Sewerage, a system of drainage under the streets of towns for carrying off the surface waters and the liquid refuse matters from houses. The necessity of underground channels of this character to the comfort and health of thickly populated places was well understood by the ancient Romans, who at a very early period adopted a regular system of drainage in their famous sewers. (See CloacAE.) These included not merely the spacious subterranean vaults by which the drainage of the pestilential marshes about the city was effected, but also the wooden pipes, clay tubes, or drains of whatever kind by which the impurities were conveyed from the houses into the main conduits. An elaborate system of sewerage has recently been discovered in connection with the Colosseum. So completely was the city underlaid by these passages, that it was designated by Pliny as urbs pensilis, a city supported upon arches. The avaskara mandira (filth temple) of the ancient Hindoos was beyond doubt connected with channels and receptacles for its accumulations.

The great canal system of Egypt, executed under Rameses I. and his successors, served extensive sewerage purposes; and probably also the magnificent canals of Assyria and Babylon, fed by the Tigris and Euphrates. The ancient Chaldean tomb mounds possess great interest on account of their system of drainage. Long shafts of baked clay extend from the surface of the mound to its base, composed of a succession of rings 2 ft. in diameter and about 1 1/2 ft. wide, joined together by thin layers of bitumen. (See Rawlinson's "Five Ancient Monarchies," vol. i., p. 89.) The recent discoveries of Dr. Pierotti among the ruins of Jerusalem have shown that the ancient city contained a complex and perfect system of aqueducts, drains, and reservoirs. The preservation of many of the aqueducts is owing to the fact that they were excavated in the solid rock, and have not been affected by the demolition of the structures above. It appears that the pool of Siloam received the washings of the temple, and the liquid was used for the purpose of irrigating the king's gardens.

The discoveries clearly show that the inhabitants of Jerusalem were fully aware of the necessity of speedily removing all decomposable refuse matter. - The system of sewerage of Paris has during the last half century been made among the finest in the world. Previous to the 14th century, Paris being walled only on the south, the drainage of the faubourgs St. Germain and St. Marceau was poured into the Bièvre; but when this district was surrounded by ditches about 1356, in the reign of King John, the sewers of the quarter St. Germain-des-Prés were turned into these ditches, and they have since taken the same course, being carried by the vaulted sewer which starts from near the école de médecine, and empties into the Seine below the palais des arts. On the opposite bank an open sewer was covered over in the reign of Charles VI. by Hugues Aubriot, and hence he is said to have commenced the system of covered sewers in Paris. The open sewer of St. Catharine was very offensive to the inhabitants of the palais des tournelles, and as a consequence Francis I. bought the site of the Tuile-ries. As late as 1663, in the reign of Louis XII., there were only 1,207 toises of covered sewers in Paris, and 4,120 toises of open sewers or ditches.

The sewer formed by the ancient rivulet of Ménilmontant, then and now called the grand égout de ceinture, was not walled and covered till 1740. In 1805 Napoleon authorized necessary repairs and the extension of the covered sewers of Paris. In 1806 there were about 23,000 metres of covered sewers, but in 1862 they amounted to about 226,000; at present (1875) the total length of covered sewers is about 400,000 metres, or nearly 250 m. To organize the great system of sewerage, Paris was divided into five basins, of which three are on the right and two on the left bank of the Seine. Six principal galleries cut the city at right angles, and receive 15 secondary galleries, and these in turn numerous minor galleries. Besides these galleries, the old grand sewer, the égout de ceinture, serves as a collector, into which the others empty. This grand sewer now starts from the rue des Coutres-Saint-Gervais, follows the rues Vielle-du-Temple and Filles-du-Calvaire, crosses the boulevards, follows the rue des Fossés-du-Temple, traverses the lower end of the boulevard Prince Eugène, continues its course through the rues du Château-d'Eau, des Petites-Écuries, Richer, de Provence, and de Saint Nicolas-d'Antin, and falls into the general collector at Asnières, under the boulevard Malesherbes. Formerly it emptied into the Seine at Chaillot. The principal galleries on the left bank are: 1, that along the line of quays; 2, that which follows the boulevard Saint-Michel; and 3, the vast subterranean canal which receives the Bièvre. These sewers also fall into the general collector at Asnières, under the place de la Concorde, crossing the Seine by means of a reversed iron siphon over a yard in diameter and 217 yards long, entering the river about 6 ft. below low-water mark.

