Fig. 147.—Storm-Water Stand-by Tanks.
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.
MEASURING APPARATUS.
In recent years the provision of proper means of measuring and recording the flow of sewage at disposal works is becoming more general, but there are still a very large number of works at which it is impossible to obtain any trustworthy information as to the volume of sewage treated. As long as all works smoothly, and there is no trouble with the effluent, it is considered superfluous to trouble about the quantity of the liquid which passes through. When, however, difficulties arise, and it becomes necessary to investigate the cause of the trouble, it is of the utmost importance to be in a position to ascertain the daily volume of the sewage arriving at the works and the variations in the rate of flow, as well as the quantities dealt with by each separate tank and filter. It is also of great assistance, in making investigations at such times, to have a definite record of the volumes treated day by day during the preceding six months; indeed the possession of a complete record of the daily flow of sewage over the whole period during which a sewage works has been in operation, is a valuable asset not only to those responsible for the works themselves but also to the authorities who control the streams and watercourses, and to investigators in search of information to be used for the public benefit. The initial cost of suitable measuring and recording appliances is not excessive, but, when once the works are completed and in good working order, the local surveyor or manager has great difficulty, and usually finds it impossible, to persuade his committee that the outlay is justified. It is, therefore, desirable that engineers should in all cases make provision for such apparatus in the contract for the construction of the works, and thus make sure that it will be available when required.