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.
Sections of the Warren and Jewell filters used at Pittsburg are presented herewith. The filters here shown are practically identical with those used at Lorain and Louisville, and nearly all the exact information regarding mechanical filters relates to filters of these types. These sections show clearly the constructions used at Pittsburg and Louisville, but there are some points in connection with the designs of these filters which require to be considered more in detail.
The simplest idea of a mechanical filter is a tub, with sand in the bottom and some form of drainage system. Water is run over the sand, passes through it, and is collected by the drainage system. When the sand becomes clogged it is washed by the use of a reverse current of water. This reverse current of water is so rapid as to preclude the use of a drainage system consisting of gravel, tile-drains, etc., such as are used in sand filters operated at lower rates, and instead metallic strainers in some form are used. The sand comes directly against these strainers, which are made as coarse as it is possible to have them, without allowing the sand to pass.
The rate of washing is usually from five to seven gallons per square foot per minute. In the Warren filter the openings in the strainers at the bottom are 6 to 8 per cent of the total area, and during washing the water has an average velocity of 0.20 foot per second upward through them. This velocity is so slow that the friction of the water in passing through the openings in the screen is practically nothing. A result of this is that if there is any unequal resistance of the sand to the water, the bulk of the water goes up at the points of least resistance in the sand.
Fig. 22.—Section of Jewell Mechanical Filter used in Pittsburg Experiments.