damming back the sewage and then liberating it when it has attained a sufficient head, or depth, in the manhole. Special flushing gates, sluices, etc., are fixed in manholes for the purpose. An old-fashioned method was to use a board across the outlet of the manhole, fitting into grooves at its ends and lower edge, but a few inches away from the mouth of the outlet. The sewage accumulated behind the board, which was then pulled up by means of chains in order to liberate it. This had the advantage that if the sewage was allowed to rise above the top edge of the flushing board, it simply flowed over and through the sewer. If a sluice or gate is used, a sort of inverted ramp should be provided as an overflow in case of neglect to open the sluice, the manhole end of the ramp being always open.<Callout type="tip" title="Tip">This method ensures that excess sewage can flow naturally if the sluice is not opened.</Callout> Flap Valves for Flushing. The arrangement of the base of the manhole when a flap valve, F, is used is shown in Fig: 410. The floor is slightly sunk in front of the outlet, to permit of the flap readily opening. A strong chain is attached to the flap and fastened loosely near the top of the manhole. The chain shown in the sketch is unnecessarily heavy, but a strong one is essential. The overflow is omitted, but the mouth of it would be about at O, and the pipe would pass down to join the outlet sewer a few feet from the manhole.<Callout type="warning" title="Warning">Using an excessively heavy chain can hinder quick operation.</Callout> Flap valves sometimes have floats attached, which are intended to lift them automatically when the sewage reaches a certain height, but the plan is not very reliable. Tipping Tanks, Tipping tanks are also used for flushing sewers, but they are only suitable for small volumes of flow.<Callout type="important" title="Important">Tipping tanks should be used sparingly as they are inefficient for large flows.</Callout> Penstocks for Flushing. Another method is shown in Figs. 412 and 413, which show a longitudinal and cross-section respectively of the base of a manhole fitted with a penstock or sluice, S. The latter is merely an iron plate or door, fitting into a grooved frame of iron and capable of being rapidly opened by the pull of a chain from above. The frame in the example shown is fixed a little away from the outlet so as to allow the sluice to form an overflow.<Callout type="risk" title="Risk">Improperly placed frames can lead to incomplete flushing.</Callout> For properly cleansing the sewers it is necessary to introduce plentiful supplies of water, and to do it systematically. In dry weather, more frequent flushings are, of course, necessary, and in times of epidemics disinfecting liquids may be added to the water.<Callout type="beginner" title="Beginner">Regular flushing helps prevent the buildup of harmful bacteria.</Callout> Storm Water Overflows. When the combined system of sewer- age is in use, it would be very costly to construct long lengths of main sewer large enough to carry away all the rainfall which finds its way into the sewers during a heavy storm, while it would also be a severe tax on the sewage disposal plant to deal with such large volumes. It is usual, therefore, at selected points not far removed from streams or other watercourses, to put in storm overflows, by means of which excess of rainfall is discharged into relief sewers leading directly to watercourses.<Callout type="gear" title="Gear">Overflow chambers and siphon spillways require careful planning and robust construction.</Callout> Provided that each overflow is at a suitable level, the water- course will not be unduly polluted thereby; for during the early part of the storm, when the street washings and other objectionable matter will be carried into a sewer, the water will be rising toward 372 SEWERAGE the overflow level. When that level is reached the storm-water should be running clear and the sewage will be so diluted by it that it is not likely to cause a nuisance in a stream to which it is discharged, especially if care is taken to avoid taking light floating debris and heavy solid matter over the storm-water overflow.<Callout type="risk" title="Risk">Improperly placed weirs can lead to pollution.</Callout> The Ministry require that the overflow level shall be at such a height that it will not function unless the discharge of the sewer is more than six times the mean dry weather flow. One simple method of doing this is illustrated in Fig. 411, a relief sewer being built from the main sewer to a river. A flap valve is shown at its outlet, to prevent back flow when the level of the river is unusually high.<Callout type="tip" title="Tip">Flap valves ensure that water only flows out during necessary overflows.