Sewer, a channel which serves to carry away waste water and liquid refuse, trade effluents, rainfall, etc.; the term drain is applied to a channel which carries off the drainage of one building only, and which communicates with a cesspool or similar receptacle, or with a sewer; a sewer being the larger duct which receies as tributaries the various drains which communicate with it. In some towns what is known as the separate system of sewerage is adopted, the rainwater being carried away by a series of ducts distinct from those which carry household and trade effluents. Where the separate system is not adopted, the capacity of the sewers must be so regulated as to enable them to remove storm waters. It is usually calculated in this country that the sewer should be capable of dealing with a maximum rainfall of one inch per hour, over and above the waste matters derived from other sources. The smallest sewers are usually made of earthenware pipes, varying from nine inches to two feet in diameter: the larger main sewers are made of brick, set in cement upon a bed of concerte; in vertical section their form is generally oval or egg-shaped rather than circular, this method of construction rendering them less liable to be silted up when only a small volume of sewage is flowing through them. Sewers are laid in straight lines manholes being provided at the various junctions so as to facilitate inspection and to allow of the operations of flushing being performed. The fall of a sewer varies from about 1 in 100 in the smaller to 1 in 750 in the larger channels.
The composition of sewage is, curiously enough, very little altered by the fact of the exclusion or presence of water-closet discharges; the average compoition of sewage from water-closet towns and from towns in which dry systems of removal prevail not being strikingly dissimilar. Sewage contains, on an average, some twelve hundred or thirteen hundred parts of solid residue in 1,000,000 parts by weight, the amount of the suspended solids being rather greater than the amount of those which are held in solution. The most valuable constituents of the sewage from a manurial point of view are the nitrogenous compounds, potash salts, and the phosphates. It has been calculated that, on an average, the annual quantity of sewage per unit of population is 100 tons, and were this made to yield its theoretical value as manure, it would amount to nearly £ per head.
The air contained in sewers differs somewhat in composition from the air of the atmosphere, gases being continually given off by the sewage and some percolation of ground air into the sewers usually occurring. Where the fall of a sewer is insufficient, and particularly when the level of a sewer is not properly regulated, or has become affected by subsidence (allowing of the collection of stagnant pools of sewage), this accumulation of foul gases in the body of the sewer is especially favoured. Under such circumstances the sewer-air may be a serious source of danger to those who work in the sewers. There is risk, too, of its finding its way into houses with imperfect drainage anangements, and wherever means of escape are provided for the gases complaints of nuisance are almost sure to arise. With a view to securing some interchange of air in sewers and so making it safe for flushers and sewermen to enter them, and with a further view to allowing of the escape of air at times when there is an increased flow of sewage in the sewers, and particularly after rainfall, it is necessary to provide means of ventilation. Gratings situated in the centre of the roadway are usually employed for this purpose, the distance between such gratings being one hundred yards or thereabouts. In some instances shafts are carried up the sides of houses with a view to causing the sewer-air to escape at points where it will not cause offence. Sometimes the air from sewers has been extracted and passed through a furnace so that it may be rendered innocuous. The method of ventilation by gratings in the middle of the roadway is, however, very rarely productive of annoyance when the sewer is in a satisfactory condition.
The methods of disposing of town sewage have received a good deal of attention during recent years, especially since the undesirability of passing sewage directly into streams has been insisted upon. Some form of chemical treatment (the essence of which consists in precipitating the organic matter by the addition of agents such as lime, sulphate of alumina, etc.) is usually adopted. The effluents after such treatment are in many cases still a source of injury to the streams into which they are discharged. Filtration of sewage is sometimes had recourse to, the material being discharged over a porous soil, and the flow being from time to time suspended in order to permit fresh supplies of oxygen to obtain access to the filter. The method of broad irrigation, in which the sewage is distributed over a sewage farm and utilised as manure for certain crops, has also been employed in several instances. This is no doubt the best means of dealing with sewage when land in sufficient quantity and of suitable character is obtainable,