This sludge contains about 90 per cent. of moisture, and if left to dry atmospherically, a thin crust forms over it, thus protecting that which is underneath, and it will not dry for many months. In some cases it is sought to dispose of the accumulations of sludge by digging it into the land; in others it is mixed with house ashes, &c., and sold as manure. Its bulk, however, in proportion to its manurial value is so excessive as to render it almost valueless, and it is difficult to get rid of it for this purpose even when fortified with ammonia or other chemical. A frequent practice now adopted is to reduce its bulk by exposure or by presses to a semi-dried condition; in other cases it is dried to a powder by heat, and General Scott has patented a method where, in connection with the lime process, it is burned and manufactured into cement. Sometimes it is squeezed in presses, such as Needham and Kite’s or Johnson’s, or it is filtered by Milburn’s or Weare’s apparatus. For drying the sludge by heat, Borwick’s or Forrest’s machines have been used with some success, but there is no doubt that the slimy, glutinous, albuminous, offensive mixture technically known as sludge is a difficult matter to dispose of in all sewage works. In connection with the chemical treatment of sewage, it is of advantage to pass the effluent over land filters on the intermittent downward filtration principle. Another very effective plan is to pass the effluent (or even crude sewage) through land which is thickly planted with the Anacharis or American weed, duckweed, sedges, rushes, reeds, &c., or through beds of osiers or alder trees. An acre of land thus planted is said to purify more than three million gallons of sewage per diem.[192] There is no doubt that plants of this description have a powerful action in purifying sewage or foul water of any kind, and where land is scarce this method has many advantages. It is almost unnecessary to add that where sewage is treated in any other manner than that of throwing it into the sea or river, bulk is a great objection, especially if it has to be pumped. To obviate this bulk the separate system is of great advantage, and Isaac Shone’s new method for ejecting sewage along pipes seems also to be a most desirable invention in connection with this subject. [187] It is computed that every ton of liquid sewage which is treated chemically costs about three-quarters of a farthing. [188] _Vide_ ‘Sewage Disposal,’ by Henry Robinson, C.E., &c., 2nd edition, p. 79. [189] I purposely omit (Kendal) 856, as this is, properly speaking, “filtration.”