Art. LII.—On Sewage Irrigation, and its results, with a Sketch of the Main Drainage Systems of London and Paris.
[Read before the Philosophical Institute of Canterbury, August 4, 1869.]
There are few subjects more interesting to the inhabitants of towns generally, than questions relating to sanitary arrangements, and properly organized systems of main drainage.
Although it might be thought that in every large town, such a system had been in partial operation since the times of the Cloaca Maxima, yet it is a curious fact that, until very recently, no large city, either in England or on the Continent, had paid any real attention to this important subject.
The author therefore proposes to state, briefly, what steps have been taken in London and Paris to secure effectual drainage, and to compare the working of two distinct systems varying in some important particulars.
Up to the year 1815 it was illegal to discharge any sewerage into the drains of the city of London. After that date it became impossible to prevent the influx of sewage matter, and in 1847 the law was reversed, and drainage into sewers rendered compulsory.
Commissioners were appointed to carry out the various works necessitated by such a change, and held office until the year 1856, when the present Board of Works was constituted.
The Board, after full investigation, resolved to adopt the scheme elaborated by their own engineer, Mr. Bazalgette, under whose most able administration the works were commenced in the year 1859, and will probably be completed in the course of a year or so, contemporaneously with the Thames embankment.
In Paris the cholera attack of 1832 first opened the eyes of the inhabitants to the sanitary condition of the city, and such vigorous measures were adopted, that in four years their sewerage system was doubted, and within the next twenty-two years quadrupled. Paris is built in blocks, each block having its own cesspool, which is emptied at stated times, the contents deodorized and part sold. All waste water from the houses, and rainfall, passes into the sewers, which are of sufficient diameter to allow of men working freely in their interior, and of their serving as subways for the conveyance of gas and waterpipes, and lines of telegraph. They are cleaned by means of trucks running on iron rails, and in the case of the main sewers, by a species of boat propelled by the pressure of the water. The annual cost of cleansing amounts to about £30,000, whilst it is understood that little or nothing is realized by the sale of deodorized soil.
Many difficulties arose in dealing with the sewage of London, as is generally the case in every town which has been built before any definite idea has been formed as to the ultimate disposal of its sewage; one of the main difficulties being, that the discharge was affected by the tide, a considerable area being below the level of high-water.
The six questions which presented themselves were:—
At what point, and at what state of the tide, could sewage be discharged into the river, so that it should not return within the more densely inhabited portions of the metropolis?
The minimum fall of the intercepting sewers?
The quantity of sewage to be intercepted, whether it passed off uniformly day and night, or in what manner?
Was rainfall to be included, and what was its probable amount?
Having regard to all these points, how were the sizes of the sewers to be determined?
What description of pumps were best suited for lifting sewage?
After due consideration and many interesting experiments, the conclusion was arrived at, that a district of average density of population contained 30,000 people per square mile, and the sewage was proved to be nearly equal in amount to the water supply. The calculation was, that the average daily amount to be provided for would be five cubic feet per head per day. The total areas drained on the north side of the Thames amounted to about forty square miles, on the south side to about the same, with a quantity of sewage amounting to 40,000,000 cubic feet per day on the north, and 23,000,000 cubic feet per day on the south side, respectively.
In 1865 a Private Bill was brought before a select committee of ten members of the House of Commons, having as its object the utilization of the sewage on the north side of the Thames. The Board of Works had previously advertised for tenders and proposals for effecting that purpose, with a view of making the sewage repay the cost of maintaining the drains,—the cost of construction, which will amount to about £4,100,000, being provided for by a rate upon an estimated rateable value of £14,500,000. The scheme which the author is now describing was the one approved by them, and to the advocates of which they made a grant of the total sewage on the northern side, for a period of fifty years, upon certain terms. After a protracted struggle the Bill was passed, in spite of the determined opposition of the Council of the City of London, who insisted that the terms were not sufficiently favourable to the ratepayers, the maximum estimated price per ton, twopence, being, in the opinion of their advisers, far beneath the true value of the sewage.
The main works, which were estimated to cost about £3,000,000, were then commenced, and for the purpose of testing the value to the farmer, of London sewage, taken just as it came down the outfall sewer, the directors determined upon renting a small farm of about two hundred acres, in the vicinity of Barking, to which the sewage was forced by steam-power, at the rate of 175 cubic feet, or five tons, per minute. A tank, holding thirty tons, was erected, into which the sewage was delivered from the main, so that at any period in the day the quantity delivered could be accurately gauged by the manager. His record, compared with the indicator attached to the engine, gave correct and reliable data upon which the reports submitted to the public were founded.
Up to this time so little was known of the capabilities of sewage as a manure, and the quantities in, and intervals at which is should be supplied, that the directors considered they could not do better than conduct their experiments on a thoroughly practical system, and one which would bear the inspection of both farmers and business men in general; more particularly as there exists in England a strong feeling on the subject of the fouling of streams and rivers, as is shown by the recent action of the Legislature, which is doing its utmost to prevent public bodies and private individuals from turning natural watercourses into noisome and unhealthy cesspools. Oxford and Reading are at the present time liable to penalties of £50 per day, under recent Acts for the purification of the Thames, and the Royal Commission on Rivers, now sitting, will doubtless place many towns under the necessity of instantly carrying out their drainage works, with a view to the utilization and deodorization of their sewage. It thus becomes a serious question whether it will be possible so to utilize these products as to render the residuum harmless, and at the same time to make the necessary works pay a fair interest on the cost of construction.
