in view of his experience with 2-inch rods, with an unsupported length of 4 feet, believes that the Ontario construction was too light.

Sometimes instead of iron rods, wooden slats are employed, making a screen similar to those used in racks for waterpower. At Providence, for example,1 a wooden screen is described which is placed in manholes to intercept mill refuse. This same form of screen is used on the large screen chamber shown in Fig. 119. Fig. 134 shows the design.

At Pullman, Ill., an elaborate screening tank was built, a section of which is shown in Fig. 135.2 The tank is boiler iron 6 feet in diameter and 24 feet high, the bottom being set up from the ground high enough to allow a wagon to drive underneath and receive the screenings, which are allowed to fall through a door in the bottom of the tank. The screen is of rectangular mesh with-inch openings.

In front of the wheel pits of a power plant at Richmond, Va., is a fixed screen, with a mechanical cleaning device which merits attention. The screen is vertical, made up of 3-inch steel bars spaced 1ğ-inch centers, 18 feet wide and 21 feet high. In front of this screen is a movable rake, supported on shafts at the top and bottom of the screen. The cleaning device consists of a number of pieces of angle iron, fastened to endless chains, which are revolved by sprocket wheels 1 City Report, 1894. 2 Rafter and Baker, p. 460.

Fig. 135

on the shafting, through three vertical legs. Riveted to the horizontal leg are projecting teeth, so spaced that they fit between the bars, just passing the cross-bars through which the screen bars are fastened. These teeth are ofX 1-inch iron. Fig. 136 shows the general arrangement.1

The third type of screens, characterized as mechanical, are commonly used in England, but have found little favor in this country. Mr. John D. Watson, engineer in Birmingham, has recently installed a mechanical screen described as follows:

The screens are perforated, flexible, endless metal belts inclined at an angle of 30 degrees and running over a horizontal revolving drum at each end, the lower

1 Eng. Rec., Vol. 49, p. 12.

end immersed in the sewage. The drums are placed transversely across the channel through which the sewage is passed, and are operated by a Poncelet water wheel, driven by the flow of the sewage, the speed at which they revolve and the capacity of the screens varying with the changes in the amount of sewage flowing. The intercepted material is lifted out of the sewage and carried around the drum, where a rotary brush cleans it off and transfers it to a worm conveyor placed transversely in the rear of the screens and discharging in a barrow or truck at one end.

Fig. 137 shows diagrammatically the general arrangement.

In conclusion it may be said that the importance of screening as the first step towards the purification of sewage is becoming

more and more recognized. Before any biological process can be successfully carried on, all coarse and unresponsive material must be eliminated; and while grit chambers or roughing filters may be used, engineers are appreciating more and more the efficiency and economy of the use of screens. At Columbus, Ohio, for example, the engineer in charge of the experimental plant, after experimenting with various types of screens and size of mesh, adopted two screens of diamond mesh wire cloth woven with No. 12 wire. The first screen had a clear opening of inch, and the second of 3 inch, and the action of the screens was considered to be of great importance as a part of the entire method of treatment.

CHAPTER XI.

STORM-WATER OVERFLOWS AND REGULATORS.

IN the construction of combined sewers, that is, sewers which carry both storm water and house drainage, there are two methods or opportunities for reducing the expense involved in the construction of large storm sewers: First, by diverting the storm water, in excess of a certain amount, from the trunk sewer into a convenient stream, thereby avoiding the first cost of a long and large trunk sewer; and second, by diverting the excess storm water before it passes through a pumping station or on to a purification bed, thereby avoiding the continual expense of handling a large amount of storm water.

To illustrate a suitable use of these storm-water overflows, the following example is given:

The city of Rochester has a long intercepting sewer, surrounding the city on three sides, as shown in Fig. 138. This sewer collects a large part of the city sewage, both domestic and storm water, and prevents the contamination of the small streams shown. When this sewer has reached the point A, the diameter is 8 feet, and the capacity is 340 cubic feet per second, although the house-sewage flow is only 10 cubic feet per second. In order to avoid the cost of building this large sewer further, it is reduced to 4 feet in diameter, and capacity of 40 cubic feet per second, and provision made by which the difference between 40 cubic feet and 340 cubic feet can escape through a special channel into the waters of Thomas Brook, as shown. This has worked well for ten years, but there are indications that the overflow comes into more frequent use than was intended, that the result is likely to be a nuisance in Thomas Brook, and that some remedy must soon be provided. A second overflow is also provided, at the

point B, under similar circumstances, also at three other convenient points.

The propriety, from the sanitary standpoint, of the use of this arrangement depends on the local conditions. If the

B

Fig. 138

stream is small, tortuous and sluggish, with shallow ponds, the storm overflow should not be used. But if the discharge is to be into a large river, already organically polluted, and thereby unfitted for drinking water, such a device is proper and economical.

This separation of sewage from storm water is accomplished