In many of the galleries there are railways, on which cars are run in cleaning the sewers, and also for carrying visitors. The water and gas mains are also carried in the galleries covering the principal sewers. - In England sewer commissioners were appointed in the reign of Henry VI., but their powers were restricted to surface drainage and sea walls, the subject of municipal sewerage being left principally to local commissions, and in some of the cities to corporations empowered by special acts of parliament. The drainage of London was provided for by legislative enactments commencing in 1225, and the whole subject was thoroughly revised by Sir Thomas More in the celebrated "bill of sewers," passed in 1531. The use of the sewers of London, even up to the present century, was limited to the removal of the waters that ran in the gutters of the streets, including those thrown out from the houses; and in the reign of George III. an act was passed prohibiting the discharge of other matters into them under penalty of a fine. Every house was provided with a cesspool, and this was occasionally discharged by the night carts which conveyed away the offensive matters.

Upon the gravelly soil to which the city was then limited this answered very well; but its extension over the more impervious clay beds, and the introduction of abundant supplies of water into every house, followed by the invention of water closets, led to a new use of the sewers, and to various results not at all contemplated in their original construction. The refuse matters of the cesspools, instead of being transported into the country to serve as manure, were turned into the river Thames, polluting its waters, while the sewers themselves in the lower parts of the city were incompetent to discharge the increased burdens, and the ventilating flues through the streets became avenues of the most poisonous gases. A reconstruction of many of the sewers upon a larger scale became necessary, and they were laid out upon a regular system, coming down to the river from each side for a distance of 6 m. along its course. Their total length exceeded 2,000 m., and when they were completed London was regarded in 1855 as one of the best drained cities in the world. The principal sewers were of extraordinary dimensions, several being from 12 to 14 ft. high and 6 1/2 ft. wide. Notwithstanding the great scale of this drainage system, the most serious difficulties were experienced in its operation.

The outlets of the sewers, in order to get sufficient fall for discharge, were placed but little above low-water mark, the surface of some portions of the city itself being below high-water mark. Consequently the sewers were closed by the tide except at low water, and the gaseous contents of the sewage turned back into the city and up the drains into every street and house. The discharge moreover taking place only about the time of lowest water, the accumulated contents of the sewers were kept up the river until the ebb, and when at last carried down the stream they were replaced by the same amount of fresh filth. Foul banks of black mud, from which most offensive odors emanated when laid bare at low tide, collected along the banks and in the shallow parts of the river. In 1858 it was decided to adopt some active measures for the abatement of the nuisance. It was attempted to disinfect the sewage by discharging every day during the warm weather immense quantities of lime and chloride of lime into the river.

The quantities of these thus thrown in in the summer of 1859 amounted to 110 tons of lime and 12 tons of the chloride every day, at a weekly cost of about £1,500; and in that season £20,000 more were also expended in flushing the sewers in order to aid in expelling their contents at extreme low water. Plans were also sought for from scientific and practical men by which the serious difficulties encountered might be permanently overcome; and at last one submitted by Mr. Bazalgette, chief engineer of the board of works, was adopted and executed. The main feature of this plan consists in a series of three grand parallel main sewers at different levels and distances from the river, and on each side of it, which cross the old sewers and outfalls at right angles and intercept the contents of the old system, conveying them on the north of the Thames down the river to Barking, a distance of 7 m., and on the south side to Crossness in the Erith marshes, 7 3/4 m. At this point the sewage is received in enormous reservoirs, which principally discharge at favorable conditions of tide into the Thames; but a portion is used in making what is called "native guano" by the "A, B, C" process.

