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has lately been introduced into some parts of London, and which has been so strongly advocated by Mr. Chadwick, will not only prove to be economical in many ways, but will also be productive of great sanitary advantages. The smooth and impermeable street surface supplied by the Val de Travers Asphalte is durable, elastic, and inodorous, and can easily be cleaned by jets of water; but a less costly surface, which will also be smooth, non-absorbent, and washable, will suffice for streets in the poorer localities, where traffic is inconsiderable. According to Mr. Chadwick, such a surface can be obtained by the use of General Scott's "Selenite," while some of the concretes or gas-tar asphaltes would be sufficiently durable for pavements and back courts.

CHAPTER XI.

PURIFICATION AND UTILISATION OF SEWAGE.

IT has already been shown, in the previous chapter, that of all methods of sewage-removal, the water-carriage system is the only one which meets the requirements of large towns. It is the speediest, cleanest, and, in the long run, the most economical, method which can be employed on an extensive scale, and its general sanitary advantages are now placed beyond dispute. But no sooner had this difficult hygienic problem been solved by engineering skill, than another of even greater difficulty arose. The eagerness of early sanitary reformers to get rid of human refuse at any cost blinded them to the fact that, by pouring sewage into the nearest watercourse, they were merely removing the evil from one place to take effect somewhere else. No consideration was paid to the probable results of the method on the future water-supply of increasing populations, nor to other serious consequences which speedily began to declare themselves. were in reality converted into sewers, and the communities down stream, while they loudly complained of the annoyance and danger to health, added to the nuisance by following the general example. After a time it was discovered that the mouths of navigable rivers were being silted up, that valuable stocks of fish

Rivers

were destroyed, that water-supplies were contaminated, and that riparian rights were in every sense grossly violated. Such were some of the more important evils resulting from river-pollution, and eventually legal prohibitions were issued in many places to prevent their continuance. These prohibitions have multiplied, until the sanitary authorities throughout the country are at last compelled to purify the sewage of towns before it is discharged into any watercourse at a distance from the sea, or run the risk of incurring legal penalties.

Meanwhile, there was an increasing number of economists who rightly maintained that sewage was not only wasted, but worse than wasted, when discharged into rivers, and that, on account of its manurial value, its proper destination was the soil. Hence has risen the large question of the utilisation of sewage, the merits of which will be best understood by considering first the composition of town-sewage.

SECTION I.-TOWN-SEWAGE.

In addition to excretal matters, town-sewage contains the effete products of various trades and manufactures, animal and vegetable débris, mineral detritus. from roads and streets, and the like, all of which are held in suspension or solution by an amount of water varying according to the water-supply in the first instance, and depending, in the second place, on the rainfall and amount of subsoil-water entering the sewers at different times of the year. This varying amount of water is one of the chief difficulties to be encountered in the utilisation of sewage, and, apart from other considerations, it has led Mr. Menzies and other eminent

engineers to recommend the introduction of the pipesewer system, which has already been described, into all towns where sewerage-plans have yet to be carried out. The sewage, delivered from pipe-sewers, consisting almost exclusively of excretal matters, slops, and the water-supply, can of course be readily estimated in all cases, and is much more easily dealt with. But with common drain-sewers, which receive in addition the rainfall and subsoil-water, not only is the extent of dilution much greater, but it is constantly varying in amount. Thus, to quote the data given in the Third Report of the Sewage of Towns Commissioners (1865), it is considered that 60 tons per head per annum (36 gallons per head daily) is the average amount of normal or dry-weather sewage in the metropolis, but this amount is further increased by the rainfall and subsoil-water from two-thirds to an equal volume. With pipe-sewers, however, the amount of sewage equals the amount of water-supply, and in towns supplied on the constant system, this ought not to exceed 20 gallons per head daily, or about 33 tons per head per annum. In the face of such considerations as these, the sanitary and practical importance of Mr. F. O. Ward's famous alliterative dogma of " the rainfall to the river, and the sewage to the soil," becomes at once apparent.

But, with either system of sewers, the value of the sewage may be said to depend entirely on the excretal matters, and the amount and relative value of these will be gathered from the following data:

According to Mr. Lawes, the subjoined table represents, as the result of numerous analyses, the average amount and composition of excretal matter discharged by a male adult daily :—

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In a mixed population, the actual amounts per individual will obviously be considerably below this average, and, according to Dr. Parkes, they may be estimated at 21 oz. fæcal matter and 40 oz. urine daily, an estimate which gives 25 tons solid fæces for every thousand inhabitants annually, and 91,250 gallons of urine. But the above table also shows that the manurial value of the urine voided in the twenty-four hours greatly exceeds that of the fæces passed in the same time. Indeed, the relative value, as determined by numerous analysts, is approximately as 6 to 1.

The actual value of both urine and fæces in sewage has been estimated by Messrs. Lawes and Gilbert at 6s. 8d. per individual per annum, supposing that 10 lbs. of ammonia is a fair estimate of the amount voided in that time. When the sewage averages 24 gallons daily per individual-that is, 40 tons per head per annumits money value, according to this estimate, would be 2d. per ton, and the value per ton will decrease in proportion to the rate of dilution above this average. It may be added that this estimate corresponds very closely with the money value of average sewage given in the First Report of the Rivers Pollution Commissioners (1868); for it is there stated that the value of the "dissolved constituents in 100 tons of average sewage is about 15s., while the suspended matters only contain

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