The first step is to lay out the pipe or conduit system. For this the topographical map already mentioned will be found useful. This, however, should be supplemented by a profile of all the streets in which sewers are to be laid, in order to determine the proper grades. In laying out the pipe lines, special diagrams and tables which have been prepared for this purpose may be used. In the separate system it is generally best to use 8" pipe as the smallest size to lessen the risk of stoppage, although 6" pipe is ample for the volume of sanitary sewage from an ordinary residence street of medium length. Pipe sewers are generally made of vitrified clay, with a salt-glazed surface. Cement pipe is also used in some cities. The size of pipe sewers is limited to 30 inches in diameter, owing to the difficulty and expense of making the larger pipe and the comparative ease of laying brick sewers of any size from 24 or 30 inches up. In very wet ground, cast iron pipe with lead joints is used, either to prevent inward leakage or settling of the pipe.

The pipes should be laid to grade with great care and a good alignment should be secured. Holes should be dug for the bells of the pipe, so that they will have solid bearings their entire length. If rock is encountered in trenching, it will be necessary to provide a bed for the pipe which will not be washed into fissures by the stream of subsoil water which is likely to follow the sewer when the ground is saturated.

Underdrains

Where sewers are in wet sand or gravel, underdrains may be laid beneath or alongside the sewer. These are usually the ordinary agricultural tiles, from 3 inches in diameter upward. They have no joints, being simply hollow cylinders, and are laid with their ends a fraction of an inch apart, wrapped with a cheap muslin cloth to keep out the dirt until the matter in the trench becomes thoroughly packed about them. These drains may empty into the nearest stream, provided it is not used for a public water-supply.

Manholes

These should be placed at all changes of grade and at all junctions between streets. They are built of brick and afford access to the sewer for inspection; in addition to this they are sometimes used for flushing. They are provided with iron covers which are often pierced with holes for ventilation.

Sewer Grades

The grades of sewers should be sufficient where possible, to give them a self-clearing velocity. Practical experiments show that sewers of the usual sections will remain clear with the following minimum grades: Separate house connections, 2 per cent; (2-feet fall in each 100 feet of length) small street sewers, 1 per cent; main sewers, 0.7 per cent. These grades may be reduced slightly for sewers carrying only rain or quite pure water.

The following formula may be used for computing the minimum grade for a sewer of clear diameter equal to "d" inches and either circular or oval in section.

Minimum grade, in per cent = 100

5d + 50

Flushing Devices

Where very low grades are unavoidable and at the head of branch sewers, where the volume of flow is small, flushing may be used with advantage.

In some cases water is turned into the sewer through a manhole, from some pond or stream, or from the public water-works system. Generally, however, the water is allowed to accumulate before being discharged, by closing up the lower side of the manhole until the water partially fills it, then suddenly releasing it and allowing the water to rush through the pipe. Instead of using clear water from outside for this purpose, it may be sufficient at some points on the system to simply back up the sewage, by closing the manhole outlet, thus flushing the sewer with the sewage itself. Where frequent and regular flushing is required, automatic devices are often used. These usually operate by means of a self-discharging siphon, although there are other devices operated by means of the weight of a tank which fills and empties itself at regular intervals.

House Connections

Provision for house connections should be made when the sewers are laid, in order to avoid breaking up the streets after the sewers are in use. Y branches should be put in at frequent intervals, say from 25 feet apart upwards, according to the character of the street. When the sewer main is deep down, quarter bends are sometimes provided, and the house connection pipe carried vertically upwards to within a few feet of the surface to avoid deep digging when connections are made. Where house connections are made with the main, or where two sewers join, the direction of flow should be as nearly in the same direction as possible, and the entering sewer should be at a little higher level in order to increase the velocity of the inflowing sewage.

Depth Of Sewers Below The Surface

No general rule can be followed in this matter except to place them low enough to secure a proper grade for the house connections, which are to be made with them. They must be kept below a point where there would be trouble from freezing, but the natural depth is usually sufficient to prevent this in most cases.

Ventilation Of Sewers

There is more or less difference in opinion in regard to the proper method of ventilating sewer mains. Ventilation through house soil pipes is generally approved where the sewers and house connections are properly constructed and operated, and where the houses on a given street are of a uniform height, so that the tops of all the soil pipes will be above the highest windows. Where the houses are uneven in height, or where the sewerage system or connections are not well designed or constructed, it is recommended that main traps should be placed on all soil pipes, and that air inlets and air outlets be placed on the sewers at intervals of from 300 to 400 feet.

The Combined System

The principal differences between this and the separate system are in the greater size of conduits and the use of catch-basins or inlets for the admission of surface water. They are generally of brick, stone or concrete, or a combination of these materials, instead of vitrified pipe.