The work of constructing these great intercepts, pumping stations, etc, was immense; the intercepting sewers alone, 82 m. in length, cost £4,250,-000, which was raised by a third rate levied on the metropolis, yielding £180,262 per annum, principal and interest to be paid off in 40 years. It is estimated that the total cost of these great works will not be far from £30,000,000. Notwithstanding the great improvements that have been effected in England, serious defects still exist affecting the water supply of London, from sewage outfalls above the city, but which were being remedied in 1875. - In America, sewer construction has both an ancient and a modern history. The works of the mound builders prove them to have been expert in the building of reservoirs, aqueducts, and conduits; and various places between the northwest and Central America exhibit remains of their sewer constructions. The modern history of sewers in America is not marked by any special achievements, the sewer systems of her principal cities being the result of rapidly increasing necessity.

As a rule, therefore, the sewers of the cities have been built piecemeal and rarely on a far-sighted plan, and generally discharge into the nearest available body of water. - The construction of sewers depends, as to material, size, shape, etc, upon the uses they are to serve, and the conditions under which they must be built, embracing the consideration of area to be sewered, its geological and topographical characters, the amount of rainfall, the water supply, the present and prospective population, and the disposal of the sewage. The difficulties encountered are in part the character of the earth, this being often so soft as to demand artificial supports for the pipes and sides of trench, or so hard as to require expensive excavation, the difficulty of securing sufficient fall, and that of providing for varying degrees of foulness in street wash, and securing safe escape for sewer gases. For cleaning purposes large man-holes are now depended upon, together with facilities for flushing. The form depends upon the amount of surface water, of manufacturing waste and excrement, and of subsoil water.

If the sewage contains little solid matter, a flat or obtuse oval bottom will answer; but it is found that less fluid matters will be more efficiently discharged by an egg-shaped oval, which secures a greater depth and velocity of flow. An important change has of late years been made by the substitution, where the drainage is comparatively small, of small earthenware pipes for large brick and stone sewers, whereby leakage and earth contamination have been reduced. A difficulty attending their use is the removing of sections for repairs. The cloaca maxima of Rome and the great sewers of Paris are built of cut stone, as were some of the conduits of the mound builders. The great main drainage works of London are cylindrical sewers of brick. The system of Capt. Liernur, now so highly lauded and being adopted in Europe generally, has its pipes all of riveted wrought or cast iron. Sewers are generally laid along the middle of the street and at depths according to the grades. At many street corners "catch pools" are built of brickwork, which, covered by open gratings, receive the wash of the street, retaining in the bottom the mud and sand and discharging the water into the sewer.

In towns having districts whose surface is scarcely above high tide, the principle of interception requires attention from the engineer. Intercepting sewers receive the surface water, and often also the sewage, and divert it from the common sewer which passes to the lower districts. In towns where the sewage is pumped from the lower districts to higher levels on account of tide-locked sewers, the system of interception becomes of great importance and greatly reduces the cost of drainage. This is well understood in the great drainage works in the fen districts of England, where large areas are entirely surrounded by intercepting drains, which lead the water off by gravitation. The great intercepting* sewers of London, which have been mentioned, have been constructed to convey rainfall at the rate of a quarter of an inch in depth over the whole area drained every 24 hours, at the time the maximum flow of sewage is being discharged. When the flow in a sewer does not carry off the solid contents, it is convenient to clean it by washing it out, or "flushing" as it is technically termed.

The operation is performed by letting in water at the higher levels, or by retaining the sewage in sections by means of gates, and, when the portion below has become partly empty and that above nearly full, allowing the collection by its gravity to sweep away accumulations below. The grounds occupied by the annual fair at Nizhni Novgorod, Russia, have sewers built of cut stone arranged in regular streets. They are lofty and well ventilated, and are entered by stairways through towers placed at intervals. They are flushed every day with water pumped from the Volga. - The ventilation of sewers is a subject of great importance in a sanitary point of view. The effluvium from the ventilating holes in pavements has been found to taint meat in butchers' stalls in their vicinity, to avoid which connections have been made with the chimneys of manufacturing establishments. It is not improbable that the solution of the problem will be in the erection of special chimney stacks. The relative efficiency of deodorizers has received attention. In 1858 the commissioners of London sewers reported that wood charcoal has a power of deodorizing currents of foul gases which is not much diminished by long use.

