*The figures in this column should be used with caution. More complete data

ferent screens.

The

The drum screen consists of a revolving drum, of either cylindrical or truncated cone shape, the outside being a screen. sewage enters at one end and flows out through the perforations in the screen, material tending to clog these being blown into the

should be secured, in order to furnish a reliable comparison between the efficiencies of dif† Including coarse screening, settling and subsequent screening.

drum by air or water. A number of these are in use in Germany. At Reading, Pa., Atlanta, Ga., Baltimore, Md., and Brockton, Mass., is used a drum screen known as the Weand

screen.

This is constructed with a fine wire mesh protected by

an outer coarse mesh. The intercepted material is worked toward one end by an interior spiral flange, and at that end is raised by short radial plates and dropped into the end of a chute which projects into the drum. The Reading screen is 12 feet long and 6 feet in diameter. The screen, of Monel metal, has 36 meshes per inch, the protecting screen being of No. 12 copper wire, inch mesh. It is cleaned by jets of screened sewage directed against the outside of the drum.

The Riensch-Wurl screen is used in several German cities and in Daytona, Fla., and Brooklyn, and Rochester, N. Y., and is proposed for some other cities. The general shape is that of a hat with a flat brim, both composed of slotted plates, about three-fourths of the brim being submerged; the axis being set at 10 to 30 degrees with the vertical. This revolves, and as the intercepted matter is carried above the sewage surface on the surface of the plates, it is brushed off by revolving brushes and falls into cans or onto a conveyor. Each screen in the Dresden, Germany, plant contains 230,000 openings, each .087 by 1.2 inch.

Perforated plates intercept finer matters than do bars. Wiremesh screens may be made still finer. Compressed air gives drier screenings than water jets, and both clean more thoroughly than do brushes or scrapers, but apparently cause more spattering and diffusion of odor.

The disposal of the screenings from plants such as these is a difficult problem. They quickly become offensive and must be buried, burned or treated in some way within a few hours after removal from the screens.

ART. 73. TANK TREATMENT. SEDIMENTATION

The general object of tank treatment is clarification. By clarification is meant the physical removing of matters in suspension, as is done in the laboratory by the use of filter paper. These matters are of varying size and consistency, some being so fine as to be microscopic; and there are matters known as colloids which are so minute as to sometimes render it a matter of debate whether they are in solution or suspension. Some

of the matters are heavier than water, the sand and other mineral substances from the street surface especially; some are lighter than water and float to the surface, such as fats, pieces of wood, etc.; and others have a specific gravity of practically one and only gradually move either downward or upward. Some of the suspended matters are more or less soluble and would be taken. into solution if sufficient time be allowed; in fact, the amounts of matters in solution and those in suspension in a given sewage will ordinarily vary with the age of the sewage, the former increasing and the latter decreasing. Bacteria are in suspension, attached to or embedded in particles of organic matter; so that removal of such matter by clarification or otherwise will at the same time remove large numbers of the bacteria.

By running sewage slowly through a tank or basin much of the suspended matter will settle out by gravity, forming a sludge or thick liquid at the bottom. If run through more rapidly, only sand and other coarse mineral solids will be deposited. When the flow is slow, fats, pieces of wood and other light particles, including organic matter which is gasifying, will float upon the surface. The slower the flow the larger the percentage of matter which will settle out; but this percentage increases much less rapidly than the reduction in velocity, and such reduction becomes uneconomical beyond a certain point.

The simplest plan would be to discharge the sewage into the tank from a pipe at one end and remove it at the other by a pipe at the level of the contained sewage. This, however, would cause a current more or less direct from one pipe to the other, giving too great velocity to the flowing sewage and leaving much of the tank contents practically stagnant. This is avoided by admitting the sewage through several inlets across the end, or better still through an orifice or over a weir extending entirely across the end; the effluent being removed through a similar orifice or weir at the outlet end. If a weir be used for the latter, the floating scum will pass off with the effluent. This is generally not desired, and is prevented by use of a submerged orifice, or more commonly by a "scum board " which extends across the tank a short distance from the outlet and the lower edge of which

is 6 to 18 inches below the surface of contained sewage. If the submerged orifice is used, the pipe or channel which serves as outlet for this is brought up to the desired level of the tank, thus fixing this. The form of inlet and outlet and their approaches should be so designed as to distribute the flow across the entire width of the tank and also reduce the velocity of entrance as much as possible. Present practice is by no means satisfactory in this respect.

This

If the sediment and scum remain long in the tank, bacterial action begins and becomes more and more active, especially in the former. As there is little if any available oxygen in the tank, the action is anaerobic or putrefactive. This results in liquefying and gasifying much of the organic matter, a large part of the remainder being finely comminuted. As the gases form they rise to the surface, generally carrying organic matter with them, sometimes in masses of several inches area. action and the vertical currents set up tend to prevent sedimentation and also carry into re-suspension matter which had already settled to the bottom. Some part, also, of the gases is probably taken into solution in the sewage. The escaping gases may be offensive, but generally are not seriously objectionable unless the tank be very large and the air motionless, and not always then.

Several modifications of construction have been used to meet or avail of these and other conditions. One aims to permit sedimentation and also the gasifying action without any interference of the latter with the former. Another takes advantage of the fact that fine suspended matter is observed to adhere to surfaces, by introducing a great number of surfaces. The latest idea is to prevent anaerobic putrefaction by continuously forcing air into the sediment. These will be described later.

In such a tank as is described above, the sewage flows continuously, though slowly, leaving at a level only slightly lower than that of entrance. This is called a constant-flow tank. Another style of tank, called intermittent-flow, is filled and allowed to stand full for some time, when the liquid is withdrawn. This requires the outlet pipe to be as much below the