266. Composition.—Activated sludge is the material obtained by agitating ordinary sewage with air until the sludge has assumed a flocculent appearance, will settle quickly, and contain aërobic and facultative bacteria in such numbers that similar characteristics can be readily imparted to ordinary sewage sludge when agitated with air in the presence of activated sludge. Copeland described activated sludge as follows:[[172]]
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. Aërobic and facultative 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 that have the power to decompose organic matter, especially of an albuminoid or nitrogenous nature, setting the nitrogen free; and others absorbing the nitrogen convert it into nitrites and nitrates. These biological processes require time, air, and favorable environment such as suitable temperature, food supply and sufficient agitation to distribute them throughout all parts of the sewage.
Ardern states that the sludge differs entirely from the usual tank sludge. It is inoffensive and flocculent in character. The percentage of moisture is from 95 to 99 per cent. American experience has generally been that the sludge does not readily separate from its moisture by treatment on fine-grain filters, but the results in England and at Milwaukee, Wisconsin, are in conflict with this general experience. Upon standing 24 hours or more partially dried activated sludge may start to decompose accompanied by the production of offensive odors.
Duckworth states:
The activated sludge at Salford contained three times as much nitrogen, twice as much phosphoric acid and one-half as much fatty matter as ordinary sludge.
| TABLE 91 | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Composition of Sewage, Imhoff Sludge, and Activated Sludge and Effluent at Milwaukee | ||||||||||
| (W. R. Copeland, Eng. News, Vol. 76, p. 665) | ||||||||||
| Period of Test | Source of Sample | Parts per Million | ||||||||
| Suspended Matter | Nitrogen as | Nitrogen Reported as Ammonia on a Basis of Sludge Dried to 10 Per Cent Moisture. Three samples of Sludge | ||||||||
| Free Ammonia | Albuminoid Ammonia | Organic Nitrogen | Nitrites | Nitrates | ||||||
| Aug., 1915 | Sewage | 253 | 14.6 | 7.88 | 29 | 0.15 | 0.13 | |||
| Imhoff effluent | 105 | 16.2 | 6.10 | 27 | 0.19 | 0.13 | 2.87 | 3.82 | ||
| Activated sludge effluent | 14 | 3.8 | 3.19 | 6 | 0.29 | 6.00 | 5.71 | 4.97 | 7.04 | |
| Sept., 1915 | Sewage | 300 | 13.5 | 8.81 | 29 | 0.25 | 0.14 | |||
| Imhoff effluent | 116 | 15.4 | 7.10 | 27 | 0.12 | 0.09 | 3.88 | |||
| Activated sludge effluent | 8 | 5.7 | 2.22 | 9 | 0.24 | 5.01 | 8.69 | 9.00 | ||
These results have been roughly checked by American experimenters as shown in Table 91.[[173]] In the recovery of nitrogen from sewage the activated sludge process is the most promising for satisfactory results. In all other processes of sewage treatment the sludge is digested to some extent and nitrogen lost in the gases or in the soluble matter which passes off with the effluent. In the activated sludge process a negligible amount of gasification and liquefaction take place and only a small amount of nitrogen passes off with the effluent as compared with the loss from the Imhoff process as shown in Table 91. The percentage of nitrogen in dried activated sludge is shown in Table 92.
| TABLE 92 | |
|---|---|
| Nitrogen Content of Dry Activated Sludge and Sludge from Other Processes | |
| (G. W. Fuller, Eng. News, Vol. 76, p. 667) | |
| Source | Per Cent Nitrogen |
| Milwaukee (Copeland) | 4.40 |
| Manchester, England (Ardern) | 4.60 |
| Salford, England (Melling) | 3.75 |
| Urbana, Illinois (Bartow) | 3.5 to 6.4 |
| Armour and Co. (Noble) | 4.6 |
| Approximate range of all other processes | 1.0 to 3.0 |
| These figures are expressed in terms of nitrogen and not of ammonia. Nitrogen is only 82 per cent of the ammonia content. | |
Nitrifying bacteria and other species which have the power of destroying organic matter have been isolated from the sludge. An analysis of the dried sludge at Urbana[[174]] showed the following results after the weight had been reduced 95.5 per cent by drying: 6.3 per cent nitrogen, 4.00 per cent fat, 1.44 per cent phosphorus, and 75 per cent volatile matter or loss on ignition. Analyses of other domestic sewages have not shown such high contents of these desirable constituents.
The dewatering of activated sludge is a problem which offers serious obstacles to the successful operation of the process. It is its greatest disadvantage. Five to ten times the volume of sludge may be produced by the activated sludge process as by an Imhoff tank, and the activated sludge contains a greater percentage of water. According to Copeland: