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1 concerning the high cost of treatment by activated sludge. This might mislead the public and the authorities concerned.51. The figures given in Table 7 worked out at about W E 5 per million gallons, or only Id. per thousand gallons, which was really extraordinarily cheap. He wondered what industry could show comparable economy in treating 5 tons of polluted liquid for a penny. Per head of population, this worked out at a cost of only one-twentieth of a penny per day. He realized that the Author had not included capital costs, the addition of which would probably double his figures, but even so they would still be quite insignificant. Having said that, however, it was equally true that total costs could run into many thousands of pounds, and they should always be striving still further to increase efficiency wherever possible and to reduce costs in the way described.52. With regard to the depth of tanks, a great deal of work had been done on this in the early days of 30-40 years ago. In 1914 the original Manchester plant was about 4 ft deep, but for compressed air, deeper tanks were more economical and occupied far less area; this had always been a most important consideration. Manchester's final decision was 15 ft, and this depth had also been used in nearly all American plants. The limit was reached at Reading in the middle 1920s at 23 ft. but this proved to be a big failure and the pendulum quickly swung back to 10 ft at Birmingham for Bio No. 2 plant in 1928, and again for their Coleshill plant in about 1933. For Mogden, the choice was 12 ft, which had proved very successful.53. In those days there had been much controversy as to how much oxygen was absorbed from the bubble in rising to the surface and how much was taken in at the surface by the breaking of the bubble at that point. Recent work by the Water Pollution Research Board had confirmed the earlier view that in a compressed-air plant very little was taken in at the surface, and that in this case the relation of surface area to depth or tank volume was not so important. With surface-aeration plants, on the other hand, such as the Simplex, the Kessener brush, and the Sheffield system, where a predominating proportion of oxygen was taken in at the surface, the effect of the surface area in relation to volume should be far more pronounced, and the advantage of shallower tanks, as shown by recent Manchester work, had proved to be considerable. On the other hand, shallower tanks were more expensive in relation to volume and lost the advantage of economy in space which was always so important in modem plants.54. So far as capital costs were concerned, it had always been possible to operate a compressed-air plant vigorously with more power and less capacity, as advocated by Lockett right from the start, or to economize in power with a longer detention period. For example, at Mogden, where ample cheap power was available from methane gas, a fully nitrified effluent had been obtained in 8 hours, whereas at Coleshill, Birmingham, power had been reduced by 30%,...

wrote that now the Authors had shown that the extent of slime has a significant effect on the flow-carrying capacity of sewers, it seemed necessary to differentiate between the types of sewer with which the designer had to deal. Presumably the relative amount of slime to be allowed for should be greater for foul sewers on the separate system than for combined sewers. Perhaps slime could be neglected in designing surface-water sewers, though if velocities were low, some allowance should be made for silting. Could storm-water overflow pipes be treated in the same way as surfacewater sewers or was slime likely to form in them?44. If there was likely to be a significant difference between foul and combined sewers, it would be useful if the Authors could give the type of each sewer which was tested. Since 'foul' was a somewhat relative term, though, the best course might be to give the depth corresponding to dry-weather flow in each case.45. Most of the tests reported were carried out at low relative depths of flow. In the cases where the test flow was of the same order as dry-weather flow, was it likely that the pipes would have lower equivalent values of k, when flowing nearly full?46. Further investigations would seem to be desirable to find to what extent the incidence of sliming differed in foul, combined, surface-water, and storm-water overflow sewers and also in rising mains and inverted syphons. Even visual inspection of a number of pipes in each category would be of value.Mr L. B. Escritt (Assistant Senior Engineer, London County Council) wrote that the factors which influenced turbulent flow in pipes were too complex to be expressed accurately and adequately by any existing formula that satisfied theorists. They included local changes of diameter and gradient, joints and excrescences in addition to the 'roughness' usually mentioned in the literature. However, hydraulic phenomena, when plotted on logarithmic ordinates and abscissae, almost invariably gave straight lines and this was true of the flow of water in pipes. Hydraulic radius, hydraulic gradient and velocity were accurately related by the empiric formula: V = Km"P where V=velocity in ft/s, m= hydraulic radius in ft, i=fall divided by length, and K = a constant.48.