CHAPTER XI SEWERAGE (Part 1)

CHAPTER XI SEWERAGE “‘Sewerage”’ and ‘‘Sewage’’ Defined. By sewerage we mean the system of sewers for the drainage of a district, including not only the wastes from the various sanitary fittings, but also the rainwater which falls on the area sewered and is not absorbed by the ground, The dirty water carried by the sewer is “sewage”’. Early History of Sewerage. Little is known as to the early history of sewerage in Britain, but that sewers are of ancient origin is established by the fact that some of the main sewers constructed by the Romans can still be seen in Rome. Layout. In designing a system of sewerage the engineer must have regard to the position of the outfall for the discharge of sewage, the pattern of the streets on plan, the area of the land to be drained, its possible population, the nature of the surface soil, the configuration of the ground, the rainfall and the water supply. Students and others who need a deeper knowledge of detail should study the British Standard Codes of Practice on Building Drainage (C.P. 301) and Civil Engineering Code of Practice (No. 5) on Sewerage. Position of the Outfall. The position of the outfall will, of course, be the site chosen for the purification processes. The site should preferably be at a low level, so that the discharge to it may be by gravitation and the cost of pumping may be avoided, whilst land of low value would naturally be preferred, removed from habitations so that nuisance may not be occasioned, and ample in extent to provide for possible future extensions. These considera- tions may, of course, conflict with one another; for instance, it is - possible that the low-lying land to which a gravitational discharge is practicable may have a high value for industrial purposes, in which case a careful comparison would have to be made between the economy of utilising such a site, or of using some other site in an unobtrusive position, of less value, to which the sewage would have to be pumped. . Outfall and Tributary Sewers. Working backwards from the outfall, the main outfall sewer will rise at a small gradient and branch off into different districts by smaller sewers, which will receive tributary sewers serving particular streets or estates and SEWERAGE 851 many of these will, in turn, receive tributary sewers from other streets. The plan form of the complete sewerage scheme will there- fore depend upon the lay-out of the town and upon the estate planning. Size of Sewers. The diameter required for any particular sewer will depend upon: 1. The area of the district to be drained by that sewer, chiefly because the amount of storm water to be carried off will depend largely on this. The maximum rate of run-off will, however, not be proportional to that area, because in a large area the flow will be spread over a longer period of time, as will be explained later. 2. The density of population: firstly, because the discharge of foul sewage will depend upon this, and secondly, because the more dense the population the greater will be the proportion of imper- meable surfaces, in the way of streets, roofs and paved yards, and therefore the larger will be the run-off of storm water to the sewers. In reckoning density one should not have regard merely to the present population, but rather to the population which may be expected when the land is fully developed, or that is likely during the next 30 years or so. It is not an expensive matter to make a sewer a little larger than is needed for present requirements, but it is very costly to take up one sewer and replace it by a larger one. In most cases the density provisions of a town-planning scheme will be of very great assistance in estimating future population and the likely area of roofs and paved surfaces. 8. The nature of the surface soil and its degree of cultivation: some soils are more permeable than others and water falling upon them will percolate down to form underground streams. Even on relatively impermeable surfaces, if not paved, much of the storm water will run into ditches and streams and will not enter the sewers. 4. The physical features of the land. Other things being equal, water will percolate into steep land to a less degree than into flat land, because it will have less time to soak in. Also, since on steep land the water will travel at a greater velocity, the flow from distant parts of the drainage area will not lag much behind that from the nearer parts, so that the run-off will be more concentrated. 5. The amount of rainfall in the district. The mean annual rainfall is, however, of less importance than might at first be supposed, because in the design of a sewerage system we are not concerned with the total flow in a year, but with the magnitude of short storms, and it is by no means always the case that districts subject to the heaviest annual rainfalls are subject to the heaviest storms. 352 SBWERAGE 6. The water supply of the district, since all the water which goes into a house will normally come out again as sewage. We should, however, not ignore the fact that the water supply per head of population tends to increase, in some districts because of the growth of the number of motor cars and consequently of water used in washing them, and in other districts through the more universal provision of baths and water closets. In our estimate of the water supply we should include water (if any) coming from other sources than the town mains; for example, from private wells, springs and rainwater tanks. In some places, even where there are water closets, the use of cesspools may be general, in which case some of the water supplied to the houses will not be discharged into the sewers; cesspools, however, must be regarded as a temporary expedient, likely to be dispensed with when the sewerage system becomes available, so that the sewers should be made large enough to accommodate the sewage which is, for the time being, discharged into cesspools. 