The paper shows research on a closed fermentation chamber (CFC) that is an element of the Biogas Combined Heat and Power Plant located in the Gdańsk East Sewage Treatment Plant. This tank is mounted as an above-ground cylinder-shaped structure covered with a cone-shaped roof. Corrosion and leaks appeared in the tanks during many years of operation. In this work, it was decided to investigate their cause and propose a method of counteracting them. The article presents the research on the influence of individual operational and environmental loads on the stress distributions in the tank structure. Numerical simulations using the Finite Element Method were carried out for shell and solid models. