. News-Record, Vol. 80, p. 319) | ||||||
| Paddock’s Island | Deer Island | Calf Pasture | ||||
| Sedimentation | Sedimentation | Sedimentation | ||||
| Plain | Acidulated | Plain | Acidulated | Plain | Acidulated | |
| Pounds of SO2 used per million gallons of sewage treated | 818 | 1513 | 1189 | |||
| Dry sludge per million gallons | 782 | 959 | 1709 | 1939 | 1208 | 1427 |
| Per cent Nitrogen in sludge | 3.10 | 3.38 | 3.57 | 3.45 | 3.18 | 2.83 |
| Per cent fats in sludge | 27.30 | 27.30 | 24.60 | 19.40 | 24.30 | 26.30 |
| TABLE 99 | |||||
|---|---|---|---|---|---|
| Character of Miles Acid Sludge at New Haven | |||||
| (Eng. News-Record, Vol. 81, p. 1034) | |||||
| East Street Sewer | Boulevard Sewer | ||||
| Length of run in days | 25 | 24 | 44 | 70 | 29 |
| Total sewage treated, thousand gallons | 260 | 239.4 | 407.8 | 602.2 | 145.5 |
| Gallons wet sludge per million gallons sewage | 3750 | 4025 | 3200 | 2600 | 5375 |
| Specific gravity | 1.067 | 1.048 | 1.054 | 1.061 | |
| Per cent moisture | 86.6 | 88 | 86.3 | 85.7 | 92.5 |
| Pounds of dry sludge per million gallons sewage | 503 | 483 | 439 | 368 | 403 |
| Ether extract, per cent dry sludge | 23.7 | 24.0 | 29 | 32.6 | 30.9 |
| Ether extract, pounds per million gallons | 119 | 116 | 127 | 120 | 124 |
| Volatile matter, per cent dry sludge | 47.2 | 51.2 | 57.3 | 63.8 | 78.5 |
| Nitrogen, per cent dry sludge | 1.6 | 1.6 | 2.4 | 2.0 | 3.0 |
The conclusions reached as a result of the New Haven experiments are:[[193]]