</Callout> The level of the invert of the relief sewer at its upper end is such that when water is at this height the main sewer is carrying six times the dry weather flow. This arrangement, however, will seldom be satisfactory, because it is unlikely that sufficient water could get into the relief sewer, in a heavy storm, to prevent the main sewer getting completely full and surcharged; in that case, although the relief sewer would be running full bore, much more than six times the dry weather flow would still be left passing on to the outfall, or to the next overflow.<Callout type="important" title="Important">Ensure sufficient capacity for storm overflows.</Callout> It is therefore usual to install a long overflow weir, or two such weirs, in a special manhole called an overflow chamber, as shown in Fig. 414. The object of having two such side weirs is, of course, to reduce the size and cost of the chamber.<Callout type="risk" title="Risk">Improperly sized weirs can lead to surcharging.</Callout> In the illustration S is the main sewer leading out of the chamber; this can be of less diameter than the sewer flowing in, because of the help given to it by the relief sewer. Inside the chamber the water flows in a nearly flat channel, at each side of which is a weir W. The sewage which falls over the left overflow passes directly to the relief sewer R, whilst that which falls to the right does so after flowing through the passage P.<Callout type="important" title="Important">Properly designed weirs ensure efficient storm water management.</Callout> If desired, the tops of the weirs may be adjustable iron plates. Whilst the provision of a storm water overflow of this type can be made, by placing it at the correct height, to function only when the inflow in the main sewer exceeds six times the dry weather flow, it would be wrong to suppose that all that excess will pass over the weirs.<Callout type="risk" title="Risk">Overflows may not handle all storm water.</Callout> As the water level rises higher above the sill of the weir the discharge of the overflow will increase, but so will the discharge of the outgoing main sewer. Siphon Spillways. To overcome this objection, in some recent schemes siphon spillways, such as that shown in Fig. 415, have SEWERAGE 3873 been introduced.<Callout type="tip" title="Tip">Siphon spillways ensure efficient storm water management without surcharging.</Callout> When the main sewer, S, is carrying six times the dry weather flow the water level in it is at A and any small rise in that level will cause water to flow over the throat of the spillway; under this condition there is no siphonic action, because the throat is kept at atmospheric pressure by the air pipe P. If, however, the water level rises to B, the mouth of the air pipe will become sealed, with the result that air will be carried away from the throat by the flow of water and siphonic action will begin.<Callout type="important" title="Important">Siphon spillways prevent surcharging while managing storm water efficiently.</Callout> The siphon will then discharge full bore, the head on the siphon being the difference between the water level in the main sewer and that in the relief sewer, R. Because the siphon inlet is well submerged, except at the tail end of the discharge of storm water, very little of the light floating matter will enter, whilst, because it is so high above the invert of the main sewer, none of the heavy solids will do so.<Callout type="risk" title="Risk">Improperly designed siphon spillways can lead to surcharging.</Callout> Use of Leap Weirs. One defect in the separate system of sewerage is that the first washings of streets, roofs and yards in a storm, especially after a drought, are apt to be very foul and therefore unfit for discharge into a river without treatment.<Callout type="warning" title="Warning">Storm water should not contain untreated sewage.</Callout> To provide against this the leap weir, illustrated diagrammatically in Fig. 416, may be useful, though it is in fact seldom installed. When only a small quantity is flowing in the rainwater sewer, the velocity will be low and the water will fall almost vertically into the main sewer, marked M.S., of the foul sewage system.<Callout type="important" title="Important">Leap weirs prevent untreated storm water from entering the main sewer.</Callout> If the rainwater sewer is fairly fully charged the velocity will be much higher and the water will leap across the gap to the lower rainwater sewer, R.W.S. It is obvious that the success of this method depends on the proper proportioning of the width of the gap and the amount of fall to the lip of the weir.<Callout type="risk" title="Risk">Improperly designed leap weirs can lead to surcharging.