EFFECT OF RATE UPON EFFICIENCY OF FILTRATION.
The effect of the rate of filtration upon the quality of the effluent has been repeatedly investigated. The efficiency almost uniformly decreases rapidly with increasing rate. Fränkel and Piefke[10] first found that with the high rates the number of bacteria passing some experimental filters was greatly increased. Piefke[11] afterward repeated these experiments, eliminating some of the features of the first series to which objection was made, and confirmed the first results. The results were so marked that Piefke was led to recommend the extremely low limit of 1.28 million gallons per acre daily as the safe maximum rate of filtration, but he has since repeatedly used 2.57 million gallons.
Kümmel,[12] on the other hand, in a somewhat limited series of experiments, was unable to find any marked connection between the rate and the efficiency, a rate of 2.57 giving slightly better results than rates of either 1.28 or 5.14.
The admirably executed experiments made at Zürich in 1886-8 upon this point, which gave throughout negative results, have but little value in this connection, owing to the extremely low number of bacteria in the original water.
At Lawrence in 1892 the following percentages of bacteria (B. prodigiosus) passed at the respective rates:
| No. of Filter. | Depth. | Effective Size of Sand. | Rate. Millions gallons per acre daily. | ||||
|---|---|---|---|---|---|---|---|
| 0.5 | 1.0 | 1.5 | 2.0 | 3.0 | |||
| 33A | 60 | 0.14 | 0.002 | ..... | ..... | 0.040 | ..... |
| 34A | 60 | 0.09 | 0.001 | 0.005 | ..... | 0.020 | ..... |
| 36A | 60 | 0.20 | ..... | ..... | 0.050 | ..... | 0.050 |
| 37 | 60 | 0.20 | ..... | ..... | 0.010 | 0.130 | ..... |
| 38 | 24 | 0.20 | 0.018 | ..... | 0.140 | 0.110 | 0.310 |
| 39 | 12 | 0.20 | 0.014 | 0.070 | ..... | 0.080 | 0.520 |
| 40 | 12 | 0.20 | ..... | 0.070 | ..... | 0.090 | ..... |
| 42 | 12 | 0.20 | 0.016 | ..... | ..... | 0.150 | 0.550 |
| Average | 0.010 | 0.048 | 0.067 | 0.088 | 0.356 | ||
These results show a very marked decrease in efficiency with increasing rates, the number of bacteria passing increasing in general as rapidly as the square of the rate. The 1893 results also showed decreased efficiency with high rates, but the range in the rates under comparable conditions was less than in 1892, and the bacterial differences were less sharply marked.
While the average results at Lawrence, as well as most of the European experiments, show greatly decreased efficiency with high rates, there are many single cases, particularly with deep layers of not too coarse sand, where, as in Kümmel’s experiments, there seems to be little connection between the rate and efficiency. An explanation of these apparently abnormal results will be given in Chapter VI.
It is commonly stated[13] that every water has its own special rate of filtration, which must be determined by local experiments, and that this rate may vary widely in different cases. Thus it is possible that the rate of 1.60 adopted at Hamburg for the turbid Elbe water, the rate of 2.57 used at Berlin, and about the same at London for much clearer river-waters, and the rate of 7.50 used at Zürich for the almost perfectly clear lake-water are in each case the most suitable for the respective waters. In other cases however, where rates much above 2.57 are used for river-waters, as at Lübeck and Stettin, there is a decided opinion that these rates are excessive, and in these instances steps are now being taken to so increase the filtering areas as to bring the rates within the limit of 2.57 million gallons per acre daily.
From the trend of European practice it would seem that for American river-waters the rate of filtration should not exceed 2.57 in place of the 3.90 million gallons per acre daily recommended by Kirkwood, or even that a somewhat lower rate might be desirable in some cases. Of course, in addition to the area necessary to give this rate, a reserve for fluctuating rates and for cleaning should be provided, reducing the average yield to 2.00, 1.50, or even less. In the case of water from clear lakes, ponds, or storage reservoirs, especially when they are not subject to excessive sewage pollution or to strong algæ growths, it would seem that rates somewhat and perhaps in some cases very much higher (as at Zürich) could be satisfactorily used.