In connection with Sewage Disposal Works, the term “storm-water” is generally understood to mean the extra volume which reaches the works in times of rainfall, in excess of three times, up to and including six times, the average dry-weather flow; so that the volume of storm-water for which provision should be made is equal to three times (volumes) the daily dry-weather flow. Prior to the publication of the fifth report of the Royal Commission, it was usual to provide a rough straining filter for the storm-water, or to reserve a portion of the land for the purpose of dealing with it by broad irrigation. In either case the area of filter surface or land required was 1 superficial yard for every 500 gallons of storm-water (^{D.W.F. × 3}/₅₀₀ = super-yards). As the result of their investigations, the Royal Commission came to the conclusion that “storm-water” filters, as generally constructed under these conditions, were useless for the purpose for which they were required, and this confirmed the views of most engineers of experience. Where suitable land can be secured for the purpose, and arranged in such a manner that it is reserved solely and entirely for treating the storm-water, this method may still be adopted. If this is not possible, stand-by tanks may be constructed for the purpose of receiving the storm-water. These tanks are to be not less than two in number, and should have a total capacity of not less than one-quarter of the average daily dry-weather flow. The only overflow at the outfall works from which storm-water may be discharged direct to the stream, or other final effluent outlet, must be from these stand-by tanks, and it should not come into operation until these tanks are full. Having regard to these recommendations, it is necessary in every scheme to provide at least two special storm-water stand-by tanks, with a total capacity of ¼ D.W.F.; and the drawing, Fig. 147, illustrates a simple method of constructing these. In this case the inlets are in the form of weirs, running the full width of the tank, so that if the channel leading to these tanks is in communication with, and at the same level as, the inlets to the detritus tanks, and the latter are provided with slotted doors (see Fig. 10, page 19), the weir at the inlets to the stand-by tanks will act as the actual storm-overflow, and, being of considerable length, the maximum height to which the water will rise in passing over this weir will be very small, and will thus have very little effect upon the rate of flow to the sedimentation tanks.

As the only overflow discharging direct to the stream must be from these tanks, and must only come into operation when they are full, the outlet is also constructed in the form of a weir discharging into a channel from which a pipe would be laid to the stream. A further requirement in connection with these tanks is that they should always be kept empty, ready to receive the excess of storm-water at any time. From this it is evident, that these tanks must be emptied after each heavy shower or storm which increases the rate of flow of sewage to the works beyond three times the dry-weather flow. Unfortunately, no directions are given as to the manner in which this is to be accomplished. In the absence of any definite statement to the contrary, it might be inferred that, after the overflow from these tanks has ceased, their contents may also be discharged direct to the stream. As, however, this would necessitate outlets at or near the bottom of the tanks, there would appear to be a possibility of the suspended matters deposited in the tanks being discharged to the stream—the very thing the tanks are designed to prevent. With such an arrangement, also, there will be a risk of the man in charge of the works, either wilfully or by an oversight, leaving the outlets at the bottom of the tanks open, and thus permitting the storm-water to pass direct to the stream without the settlement which it is anticipated by the Royal Commission (Fifth Report, page 233, par. 352) will be provided for all storm-water arriving at the works.

It is obvious that these tanks must be emptied after every heavy shower or storm, and that facilities must be provided both for drawing off the supernatant water and for removing the deposit which will accumulate at the bottom. In the author’s opinion, the only safe method is to provide floating arm outlets for the supernatant water, and to discharge this to land or to a special filter for further treatment, or, better still, to pump it up to the detritus tanks to be treated again with the ordinary sewage. In schemes where the whole of the sewage is pumped at the works, the contents of these storm-water stand-by tanks should certainly be discharged into the pump-well, as this would not involve the provision of special pumping plant. With regard to the sludge from these tanks, this should be drawn off by means of special outlets, and dealt with on sludge draining beds in the manner previously described (page 83).

The difficulties which frequently arise in designing suitable and convenient methods of dealing with storm-water, render it desirable that very careful consideration should be given to the question as to whether it would not be more satisfactory, from the point of view of both economy and efficiency, to omit the stand-by tanks, and increase the capacity of the filters required to deal with the ordinary sewage, to such an extent that they will be capable of dealing with the whole volume of sewage and storm-water combined up to six times the dry-weather flow, and thus obviate the necessity for any storm-overflow at all at the outfall works. If this idea were universally adopted, it would necessitate greater care in the construction of any storm-overflow required on the line of the outfall sewer itself before it reaches the works, but there are (or should be if the sewers were properly constructed) so few cases where the excess of flow, even during the heaviest rainfall, ever reaches six times the dry-weather flow, that the extra cost involved cannot be considered excessive if the greater certainty of securing satisfactory results at all times is taken into consideration.