(144) Storage Sewage Reservoirs.

In those towns which discharge their sewage into the sea it is necessary to provide either a sufficient storage reservoir, to pond up the sewage during the time in which otherwise it would be tide locked, or to construct a tank sewer, which is practically a longitudinal reservoir; many engineers prefer to make a tank sewer-that is a very capacious sewer at the lowest practical level -rather than a covered reservoir, and it certainly seems that the tank sewer can be flushed and cleansed easier than a storage tank. In all these cases there must be special ventilating arrangements and efficient traps to prevent the sewer gases becoming a nuisance.

(145) Road Gullies.

Road gullies connected with the sewers, are of two kinds, viz., those which are intended to pass everything into the sewers, and those intended to intercept mud, sand, and the detritus of the roads generally. The former are to be found only in towns where the sewers have abundant fall and flushing water; in most towns the mud must be intercepted. This is accomplished by making a catch-pit below the point of overflow into the sewers of considerable depth and size, and removing the deposit from time to time; Fig. 39 is a diagram of an ordinary road gully.

FIG. 39.

:

(146) Ventilation of Sewers.

In the metropolis this is almost entirely effected by direct openings into the sewer at frequent intervals along the centre of the roadway these openings are never offensive along the great main arteries of sewage where there is an ample air current in the sewers themselves, but on the other hand, in certain positions where the sewers are somewhat flat, in the proximity of dead ends and under other local conditions, the sewer openings are a cause of danger and offence.

Charcoal boxes have been tried, various ingeniously arranged trays of charcoal being placed in sewer ventilators and the sewer

gas passing through the boxes has to some extent been deodorized, but on the other hand a serious mechanical impediment to the diffusion of the sewer gas into the open air is in this method introduced, besides which the charcoal soon gets useless and frequent renewal is necessary. For these reasons charcoal boxes are condemned.

The Metropolitan Board of Works, a year or two ago, recommended a strip of flannel to be hung in the sewer ventilating shaft and a jar containing a concentrated solution of sulphurous acid gas (SO) hung over the flannel; by carefully adjusting the stopcock of the jar, the solution dropped upon the flannel keeping it always moist and filling the shaft with sulphurous acid gas. The writer has seen this tried, and with attention it may be made to act fairly well.

A better plan is disinfection of the sewer air by Reeves' apparatus. In this ingenious invention an apparatus in shape and appearance like a filter is placed over the ventilating hole; this has two chambers, the upper one is filled with strong sulphuric acid, the lower with dry potassic permanganate or sodic permanganate; the sodic permanganate chamber is connected with a water supply and the cock is so regulated that a small stream of water washes through the permanganate, the solution escaping on to a splash plate below the apparatus; on to this same splash plate is delivered the sulphuric acid, the result of the reaction being permanganic

FIG. 40.

acid, free oxygen, and sulphur dioxide. The sewer air passes through this atmosphere and a strongly disinfecting liquid enters

the sewer. The writer has investigated carefully this apparatus, and it is certainly effectual when in perfect working order, but requires attention.

Another method which is on its trial is Keeling's patent sewer gas exhauster and destructor; as shown in the diagram (Fig. 40), a large burner is placed within the shaft, and the arrangement is such that the sewer gas either passes through flame or other surfaces brought to a high state of heat. The claim of the inventors is that it carbonises any organic matters which the sewer gas may

contain.

(147) The Outfall of Sewers.

The outfall of an inland sewer should be protected by a flapvalve, otherwise winds are liable, should they blow in the right direction, to sweep any sewer gas back towards the town; the position of the outfall should of course be such as not to create a nuisance. An outfall should never deliver direct below the surface of a river, save in cases where there is ample fall, otherwise the sewage is more or less locked up in the sewers. The position of an outfall delivering sewage into the sea requires to be placed so that it is under water at all periods of the tide, that it does not impede navigation, that it does not deliver the sewage so that the ebb tide carries it in front of the town, and lastly, that it is not placed in such a position that the sewage is ultimately deposited on the shore of the town. All this requires often prolonged and special study, by means of floats and other appliances on the spot, of the various tides, currents, eddies and the effects of particular winds. Sea outfalls are almost always constructed with proper valves or sluices to prevent back currents-the inflowing of the sea up the sewers.

CHAPTER XVII.

THE SEWERAGE OF THE METROPOLIS.

(148) Interception.

EVERY student of practical hygiene should make himself acquainted with the main features of the drainage of the metropolis, not simply on account of the magnitude of the work, nor the interests involved, but because it is at the present time the finest example of the important principle of " INTERCEPTION."

The principle of interception is this, that where there is a small longitudinal fall, and comparatively great lateral falls, and moreover the town to be drained is of considerable size, it is more advantageous to intercept the sewage of the higher zones by a special channel, called "an intercepting sewer." In this way a town is divided up into small and manageable areas.

In sea-side places interception is specially advantageous, for the lower levels are tide-locked for many hours of the day, while if an intercepting sewer carry the sewage from the higher levels to some distant point, there is no interruption of the flow from the portion of the town thus drained.

Nevertheless it cannot be said that these considerations had as much weight with the designers of the metropolitan system as the stinking state of the river Thames, the whole drainage at one time emptying at different points close along the shores of the great city itself, until huge intercepting sewers carried it elsewhere at a safer distance into the same river.

(149) Size of the main London Sewers.

The general history of the various commissions, schemes, and legislation on metropolitan sewerage will be found in the First

Report on Metropolitan Sewage Discharge of the Royal Commission, London, 1884, to which the reader may be referred.

The designers of the London main drainage, although recognizing the many advantages of a separate system for sewage and for rainfall, were compelled to undertake the problem of removing rainfall and sewage in one system, which, at all events, would have the advantage of using the rain and storm waters to effect the flushing out of the sewers. The principle upon which the size of the main sewers was constructed is thus given by Sir Joseph Bazalgette :

"It appeared from tables of rainfall that there were only from 14 to 21 days in the year on which the rainfall exceeded the rate of a quarter of an inch in 24 hours; therefore we said, if we can divert the sewage from the river (within the metropolis) in dry weather altogether on all those days, excepting the 14 to 21, according to the season (a very wet season has been 30 days), we shall sufficiently divert the sewage from the river to make it clear."

The average amount of sewage per head was calculated to be 31 gallons, and in another part of his evidence Sir Joseph Bazalgette said: "We take the sewage at so many gallons per head, we take the maximum flow at double that quantity, and then we take a quarter of an inch of rain running off added to that, and taking the populations and the areas, we arrive at each point at the quantity to be intercepted."

(150) General Principle of the Metropolitan Sewer System.

The general principle of the metropolitan main drainage system. is that of interception.

According to the natural hydrographical features of the district, the portion of the Thames basin on which the metropolis stands is drained by transverse streams flowing into the river on each side; and the sewers, which have been formed from time to time to effect the artificial drainage, have been made to follow the lines of these streams.

In order, therefore, to prevent these sewers from discharging into the river, the principle has been adopted of intercepting them by lines of conduits lying nearly at right angles to them, i.e. in the same general direction as the course of the river, but inclining