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

Relatively recently, egg-shaped containment shells have been adopted as sludge digestor tanks at sewage treatment installations, in place of the conventional squat cylindrical tank with a dome-shaped roof closure. The advantages are superior mixing efficiency resulting in less accumulation of deposits at the bottom, easier removal of the deposits that do settle at the bottom and of the crust that forms at the surface of the sludge, and reduced heat 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. Despite many egg-shaped sludge digestors having been built in various countries worldwide, their widespread adoption is hampered by the lack of detailed and systematic information on their structural analysis and design. The present study attempts to reduce this gap in the literature by (i) developing a simple, effective and systematic structural-analysis procedure for egg-shaped sludge digestor tanks of a certain practical combination of shell geometries, (ii) presenting usable closed-form analytical results for stresses and deformations throughout the digestor, and (iii) indicating design implications for the information of the structural engineer. In this paper, the first of two, membrane effects are evaluated, and some useful results are presented and illustrated through an example. The follow-up paper (“Stresses and deformations in egg-shaped sludge digestors: discontinuity effects” J Eng Struct 23 (2001) 1373) will evaluate the discontinuity effects at the junctions of the shell components making up the digestor, present detailed analytical and numerical results, and make design recommendations.