Fig. 121b.
| T = tank. | C = carrier. |
| P = main supply pipe. | P1 = branch supply pipe. |
| V = regulating valve. | W = washout valve. |
| F = filter. | E = effluent channel. |
| H = humus tank. | |
Apart from the question of continuous versus intermittent methods of feeding percolating filters, there still remains the problem of deciding whether the actual connections between the tanks and the filters shall be in the form of channels (carriers) or pipes. For fixed sprays and revolving distributors, which involve the use of pressure due to a column of liquid, the actual connection to the apparatus must be in the form of a pipe extending at any rate to the outside of the filter. Whether it should be continued beyond this point in pipe form or in a channel depends entirely upon local conditions, such as the slope of the ground, the planning of the filters, and the relative cost of the two methods. There is one thing to be said in favour of pipes, i.e. that there is less chance of a nuisance being created by the evolution of foul gases from the sewage than in the case of open channels. On the other hand, channels may be, and frequently are, covered in. Whichever method is adopted, care should be taken to provide a valve or penstock on the connection to each separate filter, in order to be in a position not only to throw any filter out of work when desired, but also to regulate the rate of supply to each filter independently of the others. Washout valves should also be provided on the channels or pipes, in suitable positions. Where the filters are arranged in groups, as suggested in [Figs. 75] and [77], the intervening spaces may be used to accommodate a simple receiving tank, or a dosing tank, to which the sewage or tank effluent may be conveyed by an elevated channel or by a pipe under pressure. The connections to the adjacent filters may then be provided in the form of pipes leading from this central tank direct to the distributor at the centre of each filter. If the receiving tank or dosing tank in such cases is built upon piers, the floor of the tank will come above the level of the surface of the filters, and the space below may be utilised for an effluent receiving chamber or even for a settling tank to arrest the solids in suspension in the effluent. There are many ways in which the supply to filters may be arranged, and the preceding notes are merely intended as suggestions, which may be elaborated as found desirable to suit conditions which vary almost in every case.
Fig. 121c.
| T = tank. | D = dosing tank. |
| P = supply pipe. | F = filter. |
| E = effluent channel. | H = humus tank. |
[Figs. 121a] and [121b] have been prepared to show conditions under which channels and pipes respectively would be suitable for supplying the tank effluent to the filters. [Fig. 121c] is the plan of a works where a dosing tank is used to deliver the tank effluent intermittently through separate supply pipes to each of the filters. If the valves on the supply pipes are properly adjusted, each filter will receive an equal proportion of each discharge from the dosing tank. An alternative method is to provide an apparatus by means of which each separate discharge from the dosing tank is delivered in rotation to each of the filters.
In cases where the appearance of the works must be taken into consideration, the method of arranging the tanks and filters suggested in [Fig. 121d] may be adopted. By this means the space covered by the works is utilised to the utmost, and the tanks are of the form by which uniformity of distribution and the maximum reduction of velocity is secured. On the other hand, it is probable that the slope of the site will only rarely suit this arrangement.