in consequence of which, internally-fired Scotch boilers have been adopted, the combustion chambers being made of steel plates with water spaces, and with no brick-work except in the bridge walls and around the fire doors.

The amount of sludge removed by the various processes has been referred to in the previous articles. Average raw sewage contains about 200 parts per million of suspended matter, or, say, one cubic yard per million gallons, and double this of compressed sludge or ten to twenty times that amount of wet sludge. A very large percentage of this remains to be disposed of, whatever the treatment, unless it be carried away with the effluent. In Worcester, Mass., one part of sludge is obtained from 90 parts of sewage, there being one ton of solid matter to 750,000 gallons of sewage, 34 per cent of this being organic matter. With a lime precipitant there would be about 0.4 of a pound of sludge per capita daily. The experiments with Columbus sewage gave 5.75 cubic yards of wet sludge (87 per cent water) per million gallons removed by plain sedimentation; and about the same deposited in septic tanks, which was reduced to 2.68 cubic yards by hydrolysis. By chemical precipitation 11.4 cubic yards of sludge, 92 per cent of water, was obtained, or about the same amount of solid matter.

ART. 85. SUMMARY

The methods of treatment described produce effluents differing widely in quality. They use materials of construction of various kinds; some require areas many times larger than others. Some involve a fall or loss of head of only a few inches, others of several feet. Some are best adapted to fresh sewage, some to stale, and others to sewage containing large amounts of trade wastes. (Special methods are required in many instances for the treatment of trade wastes, especially those high in fats, fibrous or other carbonaceous matter.) Where anything approaching complete purification is necessary, a combination of two or even three methods is generally most effective and economical. Which methods should be employed can be properly decided only after a careful study of the conditions.

For a high degree of purification, only one practicable method is known-intermittent sand filtration. For high bacterial purification, either intermittent filtration or disinfection following clarification may be used. For producing non-putrescible effluents, the sprinkling filter seems to be the most effective and economical of area, although double-contact filters give excellent results and are probably cheaper in construction.

In connection with any of the above except intermittent filtration, some preliminary treatment is necessary to remove the coarser suspended matter; and this is advisable with finegrain filters also, although the sand which composes these strains out upon the surface most of the suspended matter. For such preliminary treatment, screens followed by sedimentation are generally best. The sedimentation tank may be operated as a septic tank, by which the sludge is reduced in volume, and that remaining is less offensive and contains much fewer pathogenic bacteria. Fine-grain filters used for final treatment, however, seem to clog up more rapidly and deeply with septic than with plain sedimentation effluent.

The structural features require engineering skill combined with knowledge of the principles of the physical and bacterial actions taking place. The best arrangement of the several parts will usually be determined to some extent by the topography of the site. The fact that the preliminary processes require less area than the final has suggested a general circular form, the sewage progressing radially from the center outward through concentric tanks or beds; but in most cases the arrangement is such that the sewage advances in parallel lines from one end of the plant to the other.

The matter of applying the sewage to filters is a detail which varies in different plants. In the case of sprinkling filters, pipes may be laid on the bottom, with a riser to carry each nozzle set vertically in a T; or the distributing pipes are sometimes laid on the surface of the bed, with the sprinkler heads screwed directly into them; but the more common practice in this country is to set them on concrete posts of such height that the pipe is just below or at the surface of the bed, and attach the sprinkler nozzles

either directly or by means of short nipples. The pipes should be arranged in straight lines with removable plugs at each end so that they can be cleaned out if necessary.

Contact beds are not always provided with arrangements for distributing the sewage; but fine-grain filters, if filled by a slow stream from one point only, would be apt to pass most of the sewage through the bed near that point. For this reason distributors, generally in the form of troughs, are laid across the bed. Wooden troughs are most common, and last four or five years. Split sewer pipe are used, also. These must be laid practically level so that the sewage will flow over their edges for their entire length. These troughs sometimes radiate from the inlet, or may be parallel and provided with outlets or branches at intervals so as to reach all parts of the bed, the troughs being spaced 10 to 25 feet apart.

Where the sewage is flushed on in doses, distributors are not often necessary, but a slab or apron is placed extending for 2 to 5 feet around the outlet to prevent wash. Dosing devices of various kinds have been used, perhaps the most common being siphons similar in action to automatic flush tanks. Also various arrangements of tipping tanks are used, air compressed by the rising sewage, and other contrivances for alternating the flow from one bed to another. Generally a dosing tank or chamber is provided which, as it fills each time, is discharged to different beds in succession. This necessarily involves a loss of head equal to the depth of the dosing tank.

Constant-flow tanks involve a loss of head of only an inch or two; intermittent flow, the depth of the tank. All filters require a loss of head equal to their depth, which should generally be at least four or five feet. Sludge beds must be at least two or three feet lower than the surface of the sewage in the tanks if the sludge is to be drawn out by hydrostatic pressure, and lower than the bottom of the tanks if it is to be drawn off by gravity.

Securing a uniform rate of flow at all points in each crosssection of a tank is very important, is never secured perfectly but deserves the most careful consideration. Any construction which reduces available area (such as a baffle extending above the sludge

or a scum board extending below the scum, or a slotted inlet) increases velocity, and this velocity, in slowing down again, produces eddies and cross and vertical currents which prevent sedimentation and stir up sludge. In most tanks there are currents, straight or winding, having several times the theoretical velocity of flow through the tank. Narrow tanks help to remedy this, but the perfect solution remains to be found.

After leaving the sewer of approach, the sewage should flow in open channels at all points, or in channels whose tops can be removed; or at least, if pipes are used, they should be so arranged that every point is accessible for cleaning out.

A fault in most tanks is that the channels are rough, with square angles, and discharge over weirs; each of which encourages deposits, which in turn help to make the plant offensive. Smooth channels with rounded bottoms, like a good sewer, should be provided, and the outlet should be through several branches whose bottoms are no higher than the bottom of the channel, or through a slot in the bottom, or of some other form which will prevent deposits in the channel.

Arrange all parts of the plant so that they can be flushed off and kept clean; avoid wood or other absorbent material in contact with sewage and keep all woodwork painted white or varnished; plant shrubbery around the grounds, especially to conceal unsightly parts of the plant, sod all banks, and in general try to make the plant and surroundings attractive. See that all sewage matters are confined to the insides of the tanks and filters. Water piped to the plant under pressure is desirable to assist in keeping it clean, and for breaking up and settling scum on Imhoff and other tanks.

Sewage treatment plants need not be particularly offensive in any feature except the sludge disposal, but this can hardly help but offend the senses. Fine-grain filters, however, if intelligently operated, create only dry sludge which can be burned or otherwise disposed of inoffensively; and many of these are operated near residences without creating a nuisance. But in general it will be necessary to arrange for the treatment and disposal of wet sludge at a distance from any built-up section.