It is also of interest to note that the sand filter (called a preliminary filter) in [Table 18], filled with the same kind of sand, when operated at an average rate of 50,000,000 gal. per acre daily for a year, allowed 18% of the applied bacteria to pass, in comparison with 3% found in Filter No. 6 of [Table 20], operated at an average rate of 38,000,000 gal. per acre daily.
There was one point of difference in the manipulation: the preliminary filter was washed by a reversed current of water, as mechanical filters are washed, while Filter No. 6 was cleaned by scraping off the surface layer, as is usual with sand filters. Whether the great difference in bacterial results with a relatively small difference in rate is to be attributed to this difference in manipulation the writer will not undertake to state.
If the experimental results of [Table 20] indicate correctly the conditions which obtain in filtering Potomac water, then increasing the rate of filtration so as to double it, or more than double it, would make but little difference in the quality of the effluent as measured by the usual bacterial methods. If the increase in rate were accompanied by the preliminary filtration of the water, then, presumably, there would be little change in the quality of the effluent, and the maintenance of excellent results might be incorrectly attributed to the influence of the preliminary filter.
It would also seem that the apparatus which is sometimes used for determining and controlling the rate with more than the ordinary degree of precision is hardly justified by such experimental results as those presented by the author.
In contrast to these results may be mentioned those obtained by Mr. H. W. Clark,[1] for experimental filters operated with Merrimac River water, at rates ranging from 3,000,000 to 16,000,000 gal. per acre daily. The results are the average of nearly two years of experimental work, the period having been nearly coincident with that covered by the author's experiments, and of many hundreds of bacterial analyses of each effluent, and form, with the author's experiments, the most thorough‑going studies of the effect of rate on efficiency that have come to the writer's attention.
Mr. Clark's results are given in [Table 22].
[1 Journal, New England Water-Works Association, Vol. 24, p. 589.]
| Effective size of sand. | Filter No. | Rate in gallons acre daily. | Bacteria per cubic centimeter in | Bacteria efficiency. | B. Coli in 1 cu. cm. (percentage of positive tests). |
|---|---|---|---|---|---|
| 0.28 | A | 3,000,000 | 48 | 99.1 | 5.0 |
| 0.25 | B | 5,000,000 | 85 | 98.4 | 24.0 |
| 0.22 | C | 7,500,000 | 105 | 98.1 | 25.0 |
| 0.22 | D | 10,000,000 | 110 | 98.0 | 25.0 |
| 0.22 | E | 16,000,000 | 280 | 95.0 | 38.0 |
It will be seen that the number of bacteria passing increases rapidly with the rate, and whether the total number of bacteria is considered or the B. coli results, the number passing is approximately in proportion to the rate. In other words, doubling the rate substantially doubles the number of bacteria in the effluent.
This is entirely in harmony with all the Lawrence experimental results extending over a period of 20 years. There have been occasional apparent exceptions, but, on the whole, experience with Merrimac River water has uniformly been that more bacteria pass as the rates are higher.