The necessity for some manner of dealing with the noxious vapours emanating from sewage other than that of letting it find its way from the sewers into the house drains and thence into dwelling houses, has induced the legislature of this country to introduce the following clause in the Public Health Act 1875, which imposes on every local authority the duty of causing their sewers to be ventilated so as not to be a nuisance or injurious to health.

“Every local authority shall cause the sewers belonging to them to be constructed, covered, ventilated and kept, so as not to be a nuisance, or injurious to health, and to be properly cleansed and emptied” (38 & 39 Vic. c. 55, s. 19).

The result of this compulsion upon local authorities to ventilate their sewers has been the introduction of many methods to effect the purpose, the great difficulty being to “ventilate so as not to be a nuisance or injurious to health,” the advocates of open ventilation contending that this is effected by having a sufficient number of openings in a sewer to dilute and safely disseminate the foul gas with atmospheric air so that no nuisance is caused.[193]

Many other methods have been from time to time suggested, some of which have been carried into effect, and I will now proceed to give them in detail, discussing their merits and objections in each case.

(1.) Open shafts are carried up from the crown of the sewer to the centre or side of the roadway, and there protected by an open iron grid or grating at the level of the street surface.

This is the system which has hitherto found most favour with town surveyors, and is sometimes modified or worked in conjunction with the practice of untrapping all the gully pits and buddle holes at the sides of the roadway, which is an excellent plan if the theory of the atmospheric air dilution at which this system aims is a correct one; in fact, if this dilution by air is all that is necessary to render the foul air in a sewer innocuous and inoffensive, there cannot be too many openings into it.

The objections to this system are as follows:

(a.) The foul air escaping into the public streets is often very injurious to persons passing a ventilator, and sewers are buried out of sight, but they are not out of mind so long as we are constantly and unpleasantly reminded of their existence.

(b.) It is found that a change of temperature either of the atmosphere, or of the air in a sewer, will seriously affect the action of a shaft, causing it sometimes to have upcast currents of air, sometimes downcast; the effect of this latter action, especially when it arises from the direction of the wind blowing over or into the shaft, is frequently to drive the impure gases contained in the sewer into the house drains, and from thence into the houses, unless they are so trapped and ventilated as to prevent it.

(c.) They are also affected by the fluctuations of the flow of sewage in the sewer, or by barometric changes in the atmosphere.

(d.) The situation of the open grids in the street is sometimes somewhat awkward for traffic, and horses will frequently shy at them, they also admit solid road detritus into the sewer unless they are protected by a catch plate of some description.

(e.) They are tempting places for children to play over, with what results may be imagined.

(2.) Open shafts are carried up the sides (gable ends if possible) of buildings in the neighbourhood of the sewer; these shafts may be either open at the top, or be furnished with exhaust cowls. This system is sometimes employed in conjunction with inlet shafts at the sides of the street, in the manner shown by the drawing which follows:

This method has the advantage over the first system I have mentioned of carrying the smells further from our reach, but it also has the following disadvantages:

(a.) The distance and the number of bends and elbows the gases have to traverse before reaching the external air.

(b.) The difficulty of fixing them just at the points where they are most required with reference to the gradients of the sewer, especially if they are to be constructed of such an internal diameter as will ensure their efficiency.

(c.) The great objection raised by occupiers and owners of premises against having them fixed on their premises, both on sanitary and legal grounds.

(d.) Their great expense.

(e.) The effect of weather upon their currents.

(3.) By making use of the rain-water pipes from adjoining buildings.

This method commends itself as being very economical, and the opportunities thus given for ventilation are so numerous. If sewers are to be ventilated at all[194] it would seem at first sight that there could not be too many openings from and into them, but this system has the following serious objections:

(a.) When raining, little or no ventilation can take place, and this is the very time, owing to the rising of the water in the sewer, that the gases should be allowed free egress if such is considered the best manner of dealing with them.

(b.) The position of the head of the rain-water pipe is generally the worst that could be chosen for the egress of the gases, both on sanitary and pneumatic grounds.

(c.) The joints of a rain-water pipe are usually none of the best.

(d.) The objections persons naturally have to allow the rain-water pipes of their houses to be used for such a purpose.

(4.) By utilising the lamp posts or columns adjacent to the sewer.

The objects of this system are first to obtain a constant upward current from the sewer, and secondly to secure that the foul gases and air shall be consumed and rendered innocuous by being burnt. The objections to this system are—

(a.) The lamps are only lit at night, consequently little or no ventilation would be going on during the day.

(b.) The number and diameter of the lamps are generally too small to make any appreciable effect upon the ventilation of the sewer.

(5.) By passing or filtering the foul air through charcoal placed in trays or other receptacles in shafts.

The theory of this method is admirable, as the charcoal would arrest all the impurities and flocculent organic matters contained in the foul air, and allow only the pure filtered air to pass into the atmosphere.

It has been found, however, where this method has been adopted, that the charcoal very soon becomes so caked and consolidated from damp and the vibration of the traffic, that it will neither allow the air to filter through, nor absorb the impurities contained in it; these objections and the amount of attention this system requires, has not rendered it very popular with town surveyors, although it has much to recommend it.

(6.) By making use of ordinary chimney shafts.

This method and the one which follows have some merits, the principle being that an upward current is established as an exhaust from the sewer, and also that the foul air is purified by being passed through fire, but both these advantages are only gained when the fires are actually burning; the objections to this system are—

(a.) Structural difficulties must often be encountered and overcome.

(b.) Possibility of explosion arising from leaks of gas mains into sewers (a by no means uncommon occurrence, as all who have charge of sewers frequently find to their cost).

(c.) The objections of owners and occupiers to allow their premises to be thus made use of to carry off a public nuisance for which they as individuals are in no way responsible.

(7.) By lofty shafts erected at convenient positions which are either in connection with furnaces or are simply open to the air.

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 “foetid 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.

The actual component parts, however, of any gases in a sewer must vary considerably with its conditions and locality, &c., in the same manner as they would in any public building or room, and it is impossible to tell, without costly experiments, what gases may be prevalent in any particular portion of a sewer. But whatever may be the analysis of this foul air, there can be but little doubt that it contains organic matter floating about in it as solids, and that it is excessively injurious and even dangerous to breathe, and that it should be caught and destroyed or rendered innocuous, and not be permitted to pass into and contaminate and poison the air we breathe.

Some engineers are of opinion that this foul air always finds its way to the upper portions of the sewerage system, but my investigations into this subject have led me to believe that no rule of this kind can be laid down, for with quick velocities of flow, in some sewers, the gases are carried by friction in the direction of the flow of the sewage, and do not ascend as has been imagined.

Whatever system of ventilation of the main sewers in any town may be adopted, it is imperative that the house drains connected with them should be properly trapped and ventilated, and this is in my judgment of even greater importance than the ventilation of the main sewers. On this point I have given more explanations in the chapter upon “House Drainage.”

In conclusion, let me refer my readers to a most exhaustive discussion, and very valuable information contained in the chapter entitled “Ventilation of Sewers and Drains” in Mr. Baldwin Latham’s ‘Sanitary Engineering,’ in which may be found almost everything that is at present known upon this important subject.