Fig. 139a.
Material for Filling Contact Beds.—The remarks made under the heading “Filtering Material” for percolating filters ([pages 101 to 103]), apply with equal force to contact beds. In none of the many cases which have come under the observation of the author has it been possible to obtain such satisfactory results from other material as from clinker, when used under the same conditions as to the strength and volume of the sewage treated. It is true that excellent effluents have been produced by beds filled with burnt clay, broken bricks or stones, but it will usually be found that in such cases the material is more or less clogged, and that the volume successfully treated per day is considerably less than that which would be dealt with by clinker. The reasons are the same as those already mentioned in pages 101 to 103, and there is no need to say more here than to recommend as strongly as possible the use of hard-burnt vitrified furnace clinker as already described for percolating filters.
The foregoing remarks apply more particularly to the material for coarse and medium-grain contact beds. Clinker of the kind described is equally suitable for fine-grain contact beds, but it is difficult to break it to the required grade except at great cost, and a considerable loss in bulk due to the production of fine dust. For fine-grain beds, requiring material specified to pass a ¼-inch square mesh, and to be retained on a ⅛-inch square mesh, clean coke-breeze from gas-works (not ashes) will be found to be the most efficient. The important point to be observed in preparing material for fine-grain contact beds is that, while none of the particles should exceed ¼ inch in diameter, as many as possible of the finer particles, ⅛ inch and 3⁄16 inch in diameter, should be retained, but all dust should be removed even if it necessitates washing the material for that purpose. Even the finer particles down to 1⁄16 inch diameter might be used, but it will be found extremely difficult to arrange the sifting process to arrest these without retaining the dust as well, especially if the material is at all damp, as the fine meshes required for the purpose quickly become clogged with the dust, and the sieve or screen is rendered useless.
Among other materials which may be adopted for fine-grain contact beds, broken saggars in the pottery districts, or slag in the neighbourhood of ironworks, are probably the next best, but only if they are properly graded in the manner described above. Indeed the chief difficulty in securing this fine-grade material is the preparation and grading, particularly where large quantities are required and the situation of the beds involves much handling of the material after it has been sifted. It is, however, of such extreme importance to have it as fine as possible, without including any dust, that the stipulations in the specification with regard to this material should be made very clear and definite, so that the contractor may make sufficient provision in his prices to enable him to comply with the specification in its strictest sense.
In addition to the bottom layer of floor-tiles and coarse material prescribed in the section relating to sub-drainage, it will be necessary to provide an additional layer above this, about 3 inches in depth, of a medium grade, to support the very fine material and prevent it being washed through the interstices in the coarse bottom layer, and special attention must be devoted to the method of distribution at the surface in order to avoid disturbance of the fine material.
Fig. 140.—Automatic Apparatus for Contact Beds.
Automatic Apparatus for Contact Beds.—Although contact beds can be operated by hand, this involves continuous and regular supervision by a man, and, unless he is under strict control, the proper cycle of operation may not be adhered to, and the result will, in that case, not be satisfactory. In the early days of contact beds, several types of apparatus were designed for the purpose of operating these beds automatically, so that the cost of manual labour would be reduced to a great extent, and in addition, the possibility of mismanagement avoided. Among these appliances, one of the most widely known is that manufactured by Messrs. Adams Hydraulics, Limited; and shown in [Fig. 140]. In small installations, the low draught syphons described above in connection with dosing apparatus are used to give alternate fillings to pairs of contact beds. Where more than two beds are in use the automatic air-lock feed is used. This consists of a cast-iron inverted U-pipe, both legs of which are trapped, either in self-contained castings or in separate chambers. The sewage flows through one, the others being charged with air. When the sewage in the bed has reached the proper height it overflows into a small chamber in which a dome is fixed. This dome is connected by means of an air-pipe to the cast-iron inlet feed, and as the sewage rises in the chamber round the dome, the air contained therein is forced up into the feed which it fills, thus forming an air-lock, which prevents the sewage from passing through and stops the supply to the bed. At the same time that the feed is stopped by the transfer of air in the manner described, the compression of the air in another small dome in the same pit has forced a water seal on the air-pipe leading from this dome to the feed in the next bed, and thus liberates the air-lock in the feed of that bed and allows the sewage to flow to this in rotation. The whole operation is then repeated with each bed in rotation, the last bed in the series, when full, starting the feed in the first bed again. These same feeds may be arranged to hold up sewage in a collecting or dosing tank, so as to ensure the accumulation of sufficient sewage to fill each bed at one dose within a reasonable time.