The comparative cost of disinfectants to purify a given amount of sewage in a uniform condition, made previous to the report, is shown in the following table:

Boghead charcoal (coke), 1 ton....................... $3 00

Nitric acid.......................................... 8 50

Black oxide of manganese........................... 9 25

Chloride of lime.................................... 10 75

Peat charcoal....................................... 11 00

Animal charcoal..................................... 16 75

Bichloride of mercury............................... 18 00

Impure chloride of zinc in damp powders............. 26 00

Chloride of zinc in solution (Sir William Burnett's.)........ 37 00

Sulphate of copper................................... 39 00

Other sulphates, as well as a number of other substances sometimes employed as disinfectants, proved ineffectual even in very large quantities to produce complete deodorization; but arsenious acid, creosote, and carbolic acid were found to possess great deodorizing power. Baldwin Latham has devised a combination of man-hole, street-water, and dirt catch with overflow pipe, and a charcoal ventilator protected from wet, which has been put in use in London with fair results. The gases principally found in sewers are carbonic acid, sulphuretted and carburetted hydrogen, ammonia, nitrogen, and fetid organic vapor; but it is only in the most unfavorable localities that the permanent gases are in sufficient quantity to produce of themselves bad effects. It is the poisonous germs contained in the accompanying organic vapor, and which may not have any very offensive or at least powerful smell, that confer on "sewer gas" its most deleterious properties. To prevent the entrance of sewer gas into houses is a problem which presents many practical difficulties. Various forms of traps are placed at the discharge of the house pipe into the sewer, and also in the water closets and beneath the wash bowls.

In the discharge into the sewer the pipe usually dips beneath the surface of the water in a receptacle which empties into the sewer. In water closets a "sealing pan," as it is called, holds water in its cavity when its rim is horizontal, the level of which is above the lower rim of the hopper. The joints of the hopper at the upper rim being made tight, no gas can escape into the room. The waste pipe below the hopper has a syphon crook, which when filled with water prevents the escape of gas from the pipe below. Other kinds of closets, which involve some additional expense, are preferred. A pump closet, used upon ships and below the water line, is modified for house purposes, and made to exhaust the hopper and force the contents out of the soil pipe. - Disposal of Sewage. This is one of the vexed and unsettled problems of the day, both in an economical and a sanitary point of view, and embraces the question of utilization of sewage, which also includes many plans, none of which have been confirmed as satisfactory.

It is the opinion of many that when a town is so situated that its sewage may be safely run off into a harbor or a river, so as not to become a nuisance afterward, this is the best method to pursue, as by a proper system of sewers with house connections it offers less danger than any other system of producing infection. When such disposal is impossible without raising the sewage from reservoirs by pumps and discharging it into a river which at the same time has a tide that will carry much of the sewage back to the town, as is the case at London, the question of utilization presents itself. There are five principal systems of utilization: 1, irrigation; 2, filtration; 3, precipitation; 4, dry earth; 5, the pneumatic system. Irrigation has been the most largely practised, but it is contended by its opponents that deleterious organic compounds are taken up by the growing plants as well as scattered through the air in the process, and by exhalation from the soil. Its advocates insist not only that a large yield is secured, but that the crop is grown in as healthful a condition as by ordinary culture. It consists in distributing the sewage over the surface of the soil, from reservoirs into which the sewers empty or into which their contents are pumped.

The objections are that infection is not sufficiently prevented, as before stated, and that, on account of topographical and geological conditions of the surrounding country, it is often impracticable. The advocates of filtration contend that it is economical and innocuous. Intermittent downward filtration is described as a variety of irrigation combined with deep drainage, so that the soil may be made to alternately receive the sewage and the air by which it is decomposed, freed of its poisonous qualities, and made fit to give nourishment to plants. Weare's process of filtration consists in filtering the sewage through charcoal, ashes, and soil contained in boxes, and is said to answer very well on a small scale. Precipitation is performed by several methods. The lime process of Gen. Scott, architect of the Albert hall, London, consists in mixing the sewage with a small proportion of milk of lime, agitating the mixture, and allowing it to subside. The precipitate is a very putrescible mud, and the liquid flows off in a tolerably clear condition, and has been used in the manufacture of a hydraulic mortar. But the precipitate contains only a small portion of valuable manu-rial constituents, and the drying of it is a very offensive operation.