</Callout> The leap weir is not suitable for use as a means of getting rid of excess of flow in a combined sewer, because when the velocity of the stream is high all the flow would be carried to the river, whereas the Ministry of Health require that six times the dry weather flow shall remain in the sewer.<Callout type="important" title="Important">Leap weirs are only effective for separating storm water from foul sewage.</Callout> A modified form of leap weir is shown in Fig. 417. An adjust- able iron plate, I.P., is fixed on the invert of the rainwater sewer, over the mouth of an opening, leading to a foul sewer, F.S.<Callout type="tip" title="Tip">Adjustable plates ensure effective separation of storm and foul water.</Callout> When only a small quantity of water is passing along the rainwater sewer, it will all go through the opening to the foul sewer, but when there is considerable velocity very little will be intercepted in this way.<Callout type="important" title="Important">Adjustable plates ensure efficient storm water management.</Callout> Surface Water Separator. Fig. 418 shows an appliance intro- duced at Birmingham many years ago which attempts to divert the excess over six times the dry weather flow in a manner different from any of the foregoing.<Callout type="important" title="Important">Surface water separators ensure efficient storm water management.</Callout> A horizontal flat plate, F.P., is fixed across the foul sewer, F.S., at the required height to cut the flow into two parts; one that below six times the dry weather flow, and the other, that above it. On the flat plate is fixed a vertical curved plate of iron, I.P., which diverts the upper flow into the rainwater sewer, R.W.S.<Callout type="risk" title="Risk">Improperly designed separators can lead to surcharging.</Callout> At the entrance to the manhole is an inclined wire screen, I.S., with an upright triangular side at A. This screen forms a sort of basket, which prevents rags, paper, and other rubbish collecting at the edge of the flat plate, or passing into the rainwater sewer.<Callout type="important" title="Important">Screens prevent blockages in storm water systems.</Callout> Inverted “Siphons’’ at Stream Crossings. Where a sewer has to cross a stream, railway cutting or other similar feature, an inverted siphon has often to be used. They are objectionable features and should be avoided wherever possible.<Callout type="warning" title="Warning">Inverted siphons can lead to blockages.</Callout> They usually consist of two sloping lengths meeting a flat length between them. They should be formed of cast-iron or steel pipes, treated with the Angus Smith or other process, and should be laid in duplicate.<Callout type="important" title="Important">Duplicate inverted siphons ensure continuous flow.</Callout> There should be a good manhole at each end, and if the length is very great it is wise to carry up a vent shaft at the middle to pre- vent the siphon becoming air-locked. There is always a tendency to blockage in the flat length, and it is usual to adopt some device to check this, such as putting a permanent large chain through from end to end, so that it can be pulled backwards and forwards to stir up the silt.<Callout type="risk" title="Risk">Blockages in inverted siphons can lead to surcharging.</Callout> The object of laying them in duplicate is that, in case of trouble, the sewage can readily be diverted from one to the other. Bridge Crossings. Where sewers cross bridges, they should be of iron or steel pipes, owing to the vibration.<Callout type="important" title="Important">Iron and steel pipes ensure durability for bridge crossings.</Callout> Sometimes they are accommodated between the girders of the bridge or, if this is im- possible, they are fixed to brackets at the side. Sewer Ventilation. We come next to the question of the ventila- tion of sewers, a question which has given rise to much controversy in the past.<Callout type="important" title="Important">Proper ventilation prevents toxic gas buildup.</Callout> On the one hand, if the sewage is of normal domestic character and the sewers are well constructed and have good gradients, foul air should not be generated in any very appreciable quantity and it should not be dangerous to the health of those who happen to inhale it. In such cases all that seems necessary is sufficient means of ventilation to allow air to escape from the sewer, when the water level rises through heavy rainfall, and to allow air to enter the sewer when the water level subsides.<Callout type="risk" title="Risk">Inadequate ventilation can lead to toxic gas buildup.</Callout> On the other hand, if the sewers are so flat in gradient that the discharge is much delayed and solid matters settle and decompose, 376 SEWERAGE gases will be given off which are highly offensive, and even danger- ous to the health of some persons, for some are much more ad- versely affected by bad smells than others. Much, too, depends on the character of the sewage; decomposing sewage will always give off carbon dioxide, marsh gas and ammonia, which should be classed as asphyxiating, rather than poisonous, gases, but some sewage also gives off sulphuretted hydrogen, which is definitely poisonous.