When water is too muddy to be applied directly to filters, the most obvious treatment is to remove as much of the sediment as possible by sedimentation. Sedimentation-basins are considered as essential parts of filtration plants for the treatment of muddy waters. The effect of sedimentation, as noted above, is to remove principally the larger particles in the raw water. By doing this the deposit upon the surface of the filters and the cost of operation are greatly reduced. These larger particles are mainly removed by a comparatively short period of sedimentation, and the improvement effected after the first 24 hours is comparatively slight. The particles remaining in suspension at the end of this time consist almost entirely of very fine clay, and the rate of their settlement through the water is extremely slow; and currents in the basin, due to temperature changes, winds, etc., almost entirely offset the natural tendency of the sediment to fall to the bottom. There is thus a practical limit to the effect of sedimentation which is soon reached, and it has not been found feasible to extend the process so as to allow much more turbid waters to be brought within the range which can be economically treated by sand filtration. CHAPTER IX. THE COAGULATION OF WATERS. The coagulation of water consists in the addition to it of some substance which forms an inorganic precipitate in the water, the presence of which has a physical action upon the suspended matters, and allows them to be more readily removed by subsidence or filtration. The most common coagulant is sulphate of alumina. When this substance is added to water it is decomposed into its component parts, sulphuric acid and alumina, the former of which combines with the lime or other base present in the water, or in case enough of this is lacking, it remains partly as free acid and partly undecomposed in its original condition; while the alumina forms a gelatinous precipitate which draws together and surrounds the suspended matters present in the water, including the bacteria, and allows them to be much more easily removed by filtration than would otherwise be the case. In addition, the alumina has a chemical attraction for dissolved organic matters, and the chemical purification may be more complete at very high rates than would be possible with sand filtration without coagulant at any rate, however low. Coagulants have been employed in connection with filtration from very early times. As early as 1831 D’Arcet published in the “Annales d’hygiène publique,”[32] an account of the purification of Nile water in Egypt by adding alum to the water, and afterwards filtering it through small household filters. More recently alum has been repeatedly used in connection with sand filters, particularly at Leeuwarden, Groningen, and Schiedam in Holland, where the river waters used for public supplies are colored by peaty matter which cannot be removed by simple filtration.