Sezeage for Irrigation the present day not only the nat ural watercourses, but also the sewer-waters of cities and towns, are util ized for purposes of irrig-ation. Formerly it was the universal practice to permit the sewers to discharge their volumes of impure water into the nearest watercourse, regardless of whether the volume of water in the stream was large enough to dilute the impurities below the dangerous point, or whether—as is the case in London, where the backing -up of the sewage by the tide occurred—in consequence of this, the conditions brought about were most unwholesome and intolerable. In certain locali ties in England and France the sewag-e, for the purpose of utilizing its fer tilizing- properties, is conducted in separate conduits and distributed over the fields. The most extensive system of this order, however, has lately been put in operation by the authorities of the city of Berlin, where all the sewage from the built-up portions of the city is conveyed in the sewers to pumping stations conveniently located for the purpose, whence the waste waters are distributed over a considerable area of fields devoted to the pur pose in the adjacent country. Other plans involving- the precipitation of the organic matter and its separation by chemical treatment, that it may be utilized and that at the same time the water may be purified, are also in use to a limited extent.
It appears, however, that, while methods similar to the above have been successfully employed for the disposal of the sewage of towns and small cities, until the Berlin experiment had been made successful it had not been found practicable to apply them to the treatment of the enormous volumes daily discharged through the sewers of the g-reat cities. For the
effectual disposal of the latter in such a manner as completely to gnard the health of the inhabitants no plan that is universally applicable has yet been devised. For cities situated upon or near the sea-coast, the plan of pumping the sewag-e into artificial reservoirs placed at a convenient point in the harbors, from which it is discharged into the sea and carried out at ebb tide, affords a system as nearly perfect as could be devised; but for inland cities, where the sewage must be discharged into the adjacent riv ers, the only practical plan seems to be its dilution by frequent periodical flushings, and its subsequent discharge into the stream at a point so remote as to reduce to a minimum the danger of its return by the tide. Fortu nately, the flowing water in the channels of the rivers very quickly purifies itself by aeration; and this natural process is so efficacious that even the water of the river Thames, charged with the vast volume of sewage con stantly discharged into it from the sewers of London, is found to have be come sufficiently pure for drinking-, purposes five miles below the city. Should the Berlin experiment be found to continue to answer its intended purpose—which now appears probable—it may teach au important lesson in the disposal of the sewage of great cities.