798 SEWERAGE with house connections it offers leas danger than any other system of producing infection. When^such disposal is impossible without rais- ing the sewage from reservoirs by pumps and discharging it into a river which at the same time has a tide that will carry much of the sewage back to the town, as is the case at Lon- don, the question of utilization presents itself. There are five principal systems of utilization : 1, irrigation ; 2, nitration ; 3, precipitation ; 4, dry earth; 5, the pneumatic system. Irri- gation has been the most largely practised, but it is contended by its opponents that de- leterious organic compounds are taken up by the growing plants as well as scattered through the air in the process, and by exhalation from the soil. Its advocates insist not only that a large yield is secured, but that the crop is grown in as healthful a condition as by or- dinary culture. It consists in distributing the sewage over the surface of the soil, from reservoirs into which the sewers empty or into which their contents are pumped. The objections are that infection is not sufficiently prevented, as before stated, and that, on ac- count of topographical and geological condi- tions of the surrounding country, it is often impracticable. The advocates of filtration con- tend that it is economical and innocuous. In- termittent downward filtration is described as a variety of irrigation combined with deep drainage, so that the soil may be made to al- ternately receive the sewage and the air by which it is decomposed, freed of its poisonous qualities, and made fit to give nourishment to plants. Weare's process of filtration consists in filtering the sewage through charcoal, ashes, and soil contained in boxes, and is said to an- swer very well on a small scale. Precipitation is performed by several methods. The lime process of Gen. Scott, architect of the Albert hall, London, consists in mixing the sewage with a small proportion of milk of lime, agi- tating the mixture, and allowing it to subside. The precipitate is a very putrescible mud, and the liquid Hows off in a tolerably clear condi- tion, and has been used in the manufacture of a hydraulic mortar. But the precipitate con- tains only a small portion of valuable manu- rial constituents, and the drying of it is a very offensive operation. The superphosphate of lime (or Blythe's) process attempts to recover the ammonia from the sewage. Superphos- phate of lime and a salt of magnesia are added under the supposition that an insoluble phos- phate of magnesia and ammonia will be thrown down, but analyses show that a part of the phosphoric acid runs off in the solution. It was rejected by the English sewage commis- sion. Holden's process mixes the sewage with sulphate of iron, lime, and coal dust. The "A, B, " process, as it is called, or the process with albumen, blood, clay, and charcoal, has been worked by the "Native Guano Compa- ny," and is said to be successful. It consists in coagulating blood obtained from slaugh- ter houses, together with whatever coagulable matters may be in the sewage, with sulphate of alumina, producing a fiocculent precipitate which shall drag down by the aid of the heavy particles of clay the organic solids contained in the sewage ; and the charcoal is for the pur- pose of absorbing gases. A company is work- ing at Crossness, between Plumstead and the Erith marshes, at the southern outfall of the London drainage, which is there discharged at the rate of 60,000,000 gallons daily; of this 1 per cent, or 500,000 gallons is used by the company. A 15-horse-power steam engine throws the sewage into mixing tanks made of concrete, which are about 50 ft. long, 20 ft. wide, and 8 ft. deep. The details of the process, together with the sanitary and agri- cultural results, are given in the " Quarterly Journal of Science" for January, 1878. Dis- posal by dry pulverized earth in movable box- es has many advocates, who think that it could be advantageously applied to large towns ; but it is the general belief that it is only adapted to those of considerably less than 100,000 in- habitants, requiring as it would the bringing in daily of dry earth to the amount of from 5 to 10 Ibs. for each individual. The pneumatic sys- tem of disposal and utilization devised by Capt. Liernur has been applied at Amsterdam, Ley- den, and other towns in Holland, where the disposal of excreta by water-carriage sewage is highly objectionable in consequence of the level of the canals being lower than that of the sea. It consists of the ordinary sewers for surface and house draining, and a second pipe system connected with an apparatus for manufacturing immediately and without loss the excreta into poudrette. The outlines of the process may be briefly stated as follows : The earthen hoppers of the houses are so con- structed that the excreta fall to the lowest level of the soil pipe. Each hopper is covered with a rubber-seated lid, and is connected with a ventilator which passes up through the roof. At street corners, under ground, are large iron air-tight tanks, to a single opening in each of which all neighboring house pipes converge, this opening being closed with a gate. These tanks are connected in turn with large tanks at the great central sewage works of the cities. By means of air pumps a vacuum is created in the great tanks, and the gates of the pipes lead- ing to the common receptacles being opened, the air is quickly removed from them and a vacuum created in each. The gates between them and their connecting house pipes being now opened, the matter within them is ex- hausted into the corner receivers, to be trans- ferred to the great tanks at the central station, where it is manufactured into poudrette. This is accomplished by allowing the sewage to run into retorts perfectly fresh, all houses, tanks, pipes, &c., being emptied every few hours, mixing it with a small percentage of sulphu- ric acid to fix the free ammonia, and then run- ning it into troughs in which revolve large
