Four giant eggs, 149 feet tall and 92 feet in diameter at the midsection, are part of a major wastewater treatment project in Bottrop, Germany. Each tank is an air-free chamber for the digestion of sludge coming from several nearby wastewater treatment plants. The prestressed egg-shaped digesters offer operational advantages such as minimum water surface area for scum buildup and steep cone floors to maximize sludge drawoff and eliminate the need for cleaning. The digesters are prestressed both horizontally and vertically to assure a water-tight structure.

To meet the 56 week construction schedule, two sets of custom-made forms were required. The outer form surface was made of radial steel I-beams supporting small trapezoidal sheathing panels and supported by a series of outer tension rings designed to carry the load of the freshly placed concrete. The inner formwork was of similar design, except that the connecting ring braces carried compressive stresses rather than tension. The inner form boards were set in place just ahead of the concrete placing crew, permitting access for placement in a continuous spiral around the "egg" and thus reducing cold joints in the structure to a minimum. While reinforcement and concrete were being placed in one section of the digester, crews on the ground preassembled easily lifted segments of formwork to be hoisted by crane and attached to previously placed sections.