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 | Depth. | Effective | Rate. Millions gallons per acre daily. Filter. | | Size of +-------+-------+-------+-------+------- | | Sand. | 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.