Sewers are built of glazed stoneware or of impervious brick laid on a bed of concrete to prevent sinking of any part, the parts being most solidly put together with cement. Iron and steel pipes are also used especially when extra strength is required, as when there is some danger of the pipes sinking. For most small streets circular stoneware sewers suffice. Oval brick sewers are more suitable for main streets in which the amount of flow varies greatly. The cross-section of these should be an egg-shaped oval with the small end downwards, as this ensures the most rapid current. Sewers should be laid in as straight a line as possible, and with a fall which will ensure a flow of at least 2½ feet per second. The following rule gives approximately the fall required for smaller sewers and drains:

For further particulars as to the velocity of flow see page [187].

Where a town is very flat, and a proper fall of sewer impossible, Shone’s ejectors are sometimes used to raise the sewage. The sewers are laid in sections, each section falling to a certain point, from which the sewage is raised by the ejector to a higher level and so carried to the next section of sewers. Each section has a separate system of ventilation. The provision of manholes for inspection of an intercepting trap and of ventilation of the house drain near its junction with the sewer has already been considered (page [179]).

As sewers have commonly to carry away the rain-water in addition to the waste matter from houses, their size must be regulated accordingly. The rainfall being very various, the sewers may occasionally become overcharged and flood the basements of houses. During heavy rainfall large quantities of road grit are washed into the sewers, the intercepting gully tanks at the road sides being insufficient to prevent this.

In addition to the above disadvantages associated with discharging storm water into sewers, the sewage owing to its increased bulk is more difficult of disposal, whatever method of disposal be adopted. The size of the sewers and of storm-outfalls into the nearest river or the sea must be regulated so that they are equal to these sudden strains on them; or the Separate system, by which the rain is carried in special conducts to the nearest river, must, in the alternative be adopted. The objections to this plan are that it necessitates a double system of sewerage, and does not allow of the useful scouring effect of rain on sewers. Where it is feasible, and particularly in small country towns, its adoption is advisable. In such cases the old brick drains are used for carrying off rain-water, while new pipe-sewers are employed for carrying the sewage. Such pipe sewers are not liable to become fouled, and on account of the decreased dilution of the sewage can be made smaller than brick-sewers.

Sewers being closed conduits containing sewage, it is highly desirable that the gases resulting from decomposition should be freely diluted. Such gases in unventilated sewers may find their way through intercepting traps into the house drain. The danger from this source has been considerably exaggerated. Ventilation of the sewer is, however, desirable. This has been commonly accomplished by gratings opening at intervals directly into the middle of streets. In narrow streets, the stench from street-grids is occasionally a source of complaint, and may cause malaise and ill-health. Charcoal traps placed below sewer grids to intercept offensive gases have been found to be of little use. The best plan is to do away entirely with surface ventilators, and carry up iron shafts or brick shafts lined with stoneware pipes above the level of all neighbouring houses. The only difficulty in adopting this plan is the difficulty in securing premises to erect such shafts up houses, although no danger attaches to the practice.

Ventilating shafts should be erected at intervals and particularly at the highest points of the sewage system of a town, the upper end of these shafts being remote from the windows of any dwelling-house. When sewers are laid with too steep a gradient, they act as chimneys, the gases mounting to the higher part of the town, and frustrating attempts at ventilation on lower levels. Various attempts have been made to ventilate sewers by artificial means, as by the aspirating effect of street lamps, etc., but these efforts have not been successful, as the effect of the up-current only influences a short length of sewer. The usual method of combined up-shafts and street grids answers fairly well, but when any complaint of smell from street-grids occurs they should be replaced by up-shafts. In Bristol and a few other towns no provision is made for sewer-ventilation, and no ill effects have apparently resulted. There can be no question that the importance of sewer-ventilation has been exaggerated. If the sewer has a sufficient gradient, and is properly laid, and efficiently flushed, so that no offensive deposits occur, the provision of up-shafts at favourable points is all that is necessary. Sewer effluvia have been credited with causing enteric fever, diphtheria, and other diseases. These diseases rarely if ever owe their origin to this cause. The microbes discovered in sewer-air have always been those of the outside atmosphere, and not derived from the sewage. Sewer effluvia might, however, predispose to such diseases, if exposure to them were frequent or protracted, by lowering the powers of resistance of the constitution.

Flushing of Sewers is required at intervals, in order to remove any deposits of grit or other solid matter. Flushing is effected by filling special flushing shafts placed at intervals in the sewers with clean water and then suddenly releasing this.

Whenever there is stagnation, a foul odour is certain to be emitted. The cardinal rule with regard to sewage is to keep it in rapid onward motion, until it has passed the outlet of the sewer. The introduction into a sewer of hot water or waste steam is an occasional cause of nuisance.