7. The gradient available for the sewer. It is obvious that the velocity will be increased by an increase in the gradient, so that a smaller cross-sectional area is needed to obtain the same discharge. At a later stage in this chapter we shall return to this subject of the determination of the diameter required for a sewer. Combined, Separate and Partially Separate Systems. There are three methods of arranging a system of sewers, known respectively as the combined, separate and partially separate systems. Combined System. The combined system involves the pro- vision of a single sewer in each street, there being also a single system of drains to each building. The one sewer therefore takes all the domestic sewage, rainwater on roofs and backyards, and the road drainage. Separate System. In the separate system there are two sewers, one for foul sewage and one for surface water; similarly, each house has two sets of drains, one for sewage and one for rainwater. Partially Separate System. The partially separate system is a compromise between the two just described, and consists of two sewers, one to take all the drainage from private houses, (i.e. the “soil” sewage, the waste water from baths, basins, scullery sinks and rainwater from roofs and private paved spaces), while the other sewer takes the surface water from road gullies and some- times the rainwater from the roofs of public buildings and public paved spaces. The rainwater from the roof of private houses is sometimes described as the “unavoidable rainfall”, The partially SEWERAGE 853 separate system (like the combined system), involves the house owner in only one set of house drains. Each of the systems possesses certain advantages and dis- advantages when one is compared with the other, but individual preference and local conditions enter largely into the choice of system. The quantity of rainwater to be dealt with per day is very variable, and it is difficult to design a foul sewer satisfactorily if it is liable to enormous fluctuations of volume of flow. If two sewers are used, the foul sewer can be relatively small, and the smaller a sewer is, so long as it is large enough for its work, the cleaner can it be kept. Again, if pumping has to be resorted to, the quantity to be pumped is reduced to a minimum by the exclu- sion of rainwater. It is quite unnecessary to incur considerable initial and annual expense in laying down a pumping installation of sufficient capacity to lift all the rainwater, as well as sewage, to a higher level; it is quite reasonable, if proper precautions be taken, to run the surface water to the nearest natural watercourse. On the other hand, while in rural districts, generally speaking, the rainfall is comparatively pure and so can be safely taken direct to the watercourses, in other cases it is of a very different nature. The first washings of a busy street, after a dry spell, would give a liquid that could only be fairly regarded as foul sewage. Another consideration is that if there are two sewers in the street instead of one, there must be proportionately greater obstruction to traffic by repairs. It has been said that another drawback of the separate system is that there is a possibility of house drains, conveying foul sewage, being connected to the rainwater sewer; but this is hardly a valid argument, as the local authorities have ample powers of supervision. Service Sewers. The reason that the separate system has not been extensively adopted is that, generally speaking, the local authorities have no power to compel the provision of two sets of drains to the houses. This fact has led to the fairly extensive adoption of the partially separate system. The sewage from the higher levels of a district is collected into subsidiary sewers of relatively small diameter, say from 9 inches upwards, and is discharged into main sewers at the lower levels. Intercepting Sewers. Where the levels of the district vary considerably it may be necessary to use intercepting sewers, that 18 to say, subsidiary main sewers running on different contours and draining the areas above them, all discharging ultimately into the main outfall sewer. It often happens that the sewage from a 12—D.s. B54 SEWERAGE low-level intercepting sewer has to be pumped up into the outfall sewer. Assume a town laid out on a hill sloping rapidly down to a river. If there were only one sewer, at the lowest level, it would be liable to be flooded by the surface water from the high levels; further, the sewer might be at such a level that the whole volume of sewage passing through it would have to be pumped up at the outfall, The method then adopted is to have more than one sewer to intercept the drainage above that level. Thus, in Fig. 879, the sewer on contour A would take the drainage from above its level, the sewer on contour B the drainage between the levels A and B, and sewer C the drainage between B and the river. This is the arrangement in London, north of the Thames. The three | sewers would then converge at some distance from the town and the © whole combined volume would be carried on in the main outfall sewer. Assuming, in Fig. 879, that it is necessary to keep the main outfall sewer at about the level of B, sewer A would dis- | charge into B by gravitation, and the volume passing through C€ would be pumped up into B. Pipe Sewers. Small sewers are generally constructed of iron pipes with caulked lead joints or of stoneware