------------+----------+--------------------+----------+--------+----------- | | | | |Sulphate Number | | Bacteria. | | |of Alumina of Days |Turbidity.+----------+---------+ Per cent |Per cent|used Grains Represented.| |Raw Water.|Effluent.|remaining.|removed.|per Gallon. ------------+----------+----------+---------+----------+--------+----------- JEWELL FILTER. 14 | 0.05 | 3,938 | 81 | 2.06 | 97.94 | 0.88 13 | 0.07 | 7,827 | 167 | 2.13 | 97.87 | 0.87 WARREN FILTER. 15 | 0.06 | 3,545 | 59 | 1.66 | 98.34 | 1.67 14 | 0.06 | 20,022 | 265 | 1.32 | 98.68 | 1.48 ------------+----------+----------+---------+----------+--------+----------- It will be observed that the bacterial efficiencies are substantially the same, with the lower and with the higher numbers of bacteria in the raw water. That is to say, other things being equal, as the number of bacteria increase in the raw water the number of bacteria in the effluent increase in the same ratio. A further analysis of other groups of results would perhaps show variations in one direction or the other, but on the whole it is believed that the comparison is a fair one, and that there is no well-marked tendency for bacterial efficiencies of mechanical filters to increase or decrease with increasing numbers of bacteria. AVERAGE RESULTS OBTAINED WITH VARIOUS QUANTITIES OF SULPHATE OF ALUMINA. As it appears that neither the turbidity nor the number of bacteria in the raw water has a material influence upon the percentage bacterial efficiency obtained, we can take the results given above, which include all the results obtained (except a very few abnormal ones) for computing the various efficiencies obtained with various quantities of sulphate of alumina. These results are graphically shown by Fig. 21, p. 167, on which lines have been drawn indicating the normal efficiencies from various quantities of sulphate of alumina as deduced from our experiments. In computing the amount of sulphate of alumina which it would be necessary to use in operating a plant at a given place to give these efficiencies, the quantities of sulphate of alumina shown by the diagram can be taken as those which it would be necessary to use during those days in the year when the raw water was clear, or sufficiently clear, so that the amounts of sulphate of alumina mentioned would suffice to properly coagulate it. TYPES OF MECHANICAL FILTERS.