Sewage irrigation has been carried on at Edinburgh, Croydon, Carlisle, Rugby, Watford, Worthing, the Crystal Palace, and in other places.
The experiments made at Rugby were conducted by Mr. Lawes, a manufacturer of artificial manures, and a well-known agriculturalist, who was also at the time, a member of a royal commission appointed to make experiments and report their results. They were therefore carefully conducted, and the following were the values assigned:—
£15 per acre being the value of the milk derived from one acre of ordinary meadow grass, £25, £33, and £36, were the values derived from the same grass when watered with 3000, 6000, and 9000 tons of sewage per acre. From the use therefore of 1000 tons of sewage, we get a result varying from £3 6s. 8d. to £2 6s. 8d. over and above the amount that would have been produced by the natural grass, assuming milk to be worth 1d. per pint. This gives the sewage an average value of from 8d. to 55d. per ton. The sewage which had been used was found by analysis to contain from 15 to 25 per cent. of its manurial properties, owing to the nature of the soil and the slope of the ground, and it might have been advantageously used a second time.
In Edinburgh the results have been more satisfactory with regard to the money value per acre. There the meadows are annually let or sold, the purchasers generally cutting the grass for themselves, at prices varying from £25 to £40 per acre; and at Leith, where the sewage is used a second time, at £30 per acre.
These results are, however, obtained by the use of very large quantities of sewage, as much as 20,000 tons per acre being applied, although its actual manurial value is not equivalent to more than half that of ordinary sewage, as the Foul Burn, by which it is brought down, drains a large area of open country.
At Croydon, after paying rent at the rate of £4 per acre, the gross value of the sewage is returned at from ¾d. to 1d., for Italian rye-grass, per ton, used.
The results obtained by Lord Essex at Watford, by Sir J. Paxton at the Crystal Palace, and by Mr. Mechi, and others, do not admit of accurate comparison, an exaggerated value having been put upon sewage as a manure, and consequently the outlay upon pipes, pumps, and apparatus, has usually been upon far too large a scale.
In the case of the farm now about to be described, it should be borne in mind that the object for which the farm was worked, was not so much to pay a dividend, as to prove definitely the actual value per ton of sewage delivered on a farm, and for what sum per acre a certain quantity of sewage could be economically made available.
So far, the three principal methods of irrigation have been the catch-water, the ridge and furrow, and the hose and jet. These names almost explain themselves; but that there may be no mistake, I may explain, that the catch-water is a system of contour ditches communicating with main feeders, each ditch acting as a drain to the plot of land lying above, and a feeder to that below.
The ridge and furrow is commonly used when the natural fall of the land is too slight for the catch-water system, and can frequently be made use of in conjunction with, and prior to it. It consists of a series of artificial undulations about 60 yards wide, having a fall of 1 in 140, or thereabouts.
The hose and jet is a system of underground pipes, under pressure, having valves at intervals, and junctions to which the hose is affixed, the hose itself travelling on a light carriage to prevent injury to the crop. There is also another system occasionally made use of, viz., wooden or iron troughs, but it is usually auxiliary to the other methods of distribution. In the present example the ridge and furrow, and the catch-water, were the systems employed. The area brought under their operation amounted to about seventy acres, and
the crops experimented upon were wheat, oats, mangold wurtzel, sugar beet, cabbage, onions, lucerne, kohl-rabi, potatoes, flax, leeks, celery, asparagus, strawberries, etc., but principally Italian rye-grass, a patch of Bromus Schroederi, or prairie grass, and ordinary old pasture. Upon its arrival at the farm the sewage was allowed to flow from the measuring tank into another considerably larger, whose top was truly level, thus allowing the liquid of the sewage to flow over its lips, and retaining a greater part of the sediment. This was done to facilitate the labour of cleaning the carriers, but the porous nature of the ground, and the large quantity of sewage absorbed by the carries, rendered it advisable to allow the sewage to flow, at first, direct into the carriers, which were gradually puddled by the deposit. The farm was pipe-drained, which was also an unnecessary expense with land of so light a character, and with a deep gravel subsoil. So far as experiments have gone, subsoil drainage has been found of little value in sewage irrigation, as in the extreme case of Croydon, where the soil is a stiff clay, the subsoil drains were taken up by the proprietor, who said the grass was better where they had not been laid down. This fact is opposed to the general opinion of the agricultural world, but there is little doubt that a gravel subsoil will carry away a very great additional increase to the rainfall of a tolerably dry country.
The fifty-five and a half acres of Italian rye-grass supported from 200 to 300 milch cows, which were fed upon 2500 tons of grass, 1 cwt. to 1 ½ cwt. each per day, the produce of 250,000 tons of sewage.
