2
464
Ventilation
of sewers. Vents not Ventilators required.
•
26
I have conferred with the Medical Officer of Health on this subject, and he states that he is quite prepared to take over the administration of the street sewers as well as drains, nullahs, etc. He will not require the services of an Engineer for this purpose.
At most, it will be only necessary to increase the number of Sanitary Foremen. The Foremen, now in charge of the Street Sewers, should be transferred, for in the absense of Record Drawings, they possess the necessary knowledge of the Sewerage System. As the Director of Public Works is a member of the Sanitary Board, he would be able to give advice upon any engineering points, that might arise.
75. I have fully set forth my views, as to the Ventilation of Sewers, in a report of 1890; and I have little to add to what I then wrote. The main point to be remembered is that it is impracticable to establish a constant current of air in a given direction through a sewer, by means of ventilating shafts. Sewage, unless it be stagnant, evolves an inappreciable amount of gas, though it is capable of tainting a vast quantity of air. Personally, therefore, I have never been able to recognise the advantage of sewer-ventilation, in the sense of drawing a current of air through sewers, and thus contaminating more air than necessary, with sewage emanations. It always appeared to me that the best place for contaminated air, was the sewer itself. My impression is that the idea of ventilation has, to a great extent survived from the days of large combined sewers and drains, not self-cleansing, into which men have to enter, for the purpose of cleaning them. What is wanted are Vents, not Ventilators, in order to prevent an accumulation of pressure, when there is a sudden rush of sewage. Much may be done, in this direction, by connecting across the summits of sewers, by means of pipes, laid so high as to be dry.
C
A
і
D
B
Ventilation by house ventilators,
If the
Let A.B. and C.D. be two branch-sewers, flowing to some main sewer, B.D. heads were connected across, by a dry pipe C.A; then, supposing that there were a sudden rush of sewage down, say, A.B., the air displaced would enter B.D., rise up again through D.C., and flow through the dry pipe C.A.; to replace the air dis- placed from A.B. If the heads of a large network of sewers, are connected in this manner, there can be no prejudical accumulation of pressure. It is not essential that the connecting-pipe should be dry. It may serve as a Sewer, for the houses along its length. Thus, A.C. might flow from A. to C., but it should open into a manhole at A., at a higher level than the outlet of the sewer A.B. An inspection of the sewerage plans, shews that there are many places in which the suggested arrange- ment could be carried out, at very small cost. I am of opinion that it would be desirable so to do.
76. My views as to the necessity or otherwise for the ventilation of Street-Sewers are not, I admit, universally shared by the profession; I am not now referring to House-Sewers, where very different conditions prevail. Ventilation, by gratings, in the manholecovers, situated in the middle of the street, is certainly objectionable; though in accordance with very general practice. Ventilation, by means of un- trapped gulleys, is still more so. -