Sewage Disposal Works: Their Design and Construction

by William Charles Easdale

Methods of Distribution.—Much diversity of opinion exists with reference to the question of distribution in filling contact beds. The many methods which have been tried in various places, may be arranged under four headings, viz. (a) above the surface of the material; (b) at the surface level; (c) just below the surface; (d) at the bottom of the bed. Some engineers hold the opinion that distribution over a large area of the material has no value, and that it matters little how the bed is filled so long as the liquid finds its way into the interstices of the material with as little disturbance as possible. The simplest method which fulfils these requirements is to allow the sewage to flow over the surface at the inlet end of the bed, but this soon causes the surface at this point to become clogged, and unless it is cleaned at frequent intervals the solids very quickly become washed down into the bed, and in the end this portion of the material will have to be removed and washed before it can be used again. The efforts to avoid this trouble have resulted in the numerous methods of distribution referred, to above.

Taking them in the order given, the idea of discharging the sewage above the surface level (a) by means of elevated troughs or pipes, has been to imitate to some extent the method found necessary in the case of percolating filters, and thus aerate the sewage before it enters the bed. The difficulties which arise in this case are that some extra fall is needed, and the provision of the troughs or pipes with suitable supports is costly. It is also extremely difficult to arrange the distribution by these means so that it shall be uniform over the whole area, and unless this is done it cannot be of much advantage.

Distribution at the surface level (b) may be provided by means of shallow grips in the material itself, and these have the great advantage that if they become clogged to such an extent as to prevent the sewage from freely passing into the bed, it is a small matter for the manager to cut a fresh grip in another direction and leave the first one to dry up when the sludge in it can be easily removed by hand. Another method is to use rows of stoneware channels, or wooden or iron troughs, with their edges set level with the surface of the material, so that the sewage may flow over the edges or through holes or notches in the sides. This is usually satisfactory, but it is not an easy matter to maintain all the channels at the same level, and after they have been in operation for a time it will be found that the material immediately under the troughs or channels is badly clogged, and can only be cleaned or renewed by removing the channels.

Sub-surface distribution (c) is arranged by means of perforated or open-jointed pipes, laid below the surface of the material and thus out of sight. The reasons for adopting this method are: that it avoids the unsightliness caused by surface distribution; that the surface is kept free from obstructions, and thus allows free aeration when the bed is emptied; and last, but not least, it prevents any nuisance arising from the evolution of obnoxious gases in the tank effluent whenever it is over-septicised, a not infrequent occurrence in the case of old-fashioned schemes, or in new works where the volume of sewage for which the tanks were designed has not yet reached its maximum. This method has the disadvantage that when the openings in and between the pipes become choked, more labour is involved in cleaning them than in the case of open channels or troughs on the surface.

Filling from the bottom (d) is assumed to possess all the advantages and none of the disadvantages caused by the other methods. The distribution is certainly uniform, as the liquid first fills the sub-drains and then rises at the same level throughout the whole of the material, forcing out any carbonic acid gas that may have accumulated in the lower part of the bed. As the sewage does not appear on the surface at all, there is no unsightliness and no trouble from bad odours. On the other hand, it is evident that the solids in suspension in the sewage or tank effluent are retained at the bottom of the bed, especially in the under-drains, and thus they will appear in large quantities in the effluent. Unless some special provision is made, by means of an effluent settling tank or sand-filter, to arrest these solids in suspension in the final effluent, they will be liable to cause trouble in the stream, and will, in any case, seriously affect the results of any analyses that may be made. The usual manner of arranging this method of filling, is to cause the sewage to flow into an open or covered chamber at the inlet to the bed, the walls of the chamber being provided at the floor level with openings connected to the sub-drains laid on the floor of the bed.

Whatever method of distribution is adopted, it is desirable that the surface of the filter material shall be not less than 3 inches above the highest level to which the sewage will rise, so that the liquid may not be visible at the surface.

Sub-Drainage.—Reference has already been made to the fact that lack of ample under-drains has often been the cause of the failure of contact beds in the past. The general practice for a long time consisted in placing a layer of coarse material on the floor of the bed, and providing a few rows of ordinary agricultural drain-pipes laid with open joints. In some few cases special perforated pipes were used, in others the pipes were partly embedded in the concrete floor. In the opinion of the author, however, no drains in the form of pipes are satisfactory, as they do not leave a space at the floor level as a free exit for the solids in the effluent. Where pipes are used it will generally be found after a few months’ operation that these solids, in the form of black sludge, have accumulated along the sides of the pipes and among the material at the floor level, and when this once commences the accumulation continues to take place, rising gradually in the bed until the interstices are choked to such an extent that the liquid capacity of the bed is reduced to a fraction only of its original volume. The trouble was intensified by the comparatively small number of the pipe drains usually found in the beds. It was evidently assumed that the matters would travel laterally through the layer of coarse material at the floor level. Unfortunately an additional impediment to the free flow of these matters was caused in many cases by the want of sufficient fall on the floor itself. Very little consideration will show that a large area of floor requires a considerable slope in order to produce the velocity of discharge necessary to remove matters in suspension, yet it was seldom that a gradient of more than 1 in 200 was provided, and in a few cases the surface of the floor was absolutely flat. Under these conditions it is difficult to see how any other result could be expected. It may be argued that it was not properly understood in those days that the solids in suspension (converted organic matters, the products of oxidation) must be removed if the filtering material is to retain its working capacity, but this fact has long been recognised in connection with percolating filters, which have in most cases been constructed upon complete false floors, provided with perforations, and with a suitable slope on the surface of the actual floor.

There is very little doubt that the question of providing ample means of sub-drainage deserves special consideration; and, in the author’s opinion, the floors of all contact beds should be laid with much greater fall to the outlet than in the past, and they should, in addition, be covered entirely with a false floor of special floor-tiles of the kind described in connection with percolating filters ([pages 91 to 94]). If the usual bottom layer of coarse material is then laid upon the false floor, it will be found that the beds will maintain their normal working capacity for a much greater length of time than in beds constructed on the old style. Instead of arranging the slope on the floor from the inlet end to the outlet end of the bed, it is preferable to construct an effluent channel with a suitable fall down the centre of the bed to the outlet, and arrange the floor with a cross-fall from the sides to the centre channel, which may be covered by slabs of concrete or stone laid upon the top of the floor tiles where they abut upon the edges of the channel as suggested, [Fig. 139a].