The superphosphate of lime (or Blythe's) process attempts to recover the ammonia from the sewage. Superphosphate of lime and a salt of magnesia are added under the supposition that an insoluble phosphate of magnesia and ammonia will be thrown down, but analyses show that a part of the phosphoric acid runs off in the solution. It was rejected by the English sewage commission. Holden's process mixes the sewage with sulphate of iron, lime, and coal dust. The "A, B, C" process, as it is called, or the process with albumen, blood, clay, and charcoal, has been worked by the "Native Guano Company," and is said to be successful. It consists in coagulating blood obtained from slaughter houses, together with whatever coagulable matters may be in the sewage, with sulphate of alumina, producing a flocculent precipitate which shall drag down by the aid of the heavy particles of clay the organic solids contained in the sewage; and the charcoal is for the purpose of absorbing gases. A company is working at Crossness, between Plumstead and the Erith marshes, at the southern outfall of the London drainage, which is there discharged at the rate of 50,000,000 gallons daily; of this 1 per cent. or 500,000 gallons is used by the company.

A 15-horse-power steam engine throws the sewage into mixing tanks made of concrete, which are about 50 ft. long, 20 ft. wide, and 8 ft. deep. The details of the process, together with the sanitary and agricultural results, are given in the "Quarterly Journal of Science" for January, 1873. Disposal by dry pulverized earth in movable boxes has many advocates, who think that it could be advantageously applied to large towns; but it is the general belief that it is only adapted to those of considerably less than 100,000 inhabitants, requiring as it would the bringing in daily of dry earth to the amount of from 5 to 10 lbs. for each individual. The pneumatic system of disposal and utilization devised by Capt. Liernur has been applied at Amsterdam, Ley-den, and other towns in Holland, where the disposal of excreta by water-carriage sewage is highly objectionable in consequence of the level of the canals being lower than that of the sea. It consists of the ordinary sewers for surface and house draining, and a second pipe system connected with an apparatus for manufacturing immediately and without loss the excreta into poudrette.

The outlines of the process may be briefly stated as follows: The earthen hoppers of the houses are so constructed that the excreta fall to the lowest level of the soil pipe. Each hopper is covered with a rubber-seated lid, and is connected with a ventilator which passes up through the roof. At street corners, under ground, are large iron air-tight tanks, to a single opening in each of which all neighboring house pipes converge, this opening being closed with a gate. These tanks are connected in turn with large tanks at the great central sewage works of the cities. By means of air pumps a vacuum is created in the great tanks, and the gates of the pipes leading to the common receptacles being opened, the air is quickly removed from them and a vacuum created in each. The gates between them and their connecting house pipes being now opened, the matter within them is exhausted into the corner receivers, to be transferred to the great tanks at the central station, where it is manufactured into poudrette.

This is accomplished by allowing the sewage to run into retorts perfectly fresh, all houses, tanks, pipes, etc., being emptied every few hours, mixing it with a small percentage of sulphuric acid to fix the free ammonia, and then running it into troughs in which revolve large copper cylinders heated to a high degree, and the whole working in covered jackets. A thin layer of sewage is taken on at each turn of the cylinder, and by its heat is converted into dry poudrette, which is scraped off by a knife at the end of one revolution, and falls into a trough. No water is used in the house closets except the urine. The pipes being closed below and ventilated above, effluvia cannot escape into the houses, and there is no escape of sewer gas anywhere but at the central station under the furnace fires. The system, although embracing considerable outlay, is said to be not only self-supporting, but profitable, and to be extending over Europe. - See F. C. Krepps, "The Sewage Question" (London, 1867); Baldwin Latham, "Sanitary Engineering" (London, 1873); and Dr. Whitty, "On the Water Supply and Sewerage of Jerusalem".