Discussion. A Comprehensive Scheme for Sewage Sludge Disposal in North--West England.

Abstract

I should like to compliment the Authors on covering the various aspects of a topic which poses an increasing number of problems. It strikes me as a pity that, in this case, the most economic solution proved to be final sludge disposal at sea, rather than further use on the land, unless the value as a fertilizer is not very great.65. This project provides a good illustration of the desirability of carrying out experimental work before embarking on a complex design. I should like to give a few more details concerning the operation of the viscometer developed by the British Hydromechanics Research Association (BHRA) specially for this job. MC A. Linford was responsible for the viscometer design. The most notable feature was that the three tubes were arranged horizontally, rather than vertically, as for most normal slurry viscometers. This resulted from consideration of the space available on site, as the relatively large diameter tubes necessary for use on sludge meant additional length for a given pressure drop. The three tube sizes were intended to cover the range of expected viscosities. It was not expected that settlement of solids would occur with this type of sludge, even in laminar flow, in a horizontal tube.66. Proving of this device was carried out at the BHRA on a chalk slurry with a similar range of viscosity as the actual sludge. Operation is relatively simple, and viscosity may be calculated from measurement of shear stress (PDI4L) and shear rate (8 V / D ) for laminar flow.67. Departure from ideal Bingham plastic behaviour at low shear rates may be explained by the central core or plug of fluid starting to move before that adjacent to the pipe wall.68. The Paper mentions marked differences in viscosities of sludges having similar solids concentrations. Was there any marked difference in the particle size distribution? Did the reported gas content of the sludge have any adverse effect on the accuracy of pressure measurements in the full-scale trial? 69. I was interested to note the reasons given for the choice of positive displacement primary pumps rather than centrifugal pumps. With the general trend to larger single pumping units, would the Authors envisage centrifugal pumps being considered for the primary pumping duty if the abrasive wear and screening problems could be overcome? Presumably these problems also exist to some extent with positive displacement pumps. Abrasive wear in both pumps and pipelines is currently being investigated at the BHRA. M r W. T. Greer, Depute City Engineer, GlasgowThe disposal of sewage sludge can be a considerable undertaking and the Paper deals with an alternative solution to that put forward by Dainty et a1.l 71. Without doubt, the disposal of sewage sludge to sea is a very attractive proposition to those authorities able to obtain access to shipping facilities. Sludge dumping has been going on for a long time and within my experience no cause for alarm has been substantiated.