Those who advocate intermittent supply under all circumstances would appear to have lost sight of the fact that very ample periods of rest are already provided by all revolving distributors. When working at the rate of one revolution per minute, the average time taken by each arm to pass over any one point on the surface of the filter may be taken as 0·5 second. Before the succeeding arm reaches the same point a period of 15 seconds will have elapsed, so that under these conditions the ratio of the periods of work to periods of rest is as 1 to 30. In other words, even during continuous distribution, the time allowed for rest and aeration is thirty times as much as that during which the sewage is actually being delivered to the filter, and there would thus appear to be no reason for unnecessarily adding to the periods of rest by means of dosing tanks, especially as such a course involves a greatly increased rate of distribution at all times when the discharge actually takes place.
There is, of course, the possibility of adopting the happy medium, which would consist of a dosing apparatus of such a type that it would provide a continuous supply during the maximum flow of the sewage, and only act as an intermitting appliance during the minimum rate of flow. This is probably the best arrangement to adopt in all cases where a dosing apparatus is absolutely essential.
As previously stated, however, there are many cases where the conditions render the use of a dosing tank absolutely necessary to prevent the stopping of the distributor, and these have brought about the introduction of several types of automatic syphons and valves, all arranged specially for the purpose of giving intermittent discharges to filters, contact beds, areas of lands, and so forth. There are various methods of constructing dosing tanks, all dependent upon the arrangement of the preliminary processes and the filters which follow them. In nearly all cases, however, it is found necessary to reduce the fall taken up by the dosing tank to the minimum, and the different syphons and valves have thus been designed to work with the least possible head. One point in the construction of dosing tanks should not be overlooked, i.e. the provision of a washout valve for use when it becomes necessary to clean out the tank. It may be noticed here that neither the Fiddian type of distributor, on the water-wheel principle, nor the power-driven distributors, require any dosing tank to keep them in motion during the periods of minimum flow of the sewage. In the case of the Fiddian distributor the apparatus remains standing until the buckets are filled, and each time these discharge their contents the distributor is moved forward a short distance. Under these conditions, however, the disadvantage of long intervals of rest between comparatively large discharges of sewage still remains. Undoubtedly the most even rate of distribution under all conditions is secured by the use of power-driven distributors, whether of the rotary or travelling type, but these can only be economically adapted for use in large installations or where power is available at a very low cost.
Fig. 121a.
| T = tank. | C = carrier. |
| P = supply pipe. | F = filter. |
| E = effluent channel. | H = humus tank. |
