Fig. 138.
There are doubtless other alternatives, or combinations of methods, which may be adapted to meet the exigencies of peculiar conditions of site and fall, but the foregoing details will probably suffice to suggest ideas to those in need of them in designing schemes of contact beds.
General Construction.—The most important point to be borne in mind in constructing contact beds is, that they must be absolutely watertight. Should they leak in any way, the sewage may pass away untreated; or it may find its way into adjacent beds, and thus prevent these from being properly aerated during the periods of rest when empty. It is therefore evident that they should be constructed in a substantial manner. The floors are usually of concrete, and the thickness of the floor will depend upon the nature of the subsoil. If for any reason the floors have to be laid upon made-up ground, provision should be made by means of piers or cross-walls, carried down to the solid subsoil to support the floor, independent of the made-up ground which is, in all cases, absolutely untrustworthy. The walls of the beds may be constructed either of concrete throughout or of brickwork in cement, and they should be of such a thickness that they will withstand the pressure of the head of water which would result if the beds were filled to the top of the walls.
It will not be found satisfactory to place reliance either upon brickwork or upon concrete alone to form a watertight bed, and in both cases the whole of the floors, as well as the walls, should be rendered with cement mortar in the proportion of 2 parts of sharp clean sand to 1 part of Portland cement. No rendering should be done during frosty weather or during excessive heat, as in both cases it will usually be found defective, and it is better to stop the work altogether for a time than have to patch it up afterwards.
Fig. 139.
One safeguard which can be adopted to prevent difficulties later on, is to insist upon testing all such beds with water to the full height before any of the filtering material is placed in position. Any slight defects which may appear can be made good then at very little expense. If the defects are not discovered until after the beds are filled with the filtering material, they can only be properly rectified by removing the material, and this involves a considerable outlay. In order that these tests may be carried out without friction, it is necessary to stipulate clearly in the specification for the work that each bed is to be tested with water to the full depth before any material is placed in the bed, and that the contractor must take full responsibility for making the beds absolutely watertight. It is, of course, understood that the method of construction adopted by the engineer is such that, if properly executed, the beds will be absolutely watertight; and, in order to prevent any misunderstanding, an item should be included in the quantities for the contractor to provide whatever sum he may consider necessary to allow for making these tests. It may be thought sufficient to state simply that the contractor should make all absolutely watertight, and to leave it to him to provide the means for doing so. It will, however, be found more satisfactory to all concerned to provide all means both in specification and in the quantities for attaining the desired results. It is not sufficient even to use the word “watertight” alone in this connection, as the interpretation of this word may be subject to differences of opinion which are obviated by the addition of the word “absolutely.”
The foregoing observations refer not only to contact beds but to the tanks and other portions of the work which are required to hold water.