The best information now available points to a combination of settling and decantation as a preliminary dewatering process. By this means the water will be cut down from about 99 per cent to 96 per cent. On passing the concentrated residue through a pressure filter the moisture can be cut down to 75 per cent. The press cake can be dewatered in a heat drier to 10 per cent moisture or less.[[175]]
The quantity of sludge produced at Milwaukee[[176]] is about 15 cubic yards per million gallons of sewage, the sludge having about 98 per cent moisture. On the basis of 10 per cent moisture it produces ½ ton of dry sludge per million gallons of sewage treated. At Cleveland,[[177]] 20 cubic yards per million gallons at 97.5 per cent moisture are produced. Methods of drying sludge are discussed in Chapter XX.
Chemical analyses and biological tests indicate that the fertilizing value of the sludge is appreciable. Professor C. B. Lipman states, as the result of a series of tests in which a sludge and a soil were incubated for one month, as follows:[[178]]
The amounts of nitrates produced in one month’s incubation from the soil’s own nitrogen and from the nitrogen from the sludge mixed with the soil in the ratio of one part of sludge to 100 of soil is, in milligrams of nitrate, as follows: Anaheim soil without sludge 6.0, with sludge 10.0; Davis soil without sludge 4.2, with sludge 14.0; Oakley soil without sludge 2.2, with sludge 4.0.
The effect of the sludge on plant growth is shown in Table 93.[[179]] The results represent the growth obtained after fifteen weeks from the planting of 30 wheat seeds in each pot.
267. Advantages and Disadvantages.—Some of the advantages of the process are: a clear, sparkling, and non-putrescible effluent is obtained; the degree of nitrification is controllable within certain limits; the character of the effluent can be varied to accord with the quantity and character of the diluting water available; more than 90 per cent of the bacteria can be removed; the cost of installation is relatively low; and the sludge has some commercial value.
| TABLE 93 | ||||
|---|---|---|---|---|
| Fertilizing Value of Activated Sludge | ||||
| (E. Bartow, Journal Am. Water Works Ass’n, Vol. 3, p. 327) | ||||
| Cultivating Medium | Grams Contained in Experimental Pot | |||
| 1 | 2 | 3 | 4 | |
| White sand | 19,820 | 19,820 | 19,820 | 19,820 |
| Dolomite | 60 | 60 | 60 | 60 |
| Bone meal | 6 | 6 | 6 | 6 |
| Potassium sulphate | 3 | 3 | 3 | 3 |
| Activated sludge | 0 | 0 | 20 | 0 |
| Activated sludge extracted with Ligroin | 0 | 0 | 0 | 20 |
| Dried blood | 0 | 8.61 | 0 | 0 |
| Number of heads of wheat | 14 | 15 | 22 | 23 |
| Number of seeds | 85 | 189 | 491 | 518 |
| Weight of seeds, grams | 2.38 | 5.29 | 13.748 | 14.504 |
| Bushels per acre, calculated | 6.20 | 13.6 | 35.9 | 38.7 |
| Average length of stalk, inches | 19.40 | 23.0 | 35.4 | 37.1 |
| Weight of straw, grams | 2.25 | 8.25 | 26.75 | 26.21 |
| Tons per acre, calculated | 0.18 | 0.68 | 2.23 | 2.18 |
Among the disadvantages of the process can be included, uncertainty due to the lack of information concerning the results to be expected under all conditions, high cost of operation under certain conditions, the necessity for constant and skilled attendance, and the difficulty of dewatering the sludge.
268. Historical.—The most notable work in the aëration of sewage within recent years was that performed by Black and Phelps for the Metropolitan Sewerage Commission of New York, in 1910,[[180]] and by Clark and Gage at the Lawrence, Massachusetts, Sewage Experiment Station in 1912 and 1913.[[181]] The results of these investigations showed that the treatment of sewage by forced aëration might give a satisfactory effluent, but that the time and expense in connection thereto rendered the method impractical.
It remained for Messrs. Ardern and Lockett of Manchester, England, to introduce the process of the aëration of sewage in the presence of activated sludge, as a result of their connection with Dr. Fowler, who attributes his inspiration to his visit to the Lawrence Experiment Station and observing the work of Clark and Gage. Ardern and Lockett commenced their experiments in 1913. Their results were published in the Journal of the Society of Chemical Industry, May 30, 1914, Vol. 33, p. 523. Shortly thereafter experiments were started at the University of Illinois by Dr. Edw. Bartow and Mr. F. W. Mohlmann of the Illinois State Water Survey. At about the same time an experimental plant was started at Milwaukee, by T. C. Hatton, Chief Engineer of the Milwaukee Sewerage Commission. The United States Public Health Service became actively interested in December, 1914, and on February 20, 1915, announced its intention to co-operate with the Baltimore Sewerage Commission in the conduct of experiments. In May, 1915, patent number 1,139,024 was granted to Leslie C. Frank, Sanitary Engineer of the U. S. Public Health Service, covering certain features of the process. Mr. Frank generously donated this patent to the public for the use of municipalities.