The effect of the bed upon the bacterial flora of sewage is, as was to be expected, but very slight, and it is on record now that, as far as the micro-organic life is concerned, the effluent is to all intents and purposes raw sewage. [Sidenote: Silting up of bed.] Some of the substances contained in raw sewage remain in the bed, no matter how carefully the sewage has been previously strained, and these, in combination with the slimy surface coating of the component particles, the accumulation of mineralised substances in the pores, the consolidation of the bed, the disintegration of the filling material, and the liquid retained, lead gradually but surely to the silting or sludging up of the bed. [Sidenote: Theoretical original water capacity of bed.] (_c_) _Water Capacity of Bed and Silting up._—The theoretical water capacity of the bed, previous to commencing operations, is the aggregate of the cubical space occupied by the pores or small passages between the particles forming the filling material, and the pores of the filling material itself; but in practice a certain amount of this space is occupied by air, which it is impossible to dislodge altogether in filling. The aggregate of the cubical space of the pores may be called the pore-volume. It is difficult to lay down general rules as to what the original water capacity of a bed should be expressed in per cent. of the space occupied by the filling material, but speaking within fairly wide limits the following is somewhat near the truth. [Sidenote: Original water capacity with spherical particles of uniform size.] When the particles forming the filling material are fairly spherical and of equal size, the original water capacity of a bed amounts to about 38 per cent. of the space occupied by the filling material; but as in practice it is difficult to obtain spherical particles of uniform size, the original water capacity is found to range from 35 to 45 per cent. of this space. [Sidenote: Original water capacity with particles of different sizes.] When, however, the particles are of materially different sizes, and when the smaller ones fill up the spaces between the larger ones, the original water capacity may sink down to as low as from 5 to 10 per cent. of the space occupied by the filling material. [Sidenote: Size of particles of filling material does, under certain conditions, not affect original water capacity of bed.] It has been further demonstrated that the water capacity of a bed is not affected by the size of the particles, provided the latter are spherical and of uniform size. In other words, the water capacity of two beds filled with material of different sizes is the same, provided the particles are spherical and of uniform size throughout each bed. [Sidenote: Silting up of bed during regular work.] [Sidenote: Rapid initial decrease of capacity.] [Sidenote: Consolidation of bed.]