The magnitude of the question of “sewage disposal” almost decided me to refrain from making any remarks upon it, but on reconsideration I thought a few might be serviceable.

Sewage disposal means the getting rid of the foul water contained in the sewerage system of any community.

Where a dry method is in force for the collection of the excrementitious matters it is called “interception”; the following are some of the systems which effect it:—

Privies, ashpits, middens, cesspools, pails, troughs, the “Rochdale,” the “Eureka,” the “Goux,” Fosses Permanentes, Fosses Mobiles, and Moule’s, Taylor’s, and Phillip’s earth closets.

The collection and disposal of the mass of excrement under these dry systems is found to be a very troublesome matter, and they are at their best but inferior substitutes for water carriage, nor must it be forgotten that sewers and drains are necessary even if a good interception process is in force. I shall therefore confine my remarks to the disposal of water-carried sewage.

Many books have been written, many valuable reports have been prepared and issued, lengthy papers and discussions have been frequent at the meetings of scientific societies, and almost innumerable pamphlets have been published upon this important sanitary subject. Some millions of money have also been spent in trying to deal satisfactorily with this question, not only with a view to the purification of the effluent of the sewage, but also to endeavour to make a profit out of the residuals.[187] The result of this literature, discussion, and experiment has led to the following conclusions:

No hard and fast lines can be laid down as to the best method to be adopted for the disposal of the sewage of any town, but the peculiar circumstances of each case must be considered before advice could be given on the subject: geographical position, physical arrangement, habits of the population, and the character and quantity of the sewage of the town being some of the most important.

In any case it is necessary that the transmission of the sewage to the outfall should be effected as speedily as possible, and that the position of this outfall should be such as to cause no nuisance. The contents of the sewers should, if possible, be emptied by gravitation, as pumping is a constant expense, and economy with efficiency must of course be studied.

Up to the present time the following are the methods adopted in this country for the disposal of sewage:

(1.) Passing the sewage in its crude state into the sea or tidal river.

(2.) Passing the sewage in its crude state over large tracts of land; this is called broad irrigation.

(3.) Passing the sewage in its crude state on to small tracts of land previously prepared by deep drainage; this is called intermittent downward filtration.

(4.) Mechanical subsidence of the sewage in large tanks, the effluent passing on to land or into a river.

(5.) Mechanical filtration of the sewage, the effluent passing on to land or into a river.

(6.) The introduction of lime or other precipitant into the sewage, which is allowed to settle in tanks, the effluent passing on to land or into a river.

Very little need be said upon the first of these methods. Many engineers of high standing contend that, where practicable, the sea or the tidal estuary of a river is the right place for the sewage, as no costly works are necessary, and an abominable nuisance is thus got rid of at once and for ever. To ensure this, however, great care must be exercised in the selection of the site for the outfall. Float observations should be made, not only of the surface tides and currents, but also of those at different depths, and the effect upon the sewage by its different specific gravity from that of the salt water must be allowed for, as well as the difference of level of the tides and the configuration of the adjoining coast line.

The second method, that of broad irrigation, is one that finds considerable favour with a large number of engineers and agriculturists. The great sewage-disposal cry has always been, “Put back on the land what you have taken from it, or some day there will be no beef and no bread.” The difficulty is to always find land in sufficient quantity and so situated as to be available for this purpose. Almost any soil is, however, suitable for irrigation, provided it is well and properly drained. The quantity of sewage which should be used for this purpose per acre of land varies considerably, as will be seen on reference to a table prepared by Mr. Henry Robinson,[188] where the number of inhabitants to each acre irrigated is in one case (Leamington) stated to be 55, in another (Blackburn) 208,[189] the average being 137, the number of gallons per head of population per diem being 38.

It may be useful to state here that a hundred tons of sewage will cover an acre of land 1 inch in depth, and that the value of sewage as a manure is said to vary from ¹/2d. to 2d. per ton, or, calculated in another manner, about 10d. per head of population per annum.

