273. Obtaining Activated Sludge.—After a plant is once started activated sludge is generated during the process of treatment and with careful management a stock of activated sludge can be kept on hand. When a plant is new, or if shut down for such a length of time that the sludge loses its activation, it is necessary to activate some new sludge. This is done by blowing air continuously through sewage either on the fill and draw method with periodic decantations of the supernatant liquid, or by the continuous-flow process, but more preferably by the latter. Where activated sludge is to be obtained from fresh sewage alone the time required is in the neighborhood of 10 to 14 days, and purification begins at the start. An estimate of the quantity which will be obtained can not be made with accuracy. After the initial quantity of sludge has been obtained activated sludge can be maintained during the process of aëration of the raw sewage, or by means of the reaëration tanks previously described.
The volume of activated sludge present in the aëration tank should be about 25 per cent of the volume of the tank. The volume of the sludge is measured in a somewhat arbitrary manner as the amount by volume which will settle in 30 minutes in an ordinary test tube. It is found that this is almost 90 per cent of the solids settling in 4 to 6 hours.
274. Cost.—The available information on the cost of the activated sludge process is meager and unreliable. The factors entering into the cost are: the price of fuel, the size of the plant, the period of sedimentation, the amount of air per gallon of sewage, the air pressure, and the percentage of sludge to be aërated in the mixture. In Milwaukee[[186]] the cost of construction is estimated at $44,000 per million gallons, and $4.75 per million gallons for operation. At Houston, Texas, the cost is estimated at $24,000 per million gallons, exclusive of the sludge drying plant, which may cost $40,000 per million gallons. At Milwaukee, the cost of pressing the sludge is $4.82 per dry ton and of drying is $3.93 per dry ton. The sludge may be sold at the normal rate of $2.50 per unit of nitrogen. Based on the normal value the evident profit will be $3.75 per ton. The net cost of disposing of Milwaukee sewage is estimated at $9.64 per million gallons of which $4.89 is chargeable to overhead and $4.75 to repairs, operation and renewal. In a comparison of the costs of activated sludge and Imhoff tanks with sprinkling filters,[[187]] the information given by Eddy has been summarized in Table 96. In comparing the relative areas required for different methods of sewage treatment, activated sludge should be allowed about 15 million gallons per acre per day on the basis of aëration tanks 15 feet deep. This figure represents approximately the gross area of the plants at Milwaukee and at Cleveland.
| TABLE 96 | ||||
|---|---|---|---|---|
| Comparative Costs of Activated Sludge, and of Imhoff Tanks Followed by Sprinkling Filters | ||||
| (H. P. Eddy, Eng. Record, Vol. 74, p. 557) | ||||
| Process | First Cost per Million Gallons, Dollars | Operation per Million Gallons, Dollars | Total Annual Cost at 4 Per Cent with Sinking Fund at 2.5 Per Cent per | |
| Million Gallons, Dollars | Capita, Dollars | |||
| Activated sludge | 57,100 | 20.00 | 29.85 | 1.09 |
| Imhoff tank and sprinkling filter | 78,500 | 8.50 | 21.84 | 0.80 |
REFERENCES AND BIBLIOGRAPHY ON ACTIVATED SLUDGE
The following abbreviations will be used: A.S. for Activated Sludge, E.C. for Engineering and Contracting, E.N. for Engineering News, E.R. for Engineering Record, E.N.R. for Engineering News-Record, p. for page, and V. for volume.
No. 1. Cooperation Sought in Conducting A.S. Experiments at Baltimore, by Franks and Hendrick. E.R. V. 71, 1915, pp. 521, 724, and 784. V. 72, 1915, pp. 23, and 640. 2. Sewage Treatment Experiments with Aëration and A.S., by Bartow and Mohlman. E.N. V. 73, 1915, p. 647, and E.R. V. 71, 1915, p. 421. 3. A.S. Experiments at Milwaukee, Wisconsin, by Hatton. E.N. V. 74, 1915, p. 134. 4. A.S. in America, An Editorial Survey, by Baker. E.N. V. 74, 1915, p. 164. 5. Choosing Air Compressors for A.S., by Nordell, E.N. V. 74, 1915, p. 904. 6. A Year of A.S. at Milwaukee, by Fuller. E.N. V. 74, 1915, p. 1146. 7. A.S. Experiments at Urbana. E.N. V. 74, 1915, p. 1097. 8. Experiments on the A.S. Process, by Bartow and Mohlman. E.C. V. 44, 1915, p. 433. 9. Milwaukee’s A.S. Plant, the Pioneer Large Scale Installation, by Hatton. E.R. V. 72, 1915, p. 481 and E.C. V. 44, 1915, p. 322. 10. A.S. Experiments at Milwaukee, by Hatton. Journal American Waterworks Association and Proceedings Illinois Society of Engineers, 1916. Also E.R. V. 73, 1916, p. 255. E.C. V. 45, 1916, p. 104, and E.N. V. 75, 1916, pp. 262 and 306. 11. A.S. Defined. E.N. V. 75, 1916, p. 503, and E.N.R. V. 80, 1918, p. 205. 12. Status of A.S. Sewage Treatment, by Hammond. E.N. V. 75, 1916, p. 798. 13. Trial A.S. Unit at Cleveland, by Pratt. E.N. V. 75, 1916, p. 671. 14. Air Diffuser Experience with A.S. E.N. V. 76, 1916, p. 106. 15. Nitrogen from Sewage Sludge, Plain and Activated, by Copeland, Journal American Chemical Society, Sept. 28, 1916. E.N. V. 76, 1916, p. 665. E.R. V. 74, 1916, p. 444. 16. Tests Show A.S. Process Adapted to Treatment of Stock Yards Wastes. E.R. V. 74, 1916, p. 137. 17. Aëration Suggestions for Disposal of Sludge, by Hammond. Journal American Chemical Society, Sept. 25, 1916. E.R. V. 74, 1916, p. 448. 18. Cost Comparison of Sewage Treatment. Imhoff Tank and Sprinkling Filters vs. A.S., by Eddy. E.R. V. 74, 1916, p. 557. 19. Large A.S. Plant at Milwaukee. E.N. V. 76, 1916, p. 686. 20. A.S. Novelties at Hermosa Beach, Cal. E.N. V. 76, 1916, p. 890. 21. A.S. Experiments at University of Illinois, by Bartow, Mohlman, and Schnellbach. E.N. V. 76, 1916, p. 972.
CHAPTER XIX
ACID PRECIPITATION, LIME AND ELECTRICITY, AND DISINFECTION
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]]