CHAPTER XXVII.

Comparative Cost of Treatment by Land and by
Artificial Processes.

66

In explanation of these particulars and Tables, which have both been specially prepared by the writer, the following remarks are given:-The cost is the annual cost per head of the population draining to the works, based on a daily flow of 33 gallons per head.

As regards the artificial methods of treatment, the same five preliminary processes are employed prior to the effluent being dealt with, on percolating filters in the one case, and contact beds in the other. These five are:

(a) Chemical precipitation, quiescent settlement;
(b) Ditto precipitation, continuous flow settlement;
(c) Continuous flow settlement without chemicals;
(d) Quiescent settlement;

(e) Septic tank.

With land treatment, land possessing "all kinds of good soil and subsoil, e.g., sandy loam overlying gravel and sand, was, at different sewage farms, employed in three different ways:-(f) filtration with cropping; (g) filtration with little cropping; (h) surface irrigation. Both (i) heavy soil overlying clay subsoil, and (j) stiff clayey soil overlying dense clay, were used for surface irrigation with cropping.

A TEXTBOOK ON SEWAGE DISPOSAL ΙΟΙ

The cost of land treatment is the nett cost obtained, by deducting the returns from sale of crops, from the gross cost of treatment.

The figures in the Report, dealing with comparisons in cost of treatment, have, to some extent, been based on assumptions. The relative cost of different methods would, in practice, largely depend on local circumstances. "The differences in cost are, however, small." Local considerations must always play an important part in the selection of a method of sewage treatment: these circumstances will also control the degree of purification to be aimed at. One hundred pounds per acre should be the maximum price for a site for land treatment to which the sewage can gravitate. Where, however, "It was necessary to obtain a high-class effluent, it might be cheaper to pay a somewhat higher price for good land, rather than adopt artificial treatment, which effluents are generally distinctly inferior to those obtained by the treatment of sewage on land.”

in

Explanation of Tables XI. and XII.

These figures are those of the Fifth Royal Commission; they are merely approximate, and are based on a dry-weather flow of one million gallons of sewage.

As regards contact beds, it follows that, where they are built by simple excavation and banking, the cost would be comparatively low. For the purpose of these tables the cost of double contact beds per acre is worked out at £5,700, including concrete, rendering, drains, distributing channels, engineering, and contingencies, etc. The beds are supplied with filtering material at a cost of six shillings per cubic yard, to a depth of three feet, and it is placed in position in the beds. At this rate it works out at about £1,396 per

acre.

On the basis on which the figures have been worked out, it would appear that the purification of sewage effluents from the tanks can be effected by percolating filters at a cost of two-thirds that of purification by double contact beds

CHAPTER XXVIII.

Discharge into Estuaries and the Sea.

Most towns and smaller communities bordering on estuaries and the seaside are peculiarly well situated for efficiently disposing of their sewage. There are some exceptions to this,

as we shall notice later.

Choice of Outfall.

The chief item to consider in this connection is the quesof sea currents, which may be favourable or very unfavourable, and may either cause the return of the sewage to the shores from which it is discharged, or may cause it to be cast on those of the neighbourhood.

In either of these cases, the crude sewage must be treated by one of the methods previously described; and in cases where there is a resulting effluent, steps must be taken to ensure that similar precautions are taken in its discharge as indicated above.

The following is the most useful method of ascertaining a suitable spot for the outfall. As remarked above, returning tides and sweeping currents have to be taken into consideration.

Floats," placed in the sea from the shore at the most likely spots, are used for the purpose, by means of which the action of the winds, tides, and currents may be carefully observed, with a view to discharging the sewage into the sea, at a particular time after the turn of the tide, so that the outgoing water may carry it out to sea with great velocity. The exercise of care, coupled with experience, is essential for this, because the proper choice of outfall is the chief element of satisfactory disposal of sewage. The floats are used in conjunction with specially-prepared charts. These floats, when placed in the sea, have to be carefully followed and watched, and the directions taken by them duly noted on the charts. That position on shore at which the floats were placed in the water, and from which they were carried out to sea in the most desirable direction, is the spot that should be fixed upon for the outfall. Part of these experiments should also be directed to ascertain the

most suitable time at which the discharge of the sewage should take place after the turn of the tide.

To await the hour of discharge, the crude water-borne sewage matter has to be collected in tanks, their size varying with the quantity to be stored therein. These tanks are used for storage purposes only, for the reception and retention of the sewage as it enters them from the sewers of the district served, between one period of discharge and the next. It may be necessary, where the physical features of the district draining to the outfall require it, to "lift" the sewage by means of pumping machinery, actuated by some sort of power, into the tanks to enable its proper discharge from the outfall into the sea or other water.

Typical Outfall into the Sea (Portsmouth).

Owing to the fact that practically the whole of Portsmouth lies very low, the sewage has to be pumped at the outfall. Machinery is used to do this. In ordinary weather the sewage is lifted by pumping machinery actuated by steam; and when the weather is stormy, this is supplemented by the use of large gas engines. To effect the discharge into the sea, the sewage has to be lifted some thirty feet. Its discharge takes place one hour after high water, and one and a-half hours are allowed for the purpose.

The sewage has to be collected between the periods of discharge in tanks adjacent to the outfall. One, built of concrete and brick, has a capacity of four and a-half million gallons; another, built in ferro-concrete in excavation, has a capacity of six and three-quarters million gallons. The Borough Surveyor informs the writer that no nuisance is caused to the Portsmouth or adjacent shores; and that no growths are caused at the outfall or in the sea. It is necessary, perhaps, to add that at the hour of discharge the sewage is released from each tank by the use of penstock. (Fig. 47, p. 192.)

The Protection of Edible Shell Fish Layings.

Sometimes the cultivation of edible shell fish may constitute a staple local industry, or that of some adjacent community, and, therefore, beds of oysters, mussels, cockles, etc., may be laid down on the shores. Apart from the fact that the discharge of crude or imperfectly purified