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or ejectors both to increase the rate of flow and also to dilute the sewage.

What is known as a self-cleansing velocity in the sewage flow should always be aimed at, because this will prevent the settlement of solid matter, which otherwise is sure to take place, and by being added to, as it would necessarily be from time to time, cause not only a stoppage, but a nuisance, if nothing worse. The shape of the sewer is also a factor for consideration; what is known as the egg-shape formation is a very satisfactory shape in section for the average-sized sewer. Gradients vary, but in most cases it is not wise to have them less than 1 in 100. Where a satisfactory gradient cannot be ensured, some method of retaining the flow at different points and automatically discharging it in bulk, at intervals, is useful for hastening the flow through another section. In some districts the local rainfall may be very large in comparison with others; in the British Isles, this is especially so on the western side and in the Lake Districts. The presence of trees in great quantities, especially on mountain slopes, has also an action in the causation of rainfall. The Eastern Counties, on the other hand, usually suffer more or less from a shortage in average rainfall.

Shone and Ault's Sewer Ventilating System.

Fig. 1 illustrates this system. In it "b" is the ejector chamber, through which the atmospheric air, entering at passes along the sewer, "dd, through the nozzle ," and up the outlet shaft"

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The action of filling the ejector by gravitation with the sewage from the sewer causes a partial displacement of the air therein, and it passes along the exhaust pipe "e," to the nozzle "n." The whole operation is worked automatically.

Sewer Ventilation by the Webb System.

The object of this system is to ventilate the sewers as well as to sterilise and deodorise the gas given off therefrom. For this purpose most of the surface manhole sewer grids are closed, and ventilating lamps are substituted for them. They act as sewer gas extractors, and, it is claimed, abolish the sewer gas by consuming it.

Oil lamps can be used for burning up the sewer gases, where coal gas cannot be obtained.

Fig. 12 clearly illustrates the application of this method of sewer ventilation.

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Fig. 12. Section, Webb's System of Sewer Ventilation.

Subsoil Drainage.

In some districts subsoil water has to be dealt with in large quantities, especially where the subsoil is of a porous nature. It will scarcely be necessary to say that the sewers should be so constructed as effectually to debar the entry of subsoil water, for if this can enter some portion of the sewage flow can also escape, and soak into the surrounding earth. Where the porosity of the earth in which the sewer is laid seems to call for it, concrete should be employed in which to bed the conduit. Reinforced concrete possesses valuable properties for such works as these, and is being increasingly used, especially on new systems.

Where there is a good deal of subsoil water to be dealt with, the formation of a separate system of sewers may be called for. These are by no means unmixed blessings, as there is great risk of connection being made to the wrong conduits. Where, however, they exist, the local authority should have power to enforce the provision of separate drains, and their powers should not be limited to new streets and houses.

The Fifth Royal Commission made some reference to this subject in their Fifth Report, but the inference to be drawn therefrom is that the Commissioners were not very favourably impressed with a separate system of sewers.

Where it is necessary to underdrain the site of a house the subsoil drains should be laid at such a depth as will effectually take up both the water settling through the soil, and that present in it. The depth of the outfall to the special sewer, if any, or to the ordinary sewer, must also be considered, and will frequently prevent such subsoil drains being laid more than three or four feet below the basement of the house. The pipes used for this purpose are unglazed with butt joints, and when laid are covered with clinker or rubble.

If the outlet of the subsoil drains is into the sewage drain, it must be disconnected by means of a trap and air-shaft; at the side of the inspection chamber, if any, is a suitable position. To prevent any sewage matter from entering it, the subsoil drain should enter the disconnecting chamber a little above the sewage drain.

CHAPTER IX.

Growth of Disposal by Water-Carriage.

This system is fast superseding conservancy methods of collection, because, in the latter, sewerage systems are equally needed to carry away house slops, road washings, storm waters, liquid trade and manufacturing wastes (if any), etc., and these need subjecting to some sort of treatment as well as the ordinary dry" excrementitious matter, which latter, as has already been noticed, is a more expensive process, when coupled with the disposal of the various liquid wastes mentioned above, than the disposal of mixed water-borne sewage.

Slight Septic and Mechanical Action in Sewers.

All sewage contains numerous microscopic organisms consisting of many species, and several of these classes are present in the crude water-carried sewage in the sewers; and the meaning and effect of the septic action is that these organisms carry on some work of a preliminary nature during the progress of the sewage through the sewerage system to the outfall. This process is usually spoken of as a septic action, and will be dealt with in some detail elsewhere;* suffice it to remark here that they cause the sewage to become somewhat broken down and slightly emulsified by their work in feeding on and excreting the organic solids which, in so doing, they make simpler in their nature. The extent and effect of this will largely depend upon the distance which the sewage has to travel to the treatment installation.

There is also the mechanical action which takes place in conjunction with the septic one, whereby an additional breaking down of the organic solids takes place by the simple force with which they are driven by the flow of the sewage and by coming in contact with each other, and thus becoming more or less disintegrated and emulsified by being forced and rubbed against each other, which process is described as "mechanical attrition. Lastly, all these effects are simultaneously accompanied by a putrefactive action.

* See p. 63.

CHAPTER X.

Sewage Farming.

In those places where there were no streams of water into which to discharge the crude water-borne sewage of the neighbourhood, land alone came to be utilized for its disposal; and what has since become known as sewage farming appeared to offer the only tangible solution of the difficulty for inland places to resort to. Accordingly, sewage farms were laid out in all directions, to which sewerage outfalls were directed.

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The idea underlying this method was the fulfilment of what appeared to be Nature's requirements, viz., that what came off the land should be replaced. Thus, if human excreta were replaced, it would only be following this law; because it represents the waste given off from the human system after it has derived nourishment from the vegetable or animal food which it had consumed; the former directly produced from the soil, and the latter drawing its nutrition therefrom.

This process seemed to agricultural chemists a most reasonable solution to the difficulties surrounding sewage treatment, and a proper way to assist Nature in the ordinary cycle of events, and give the land full advantage of the supposed high manurial value of the human dejecta. Hence crude water-borne sewage matter was discharged directly on to farm lands fitted for the reception of the sewage. The land has to be specially prepared by means of levelling and under-drainage to distribute the water-borne sewage evenly over all parts of it, and this is best, and of course most cheaply done, by gravitation from the outfall; otherwise pumping has to be resorted to.

As regards under-draining, it must always be borne in mind that the need for drains depends upon the volume of water-borne sewage liquid to be dealt with per acre, and not merely upon the nature of the soil and subsoil. Where the latter are porous, under-drains are not needed for a moderate quantity. With surface irrigation, under-draining is useful. Suitable crops are grown on sewage farms; of these turnips, oats, rye grass, cabbages, and peppermint usually do well;

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