The layer of sand in a slow sand filter may be considered as a very great number of small sedimentation basins communicating one with another, not in the manner of basins connected in series, but rather, as Mr. Hazen has expressed it, as a long series of compartments connected at one side only with a passageway in which a current is maintained. In any section of the sand layer there are areas through which the water passes with a velocity much greater than its mean velocity through the total area of voids, while there are other areas in which the velocity is very much less, perhaps in an almost quiescent state from time to time, greatly favoring the deposition of particles, but with a gentle intermittent circulation, displacing the settled or partly‑settled water and supplying from the main currents water containing more suspended matter particles to be removed. There is thus a considerable percentage of the total volume of voids in which the water is subjected to very favorable conditions for sedimentation, almost perfect stillness and an exceedingly small distance for a particle to settle before it strikes bottom on the surface of a grain of sand.
[1 Transactions, Am. Soc. C. E., Vol. LIII, p. 59.]
If sedimentation were the predominating factor in the purification of the water, we would then expect to find the following phenomena in the operation of the filters: A more rapid deposition of a given amount of sediment under summer temperature conditions than under winter, as the water passes through the sand, and therefore, for the former condition of higher temperature:
- (a) A greater concentration of this turbidity-producing material in the top layer of sand, or, in other words, a thinner sand layer to be removed in scraping if all the dirty sand is removed;
- (b) Because of the greater concentration, a greater rate of Increase of the loss of head, and consequently shorter periods of service between scrapings;
- (c) A higher limit for turbidity in the water applied to the filter to produce a given turbidity in the effluent.
The operation of this plant during the first year and a half offered an excellent opportunity for the study of sedimentation in the sand, and the data in [Table 30] are presented to show that certain of the phenomena of filter operation observed during this period seem to be fairly explicable by the physical theory of purification. These data are given only for the period of operation before the summer of 1907. At that time the experiments in filter cleaning already described were begun. Before that time, whenever a filter had been cleaned, all the discolored sand had been removed, leaving for the following run a new sand surface substantially in the perfect condition of a newly‑constructed filter. After that time the experimental methods of cleaning, and the new routine adopted as a result thereof, interfered with the tracing of the evidence as clearly as during the earlier periods.
| Month. | Number of filters. | Average period of service in days. | Average depth of sand removed, in inches. | Mean temperature, in degrees, Fahrenheit. |
|---|---|---|---|---|
| January | 13 | 75 | 2.09 | 39 |
| February | 6 | 98 | 2.46 | 37 |
| March | 5 | 130 | 2.66 | 41 |
| April | 8 | 149 | 2.96 | 53 |
| May | 7 | 130 | 2.80 | 67 |
| June | 11 | 124 | 2.35 | 77 |
| July | 17 | 70 | 2.12 | 81 |
| August | 2 | 49 | 1.98 | 80 |
| September | 5 | 73 | 2.48 | 76 |
| October | 37 | 70 | 1.56 | 64 |
| November | 20 | 42 | 0.81 | 49 |
| December | 14 | 57 | 0.94 | 40 |
Figure 14—Periods of Service and Depths of Scraping for Runs Ending in Various Months Covering Entire Period Oct. 1, 1905, to Mar. 1, 1907.
[Table 30] and the corresponding diagram, [Figure 14], show the general variations in the length of runs and depth of penetration, with the seasonal temperature changes. The increase in length of runs and quantity of sand removed under low temperature conditions is very marked. There is, however, a secondary maximum which appears, as the diagram shows, where a minimum for the year would be expected. This may have been an irregularity occurring this one year, which will not appear in the average of several years, and caused by some factor which has escaped observation. A careful analysis of the data at hand fails to show any explanation for it. It may exist in some of the little‑understood biological actions which have their maximum effect under warm‑water conditions, or it may be due—in some obscure way—to the liberation of air under the surface of the sand, accumulating with pressure enough to break the surface at innumerable points, thereby reducing the loss of head and extending the period of service. Some evidence was observed pointing to this explanation, but it was never conclusively proven.