The best crops for a sewage farm (in addition to nearly all kinds of market-garden produce) are rye-grass, mangolds, beetroot, cabbages, carrots, potatoes, turnips, rabi, parsnips, lucerne, beans, wheat, oats, and barley; the cereals, however, are apt to run rather to straw, and some care is also necessary not to oversewage potatoes and some other root crops. Grazing cattle or cows can also be carried on with advantage, the presence of sewage having no effect either on the milk or flesh of animals fed on sewage farms.

The third method, that of intermittent downward filtration, is really irrigation of land to such an extent as the land will filter or purify the sewage, the effluent passing off pure, irrespective of any effect upon the crops which may be growing upon the land.

Great discussions have arisen (the principal battle-ground being Merthyr Tydvil) as to the maximum quantity of sewage which an acre of properly prepared land will treat, some of the champions of this system contending that a good porous soil properly drained to a depth of six feet will purify the sewage of 6000 persons per acre, others that only the sewage of 250 persons can be so treated.[190]

There can be no doubt that earth has a most powerful deodorising power. Laboratory experiments have shown that as much as eight gallons of sewage can be filtered through a cubic yard of loamy soil in twenty-four hours, the soil being drained at a depth of six feet, the effluent therefrom having obtained a wonderful degree of purity. Much, however, must depend upon the character of the soil of the filtering area and the strength of the sewage which is being operated upon.

The following description of the manner in which the earth acts upon sewage will be of interest:

“The fÆcal matters and other impurities attached themselves to the surfaces of the particles of earth by a kind of cohesive attraction, and in this state were readily attacked by the oxygen of the air. Their organic carbon became carbonic acid, their nitrogen was converted into nitrous or nitric acid, which united with the lime, magnesia, and other basic matters present. Mechanically suspended impurities were arrested as by a sieve, and the water issued from beneath—not indeed fit for dietetic or domestic purposes, but at any rate in a fair state of purity and quite inoffensive to the senses.” (Vide W. Crookes in the discussion on the Sewage Question by Norman Bazalgette, ‘Min. of Proceedings of the Institution of Civil Engineers,’ vol. xlviii. p. 164.)

The land thus used as a sewage filter requires constant aËration by being dug over or ploughed, and if this precaution is taken, it is surprising to what a wonderful extent the land will take sewage without becoming what is called “sewage sick.” Clay soils are, however, stated to be ill-adapted for this purpose.

The next method, that of mechanical subsidence of the sewage in large tanks, has been attempted in conjunction with irrigation and filtration without much benefit, nor has the fifth method I have mentioned, viz., that of mechanical filtration of the sewage, met with any better result. Artificial filters have been constructed of burnt clay, cinders, coke, charcoal, peat, chalk, gravel, broken stone, sand, spongy iron (this is now being applied very successfully for the purification of water), straw, cocoa-nut matting, wicker-work, and wire gauze of different degrees of fineness of mesh.

The late Mr. Odams spent a considerable sum in endeavouring to strain sewage through revolving screens of wire gauze with but little success, and Mr. Bannehr has striven to achieve the same object by passing sewage over oscillating screens of the like material.

In all these cases of mechanical filtration, however, the effluent has either not been sufficiently pure or the screens and filters have become clogged and refused to act.[191]

The last method that I have mentioned for the disposal of sewage is that of precipitation, or what may be more properly called the chemical treatment of sewage.

Precipitation means the production, by the introduction of chemical substances within the body of the sewage, of certain solid compounds, which, in settling, drag down with them the suspended matters in the sewage, together with a small proportion of the polluting matters which are in solution in the sewage, this proportion varying with the quantity of solid matters deposited. The effluent from the tanks in which this precipitation takes place is then allowed to flow direct into a river or stream, or is still further purified by being passed over land or filtered through deep-drained soils.

Chemical treatment of sewage was first tried in Paris in the year 1740, and since then every effort has been made to extract a valuable and commercial manure from sewage and purify the effluent. Between the years 1865 and 1875 more than 400 patents were taken out in respect of these and other matters in connection with the sewage question.

It is almost needless to say that but few of these patents were of any practical value. Those processes which have some merit and are now best known are, I believe, included in the following list:—

List of Chemical Processes for
the Treatment of Town Sewage.