Discussion. The Use of Radioactive Isotopes for the Investigation of Sewage Treatment Plant.

Abstract

DiscussionThe Chairman (Sir George McNaughton, Vice-President) said that the Paper described a system which not only provided the sewerage-works engineer with a convenient method of checking the hydraulics of the system in a full-scale manner rather than on models, but also had many other advantages.48. The introduction of such a small amount of foreign matter into the disposal works was of great importance, because the functioning of the works was not affected in the least.49. In the past, Continental engineers in particular had been inclined to challenge their British colleagues for making sedimentation tanks too big, and the Ministry requirements in the matter had been suspect by some for many years. The engineers had justified them on the plea that the tanks had to last for a long time and that it was necessary to allow for increases in population and a host of other variables which could not be definitely settled beforehand. At the same time, some engineers had thought that there might be short-circuiting in the tanks, but they had imagined that it would be in the corners where the liquid would be quiescent; it had never occurred to them that the short-circuiting would be in the vertical plane. That was one of the most important results of the Authors' work.Mr J. Rawlinson (Chief Engineer, London County Council) said that the results of sedimentation at sewage disposal works varied within wide limits, and since the removal of the solids was a very important factor in the efficient purification of sewage, the design of the tank itself was all-important. The capital cost of providing sedimentation tanks might be as much as 25% of the cost of the works, including the removal of detritus, sedimentation, secondary treatment, and final clarification, but excluding the cost of any pumping which might be necessary and the cost of sludge digestion or gas production. Inefficient tanks were an unnecessary financial burden for local authorities to bear.51. The precise behaviour of sewage flowing through a tank had in the past been largely unknown. Mr Rawlinson did not think that many engineers had appreciated the actual path which the flow took when passing through a rectangular tank after entering it over a high weir, at or near the top water level. The total dry-weather flow at the Northern and Southern Outfall Works was about 30m.g.d., so that even a small increase in the efficiency of an important section of the works was of the greatest importance.52. The experiments with isotopes had commenced about 16 months before, and the results so far obtained indicated that the tanks were performing very satisfactorily and that the level of the weir could not be fixed at random without some loss of *