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 12-inch pipe as the smallest size, to lessen the risk of stoppage, although 8 to 10-inch 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, 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, underdraws may be laid beneath or alongside the sewer. These are usually made of ordinary agricultural tiles, 3 inches or upward in diameter. They have no joints, being simply hollow cylinders, and are laid with their ends a fraction of an inch apart, wrapped with 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 in many cases are pierced with holes for ventilation.

Sewer Grades. The grades of sewers should, where possible, be sufficient to give them a self-cleaning 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-foot 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, per cent = 1000/5d+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 waterworks 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, and 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 simply to 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 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 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 is 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 the flow should be kept as nearly the same 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 Surface. No general rule can be followed in this matter, except to place the sewers 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 sufficient to prevent this in most cases.

Ventilation of Sewers. There is more or less difference of opinion in regard to the proper method of ventilating sewer mains. Ventilation through soil-pipe foul-air outlets carried above the roof-level, with the aid of street manhole gratings, constitutes the usual procedure, though not entirely satisfactory. If air inlets and outlets were placed on the main sewers at intervals of 300 feet or so, the accumulation of air-pressures which now obtains would be prevented. The omission of all intercepting traps would result in the uniform ventilation of the public sewers through the various house pipes, and, in the opinion of some, is highly desirable

The Combined System. The principal differences between the combined and the separate system lie in the greater size of conduits in the combined system, and the admission of surface water. Combined sewers are generally of brick, stone, or concrete, or a combination of these materials, instead of vitrified pipe. Another difference is the provision for storm overflows, by means of which the main sewers, when overcharged in times of heavy rainfall, can empty a part of their contents into a nearby stream. At such times the sewage is diluted by the rain-water, while the stream which receives the overflow is also swollen.

Size, Shape, and Material. The actual size of the sewer, and also to a large extent its shape and the material of which it is constructed, depend upon local conditions. Where the depth of flow varies greatly, it is desirable to give the sewer a cross-section to sult all flows as fully as possible.

The best form of section to meet these requirements is that of an egg with its smaller end placed downward. With this form the greatest depth and velocity of flow are secured for the smallest amount of sewage, thus reducing the tendency to deposits and stoppages. Where sewers have a flow more nearly constant and equal to their full capacity, the form may be changed more nearly to that of an ellipse.

For the larger sewers, brick is the most common material, both because of its low cost and the ease with which any form of conduit is constructed. Stone is sometimes used on steep grades, especially where there is much sand in suspension, which would tend to wear away brick walls. Concrete is used where leakage may be expected or where the material is liable to movement, but is more commonly used as a foundation for brick construction.

A catch-basin is generally placed at each street corner, and provided with a grated opening for giving the surface water access to a chamber or basin beneath the sidewalk, from which a pipe leads to the sewer. Catch-basins may be provided with water traps to prevent the sewer air from reaching the street; but traps are uncertain in their action, as they are likely to become unsealed through evaporation in dry weather. To prevent the carrying of sand and dirt into the sewers, catch-basins should be provided with silt chambers of considerable depth, with overflow pipes leading to the sewer. The heavy matter which falls to the bottom of these chambers may be removed by buckets and carted away at proper intervals.

Storm Overflows. The main point to be considered in the construction of storm overflows is to ensure a discharge into another conduit when the water reaches a certain elevation in the main sewer. This may be carried out in different ways, depending upon the available points for overflow.

Pumping Stations. The greater part of the sewerage systems in the United States operate wholly by gravity; but in some cases it is necessary to pump a part or the whole of the sewage of a city to a higher level. In general the sewage should be screened before it reaches the pumps.

Where pumping is necessary, receiving or storage chambers are sometimes used to equalize the work required of the pumps, thus making it possible to shut down the plant at night. Such reservoirs should be covered, unless in very isolated localities. The force main or discharge pipe from the pumps is usually short, and is generally of cast iron put together in a manner similar to that used for water-supply systems.

Tidal Chambers. Where sewage is discharged into tide water, it is often necessary to provide storage or tidal chambers, so that the sewage may be discharged only at ebb tides. These are constructed similar to other reservoirs, except that they must have ample discharge gates, so that they can be emptied in a short time. They are sometimes made to work automatically by the action of the tide.