~Table 22.~ ===========+============+===========+=============+===========+============ | | | | | _B. Coli_ | | | | |in 1 cu.cm. Effective | | Rate | Bacteria per| |(percentage size of | | in gallons| cubic | Bacterial |of positive sand. | Filter No. |acre daily.|centimeter in|efficiency.| 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. The theory sometimes advanced, that the efficiency of filtration is controlled to a certain extent by gelatinous films, and that, as far as thus controlled, is less dependent on rate, would not seem to be borne out by these results. The Merrimac River water, carrying large amounts of organic matter, would certainly seem better adapted to the formation of such films than the clay-bearing Potomac water, comparatively free from organic matter; but it is the Potomac water which seems to show the least influence of rate on efficiency. [Footnote 1: _Journal_, New England Water-Works Association, Vol. 24, p. 589.] The experiments show that turbidity passes more freely at the higher rates with the Potomac water, as has also been found to be the case with other clay-bearing waters. In the last lines of Table 20 are given cost per million gallons for filtering at various rates. There is no discussion of these figures, and as they differ considerably from those which the writer has been accustomed to use, the calculation in Table 23, made three years ago for a particular case, may be of interest.