This is taking the whole, and striking an average, but taking that acreage, which at the same time was producing its full and proper yield of grass, it was found that 61 tons per acre was the actual crop carried. Therefore, supposing that all the fifty-five and a half acres had been of equal standing, and sown at the same time, the total yield would have been 3250 tons of grass, or about 1 ton of grass for every 100 tons of sewage, and supposing 750 tons are deducted as the natural yield of the same land under ordinary circumstances. Cow feeders, and others, give 15s. to 20s. for this grass cut and bound, so that the produce of each acre would be from £40 to £60. The laying out and drainage costs from £5 to £15 per acre, thus, inclusive of very heavy charges for labour and machinery, there remains a large margin for profit.
The mangold was sown in May, and taken up in October, having been sewaged at the rate of 1100 tons per acre. The crop averaged fifty tons per acre, doubling the yield on another part of the farm where the land was equally good, and had received twenty tons cow-house dung and five hundred weight of mixed guano, superphosphate, and common salt, per acre. All the other crops mentioned turned out very well, many carrying off prizes at the Royal Agricul. Inst. Christmas Show at Islington. The sugar beet had a higher saccharine value than any produced in England; the strawberries took the second prize at the Royal Hort. Society Show in June, 1867: the three or four acres thus planted were a wonderful sight, the berries being of enormous size and in the utmost profusion. In wheat, a dressing of 500 tons per acre produced a crop of forty-three bushels per acre, with four and a half loads of straw, whilst contiguous land under ordinary conditions bore twenty-nine bushels with three loads of straw. The cabbages also did well, being planted in August and sold in October, at £10 per acre, on the ground.
The author is indebted to Mr. J. C. Morton, the eminent agriculturalist, who had the general supervision of the farm, for some of the above figures.
Sewage irrigation carried on under the circumstances above mentioned was therefore a decided success, but it would be a mistake to suppose that all these results were due exclusively to the manurial properties contained in the sewage. It has been proved in many parts of the world, that pure water used
at the proper times, and under proper conditions of soil and climate, has a wonderfully beneficial effect upon vegetation, so that the above results must be modified, if a true value of sewage as a manure is to be deduced.
Before closing the subject, there are still some observations to be made upon the theoretical value of sewage, and upon the effects of its use as a manure, upon the health of those living in contiguity to sewaged land.
By an average of the analyses of several of the most distinguished chemists it has been found that 200 oz. of ammonia are voided annually by an individual, ⅞ths of which exists in the fluid matter of sewage, whilst the average amount found in one gallon of sewage varies from 9·7 to 3·91 grains, according to the water supply. This represents a composition in which 1000 tons of sewage is equivalent in ammonia to from 16ths to 6ths cwts. of guano. Taking guano at 13s. a cwt., the value of sewage varies from 2·44d. to 1d. per ton. At Barking, from one hundred tons of sewage were derived one ton of grass, of a value of from 15s. to 20s., which would give the practical value of 1·8d. to 2·4d. per ton, thus approximating, in a striking manner, to the theoretical values.
As regards the sanitary points in such a system, it might be reasonably expected that the continued pouring out of such vast quantities of rapidly fermenting manurial matter, the earth would by degrees become saturated, and refuse longer to carry out the powers of deodorization with which nature has endowed her; such, however, is the case in very rare instances, as it is usually hard to detect any effluvium whatever, and that which exists has nothing particularly disagreeable in its character, being merely like a concentrated essence of soap-suds. This may be partly owing to the extreme dilution, and the absence of any solid matters in the sewage, by the time it arrives at the outfalls, and the rapidity with which it finds itself on the soil before fermentation has set in, and whilst it is in the most fitting state for absorption by the growing crops. In fact, in the sewers and reservoirs themselves, after the first day or two, little inconvenience is experienced whilst the superintendent of the lower part of the sewers frequently has to take a walk of some miles up and down the sewers, or a stroll through the reservoirs, before breakfast, without being a bit the worse for it. Upon the tops of both the Barking and Crossness reservoirs are several labourers' cottages, where no illness has resulted; and at the time of the last cholera attack in London, some hundreds of men were drafted down into the author's works from the Isle of Dogs' sewer, where several had died, and a panic had arisen. There was not, however, a single fresh case after their removal, though there were many of them daily in probable contact with millions of so-called choleraic germs. It may, therefore, be fairly assumed that no evil effects can result from the use of sewage as a manure, always supposing that it is sufficiently diluted, sufficiently fresh, and sufficiently disintegrated by its passage through the sewers. Also that Italian rye grass is the crop to which it can be most economically applied in large quantities; the more particularly, as the land upon which it is grown must be re-broken up every three years, so as to ensure a full crop.
This periodical stirring would also have the effect of preventing the soil becoming too sodden, or giving rise to the generation of noxious gases.
This paper has been written with a view to lay before the meeting a slight sketch of the value of a system of Main Drainage, which shall ensure a small return to ratepayers upon any sums expended by them in behalf of the health of the general public, as well as to show the value of sewage irrigation generally, where the produce can command a ready market.