Stresses and deformations in egg-shaped sludge digestors: membrane effects

Relatively recently, egg-shaped containment shells have been adopted as slu dge digestor tanks at sewage treatment installations, in place of the conve ntional squat cylindrical tank with a dome-shaped roof closure. The advanta ges are superior mixing efficiency resulting in less accumulation of deposi ts at the bottom, easier removal of the deposits that do settle at the bott om and of the crust that forms at the surface of the sludge, and reduced he at losses. In the long term, the reduced maintenance and operational costs offset the higher initial costs of construction, making egg-shaped digestor shells a more attractive option than conventional cylindrical tanks. Despi te many egg-shaped sludge digestors having been built in various countries worldwide, their widespread adoption is hampered by the lack of detailed an d systematic information on their structural analysis and design. The prese nt study attempts to reduce this gap in the literature by (i) developing a simple, effective and systematic structural-analysis procedure for e.g.-sha ped sludge digestor tanks of a certain practical combination of shell geome tries, (ii) presenting usable closed-form analytical results for stresses a nd deformations throughout the digestor, and (iii) indicating design implic ations for the information of the structural engineer. ln this paper, the f irst of two, membrane effects are evaluated, and some useful results are pr esented and illustrated through an example. The follow-up paper ("Stresses and deformations in egg-shaped sludge digestors: discontinuity effects" J E ng Struct 23 (2001) 1373) will evaluate the discontinuity effects at the ju nctions of the shell components making up the digestor, present detailed an alytical and numerical results, and make design recommendations. (C) 2001 E lsevier Science Ltd. All rights reserved.