For a long outfall sewer with no connections this method has many advantages, notably so in the large furnace and shaft erected on the Brighton outfall sewer by the borough surveyor, Mr. Lockwood. But in a general system of sewerage, it was found by experiments made by Sir Joseph Bazalgette, C.B. that the effect of trying to reduce the gases by the action of furnaces was but very small, and that the area over which the effect extended was but very limited: in fact, a sewer may be compared to a perforated tube, the house drains, gully drains, &c., representing the perforations; the effect of an exhaustion by the action of the furnace is simply to suck fresh air into the sewer at all these points instead of removing the foul air for any distance.
High shafts at different points effect a partial clearance of sewer gases, but they are very costly, and the reasons I have previously given against the system of open shafts are practically the same in this and all other modifications of the principle.
Many other methods have been tried from time to time, either to prevent the formation of gases in the sewers, or neutralize or destroy them, some of which are as follows:
By giving a quick velocity of discharge in a sewer; by placing materials within sewers which would absorb the gases as fast as they were generated; by passing deodorants or disinfectants into sewers; by deodorizing or disinfecting all materials before they are allowed to enter a drain or sewer; by placing chemical agents within sewers to give off certain gases which would then, it was conjectured, destroy the noxious properties of the sewer gases; by introducing charcoal into sewers to absorb the foul gases; by laying pipes within the sewers for the purpose of discharging chlorine into the sewer; by employing galvanic agency to disengage or to produce ozone from the sewer gas; by passing the foul air through shafts into which water was constantly injected, and by endeavouring to extract the foul air by fans driven by machinery.
I have myself patented a plan for “annihilating sewer gases” by allowing them to be absorbed into dry earth, and have tried the system with some very marked and successful results.[195]
Having thus far considered all the known methods for dealing with the noxious emanations which proceed from sewers, the next point to consider is what these noxious emanations are, and whether they exist in all sewers.
It has been found that even in sewers of the best and most modern construction what is called “sewer gas” is generated in more or less quantity; this arises even from fresh sewage, but is far more noxious and dangerous to health when the sewage has begun to decompose. Even where the sewers are so constructed as to remove all the sewage to the outfall within 24 hours (which has been decided to be the maximum time it should take), there is still an accumulation of slime on the inner periphery of the sewers, owing to the rise and fall of the sewage line, which is constantly manufacturing gases of decomposition.
It is no doubt true that the more perfect the system of sewerage is the less foul air there is in the sewers, but in very few towns will there be found no sewers or drains where temporary obstructions of the sewage do not occur, and where gases are generated, which then find their way into other parts of the sewage system unless they are dealt with in some effective manner.
As to what is the actual composition of this foul air in a sewer little or nothing seems to be known, except that it is highly dangerous to health if breathed, and is also very offensive to the smell.
The “fœtid organic vapour,” or sewer gas proper, has for its companions in a sewer, sulphuretted hydrogen, a most poisonous as well as unpleasant smelling gas, carburetted hydrogen, due very often to leaky gas mains or services, or to decomposing vegetable matters, carbonic acid gas or carbonic anhydride (choke damp), and some ammoniacal compounds.