pipes with cement joints, as in Fig. 802, though very occasionally one of the patent forms shown in Figs. 305-316 may be used. Stoneware pipes for sewers may be obtained in 38-inch rises up to 80 inches diameter, though the larger sizes are easily damaged in transit and are not very popular. Iron pipes, on the other hand, are available from 6 inches in the same rises up to 4 feet diameter and larger. Precast Concrete Sewers. Although these large sizes in cast- iron socketed pipes are available, most engineers choose pre- fabricated concrete pipes for big intercepting or trunk sewers. They are more costly than stoneware pipes in the small sizes, 12 inches in diameter and smaller, but in the larger sizes (up to 6 feet in diameter) they prove more economical, in addition to having greater strength. Joints for Pipe and Concrete Sewers. The least satisfactory feature of the concrete tube, as ordinarily made, is the joint, which is illustrated in Fig. 886. It is known as an “ogee” joint and it will be seen that the moulded ends of the pipes are simply put together with a little cement mortar between. A rather better form is that shown in Fig. 881. Concrete tubes are also obtain- able with sockets and spigots, and are then jointed with cement mortar in the ordinary way. They are also obtainable with a **self-centring” joint, as illustrated in Fig, 882, the sockets and SEWERAGE 855 OUTLET SHONE SEWAGE EJECTOR FOR BUILDINGS TOO LOW TO DISCHARGE INTO SEWERS BY GRAVITY CHANNEL /N FLOOR OF LARGE STABLE, COVERED BY HEAVY BUCKET GULLEY FOR GRATING, (GULLEY OUTSIDE) STABLE YARD O° t__2 oe ULL ——_ aa a hs g Y 7) Z Y} Z gan Bene Y Z VY Gi Z Z : SECTION omy PLAN A-B 377 378 PLAN AND SECTION OF PETROL /NTERCEPTOR FOR MULTI-CAR GARAGES. (MOST TOWNS WILL ACCEPT AN INTER- CEPTOR WITH ONLY 2 CHAMBERS ) 356 SEWERAGE spigots being so shaped as to ensure the pipes being perfectly concentric. Foundations for Sewers. Stoneware pipes and concrete tubes may be supported by concrete, as shown for the drain of Fig. 364. This is essential if the subsoil is soft, as the weight of the sewer is then distributed over a larger area of subsoil; it is also necessary if the subsoil is a hard rock, as this cannot be cut to be smooth enough to give an even bed for the pipes. Also if the sewer is under a carriage-way it is always advisable to support it by con- crete in this way, whatever the nature of the subsoil, whilst, if the depth is so slight that the sewer may be damaged by heavy vehicles, the concrete may be extended over the sewer to form a covering arch. When the diameter of the sewer is greater than the largest size made in iron or stoneware, one has to choose between concrete tubes and sewers built up of brickwork or concrete in situ. The concrete tubes will normally be much cheaper and are much more rapidly laid, which is an important point in view of the incon- venience caused to traffic by sewerage works in streets. Egg-shaped Sewers. The shape of the cross-section of sewers is usually either circular or egg-shaped. Egg-shaped sewers are not so popular as they once were, as the same capacity in a circular iron sewer is so much stronger. Nevertheless, there are, as will be seen, certain qualities inherent in the egg-shape which are not found in the circular cross-section. Stoneware are usually obtain- able only in the circular form, but concrete tubes can be obtained in either shape; brickwork can also be built to either form. Egg-shaped and Circular Cross-sections Compared. The respect- ive advantages and disadvantages of the two shapes may be briefly stated. The circular sewer of brickwork is stronger than the egg-shaped, and slightly less expensive to construct, but the egg-shaped has some advantage from a hydraulic point of view. Figs 883 and 384 show sections of two sewers, both having the same sectional area and both containing exactly the same quantity of water. It will be seen that the water is of greater depth in the egg-shaped sewer than it is in the circular; the greater the depth of the flowing liquid, the greater its velocity, therefore the egg- shaped sewer will, when compared with one of circular section, equal in area, and containing the same volume of liquid, be the more self-cleansing. The object to be borne in mind in designing such a sewer is to give it the greatest possible hydraulic mean depth for small volumes of flow, and this condition is fulfilled by egg-shaped SEWERAGE 357 sections. Fig. 385 shows, diagrammatically, the method of drawing the section of the ordinary, or old egg-shaped type. The depth is one and a half times the greatest horizontal diameter. The top is a semicircle of radius R; the invert, or bottom, is a circular curve of $R# in radius, and the top and bottom are joined up by circular curves of radius 3R. The sketch shows the centres from which the ares are struck. Fig. 886 shows, in the same way, the method of drawing the new egg-shaped type, which is also known as the metropolitan. The bottom is more pointed, giving still better hydraulic mean depths for small flows. As in the last case, the depth is one and a half times the greatest horizontal diameter, but the radius of the invert is only half as great as in the previous case (i.e. }). This necessitates, of course, a different radius for the connecting arcs; 22R instead of 3R. Egg-shaped Sewers in Brickwork. Fig. 387 shows one method of constructing an egg-shaped sewer in brickwork. The invert is formed of terra-cotta blocks, which give a minimum of joints at this important point. They are moulded hollow, as shown, and are usually made with rebated joints at their ends, to secure perfect alignment; they may be filled with fine concrete before laying, to increase