Fig. 136.
There is still another matter which has a considerable influence upon the general design of a scheme of contact beds, viz. the slope of the ground upon which they are to be constructed. If it has a fairly rapid and even slope, the tanks and beds may be arranged close together, as shown in [Fig. 136]. The only part that needs special care in this case is the cross-wall between the primary and secondary beds, which will need strengthening, especially in its lower half, in order to resist the extra pressure it is required to take.
Fig. 137.
Where the slope is not so great, a saving in the cost of excavation may be effected by arranging the separate tiers of beds at some distance apart, as indicated in [Fig. 137], and connecting one with the other by pipes. The aim to be attained in arranging the beds under these conditions is to have the entire area of the floors on solid soil, with the walls half in and half out of the ground.
Another set of conditions occasionally met with, is where the site of the works is perfectly flat and the position of the outlet for the final effluent involves the construction of the secondary beds either wholly or partly below the surface. In such cases the primary beds will come above ground, and it will then be found economical to arrange each set of beds in two rows end to end, with a central combined supply channel and effluent carrier, the latter being formed in the space between the walls which support the former, as shown, [Fig. 138]. If there is not sufficient head to allow of the supply channel being made deep enough to serve as the dosing tank, the latter may be constructed across the ends of the settling tanks, as suggested in [Fig. 139], or in any other convenient position. A dosing tank in this position lends itself to the method of feeding the beds by means of closed pipes instead of by open channels, whether in sets of four, with a central chamber for the inlets and outlets illustrated in [Fig. 139], or in series as [Figs. 136] and [137].
