Another difference is the provision for storm overflows, by means of which the main sewers when overcharged in times of heavy rainfall may 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 of unusually large size.

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, depends upon local conditions. Where the depth of flow varies greatly it is desirable to give the sewer a cross-section designed to suit all flows as fully as possible.

The best form 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 is 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 the 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 bottoms 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. The lifts required are usually low, so that high-priced machinery is not required. 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 may be emptied in a short time. They are sometimes made to work automatically by the action of the tide.

Sewage Purification

Before taking up this subject in detail it is well to consider what sewage is, from a chemical standpoint.

When fresh, it appears at the mouth of an outlet sewer as a milky-looking liquid with some large particles of matter in suspension, such as orange peels, rags, paper and various other articles not easily broken up. It often has a faint, musty odor and in general appearance is similar to the suds-water from a family laundry. Nearly all of the sewage is simply water, the total amount of solid matter not being more than 2 parts in 1,000, of which half may be organic matter. It is this 1 part in 1,000 which should be removed, or so changed in character as to render it harmless.

The two systems of purification in most common use are "chemical precipitation" and the "land treatment." Mechanical straining, sedimentation and chemical precipitation are largely removal processes, while land treatment by the slow process of infiltration, or irrigation, changes the decaying organic matter into stable mineral compounds.

Sedimentation

This is effected by allowing the suspended matter to settle in tanks. The partially clarified liquid is then drawn off leaving the solid matter, called "sludge" at the bottom for later disposal. This system requires a good deal of time and large settling tanks; therefore it is suitable only for small quantities of sewage.

Mechanical Straining

This is accomplished in different ways with varying degrees of success. Wire screens or filters of various materials may be employed. Straining of itself is of little value except as a step to further purification. Beds of coke from 6 to 8 inches in depth are often used with good results.

Chemical Precipitation

Sedimentation alone removes only such suspended matter as will sink by its own weight during the comparatively short time which can be allowed for the process.

By adding certain substances chemical action is set up, which greatly increases the rapidity with which precipitation takes place.

Some of the organic substances are brought together by the formation of new compounds, and as they fall in flaky masses they carry with them other suspended matter.

A great number and variety of chemicals have been employed for this purpose, but those which experience has shown to be most useful are lime, sulphate of alumina and some of the salts of iron.

The best chemical to use in any given case depends upon the character of the sewage and the relative cost in that locality. Lime is cheap, but the large quantity required greatly increases the amount of sludge. Sulphate of alumina is more expensive, but is often used to advantage in connection with lime. Where an acid sewage is to be treated, lime alone should be used.

The chemicals should be added to the sewage and thoroughly mixed before it reaches the settling tank; this may be effected by the use of projections or baffling plates placed in the conduit leading to the tank. The best results are obtained by means of long, narrow tanks, and they should be operated on the continuous rather than the intermittent plan. The width of the tank should be about one-fourth its length. In the continuous method the sewage is constantly flowing into one part of the tank and discharging from another. In the intermittent system a tank is filled and then the flow is turned into another, allowing the sewage in the first tank to come to rest. In the continuous plan the sewage generally flows through a set of tanks without interruption until one of the compartments needs cleaning. The clear portion is drawn off from the top, the sludge is then removed, and the tank thoroughly disinfected before being put in use again. The satisfactory disposal of the sludge is a somewhat difficult matter. The most common method is to press it into cakes, which greatly reduces its bulk and makes it more easily handled. These are sometimes burned but are more often used for fertilizing purposes. In some cases peat or other absorbent is mixed with the sludge and the whole mass removed in bulk. In other instances it is run out on the surface of coarse gravel beds and reduced by draining and drying. In wet weather little drying takes place and during the cold months the sludge accumulates in considerable quantities. This process also requires considerable manual labor, and in many cases suitable land is not available for the purpose. The required capacity of the settling tanks is the principal item in determining the cost of installing precipitation works.

In the treatment of house sewage provision must be made for about 1/12 the total daily flow, and in addition to this, allowance must be made for throwing out a portion of the tanks for cleaning and repairs. In general, the tank capacity should not be much less than 1/8 the total daily flow.

In the combined system it is impossible to provide tanks for the total amount, and the excess due to storm water must discharge into natural water courses or pass by the works without treatment.