Sewage disposal . concrete troughs are provided in which the sewagemay flow to the extreme portions of the bed. Sometimes, as atBrockton, there is a single line of trough running down the centerof the bed, narrowing as it proceeds, and discharging a portionof its flow at each decrease in diameter (Fig. 69). In otherplants, as at Lake Forest, 111. (Fig. 70), the troughs radiate outin a crowfoot pattern. At this plant the troughs consist of twoupright sides of 2-inch plank, resting on a similar bottom plankwith 3-inch square holes at the base of the sides, spaced about 2feet apart, from which th
Sewage disposal . concrete troughs are provided in which the sewagemay flow to the extreme portions of the bed. Sometimes, as atBrockton, there is a single line of trough running down the centerof the bed, narrowing as it proceeds, and discharging a portionof its flow at each decrease in diameter (Fig. 69). In otherplants, as at Lake Forest, 111. (Fig. 70), the troughs radiate outin a crowfoot pattern. At this plant the troughs consist of twoupright sides of 2-inch plank, resting on a similar bottom plankwith 3-inch square holes at the base of the sides, spaced about 2feet apart, from which the sewage gushes out. It is well to sup- DISTRIBUTION OF SEWAGE ON INTERMITTENT FILTERS 251 port such troughs by vertical pipes or pieces independent of thesand to prevent uneven settling with the sand and consequentpooling and rotting. At many of the newer filter beds a dosing tank is provided,from which, by means of automatic devices, the allotted amountof sewage can be run upon the bed in from fifteen to thirty. Fig. 70. General View of Intermittent Filters at Lake Forest, 111.(courtesy of J. A. Alvord). minutes; and such rapid application greatly improves distribu-tion. Massachusetts engineers have generally adopted a dosingrate of 1 cubic foot per second for each 5000 square feet of area,and the beds are usually flooded at each dose to a depth of 1-3inches (corresponding respectively to 30,000-90,000 gallons peracre). Various forms of automatic dosing apparatus for intermittentfilters have been developed to a considerable degree of danger from the failure of such devices is, of course, alwaysconsiderable, and they require occasional inspection by some onecompetent to adjust the apparatus if necessary, but a fairlyaccurate automatic mechanism may perhaps be considered asreliable as the average city employee. The apparatus installed 252 INTERMITTENT FILTRATION THROUGH SAND at Lake Forest by Alvord (Fig. 71) is a simple and ingenious float in the dosing ch
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