<Callout type="important" title="Important">Proper ventilation prevents the buildup of toxic gases.</Callout> Trade effluents often contain chemicals which, on mixing with one another, may react chemically and form gases of a poisonous character. The possibilities of coal gas, leaking from gas mains, and of petrol from garages, with consequent formation of explosive mixtures with air, must also be considered.<Callout type="important" title="Important">Proper ventilation prevents the buildup of toxic gases.</Callout> In all such cases adequate ventilation is essential for the general health of the community, for the safety of men who occasionally have to enter the sewers, and for the prevention of damage to the sewers by explosion. That the danger resulting from inadequate ventilation is in some cases a very real one is illustrated by the report of a Committee, appointed by the Home Office and the Ministry of Health in 1938, to enquire into the precautions that should be taken for the safety of persons entering sewers.<Callout type="important" title="Important">Proper ventilation prevents toxic gas buildup.</Callout> They recommended that, before a man enters a sewer by way of a manhole, the cover thereof and those of the manholes on either side shall be removed for at least half-an-hour, at the end of which time tests shall be made for sulphuretted hydrogen, for asphyxiating conditions, and for in- flammable gases; unless these tests prove satisfactory no one should enter the sewer without a special respirator.<Callout type="important" title="Important">Proper ventilation prevents toxic gas buildup.</Callout> Suflicient has probably been said to show that a certain amount of ventilation is essential and that in some cases a thoroughly efficient system is highly desirable. It is, however, often difficult to secure this result without causing a public nuisance.<Callout type="important" title="Important">Proper ventilation prevents toxic gas buildup.</Callout> Combined Sewer and Drain Ventilation. For many years, a section of municipal engineers have agitated for the prohibition of the intercepting trap between house-drains and the public sewer until, to-day, many local authorities, encouraged by the Model Code, have altered their local by-laws so as to make the use of the intercepting trap optional to the householder or (more frequently) forbidding its use and that of the mica flap air inlet altogether.<Callout type="important" title="Important">Proper ventilation prevents toxic gas buildup.</Callout> When the sewers are modern and are provided with a reasonable fall, the sewer air is very little worse than that in a well-constructed house drain, and there seems little reason to object to the sewers and the drains to be ventilated as a single unit.<Callout type="risk" title="Risk">Improper ventilation can lead to toxic gas buildup.</Callout> When this is done, the local authority provides large fresh-air inlets at intervals in quiet spots and the air flows along the sewer, entering the many house drains along the route, to discharge well above eaves level, through the many soil-pipe vents of the houses served.<Callout type="important" title="Important">Proper ventilation prevents toxic gas buildup.</Callout> When conditions are good, the benefit to the sewer ventilation is inestimable and that of the many private drains is satisfactory.<Callout type="risk" title="Risk">Improper ventilation can lead to toxic gas buildup.</Callout> The disadvantage, in the view of many surveyors and architects in private practice, is in the loss of individual control of the separate house drain units. Their view is that where the local authority’s air inlets are widely spaced, nearby house drains get a plentiful supply of air and drain ventilation is good, but house drains farthest from the air inlets have the air flow reduced almost to vanishing point.<Callout type="important" title="Important">Proper ventilation prevents toxic gas buildup.</Callout> It is a problem, however, for the local authority’s surveyor or engineer, not for the individual house- holder, and if the local authority sees that air inlets are
Key Takeaways
- Use appropriate flushing methods to ensure efficient sewer cleaning.
- Implement storm overflows and siphon spillways for effective storm water management.
- Ensure proper ventilation of sewers to prevent toxic gas buildup.
- Regularly inspect and maintain sewer systems to avoid blockages and surcharging.
- Design overflow systems with sufficient capacity to handle excess rainfall.
Practical Tips
- Regularly flush s