·
would, in all cases, be the most economical source of power. This flushing tank might indeed be filled with the sewage from the Peak. I am not, however, in favour of flushing sewers with sewage. The tank is apt to be a nuisance, as it requires occasional cleaning, but the system is often resorted to and might, if con- sidered advisable, be tried.
The contents of this tank should be discharged, alternately, down the two low- level sewers, a 12" pipe, leading to each head, being provided. The flush should be discharged at or about low-water. The lowest of the two low-waters during the day should be selected, for one is often much lower than the other.
.
Flushing cannot be effective until the outfall sewers across the New Praya Reclamation are of full diameter and free from obstruction. It may also be necessary to provide a sluice in the diaphragm which is in the outfall manhole, to allow the flush to escape more freely than the actual submerged pipe will permit. When not flushing, this sluice should be closed.
8. To thoroughly flush a sewer, a quantity of water should be provided sufficient to fill it half-full, or to the depth producing a self-cleansing velocity, for about one-third of its length.
The quantity required for flushing depends not merely on the diameter and length of the pipe, but on its inclination. If a pipe has a good self-cleansing gradient, but deposit occurs owing to insufficiency of the normal sewage flow, then it will suffice to fill it to one-half or such depth as will give up a self-cleansing velocity for say one-fourth of its length. If, however, the pipe has a somewhat paltry gradient then the flush should be equal to its full contents for half its length. The bottom of the flush tank should be as high as possible above the head of the pipe and the outlet should be large enough to make the mean rate of discharge equal to the rate of discharge of the pipe to be flushed.
779
Rule as to the water requir
volume of
ed for flushing.
Alushing
9. If the experimental flushing already described proves successful, and if the Extension of necessity for flushing be not obviated by improvement in scavenging and in the system construction of sewer gullies, then let it be extended to other outfall areas. In some, namely Nos. 3 and 4 outfall areas, (vide plans 3 and 4), the sewers are so low that flushing may always be necessary or at least desirable. These should be the first to be taken in hand. In all cases, it would be easy to flush with sewage from above. It must, however, be remembered that automatic flushing tanks can- not be used for the low-level system. A flush discharged when the tide is above the level of the invert of the sewer is useless. Flushing, in the case of tide-locked sewers, must always take place at or about low-water.
1
dragging
Want of supervision.
10. I have reason to believe that chains and scrapers are dragged through Necessity for sewers, probably to their detriment, more often than is necessary. The operation sewers. has become a matter of routine perpetuated owing to inadequate supervisory staff. A single European Overseer cannot be expected to inspect all the sewers in the City, and also look after construction work. He cannot possibly supervise all the gangs' of coolies, nor can he satisfy himself, by personal inspection, that the sewers of any one district or street are in such condition as to require dragging. Consequently gangs of coolies are told off to drag in a certain district, and they do so, whether this operation is required or not. I have suggested a means, whereby the inside of a sewer may be inspected by the aid of a lamp and mirrors, without going down the manhole. Whether this arrangement will materially mend matters or not the true remedy is more skilled supervision.
tide-locked
sewers.
11. A sewer, partially or wholly tide-locked, is, according to modern experience, Objections to objectionable. Such a sewer, of necessity, violates one of the fundamental principles of sewer-design, namely, continuous onward flow, from the gully or inlet at the house, to the outfall, or place of final disposal, without stoppage or stagnation.
Though the gradient of a sewer may be sufficient, when its outlet is free, to establish a self-cleansing velocity, whenever the outlet becomes tide-locked, this condition is disturbed, the hydraulic gradient being reduced by the rise of the tide.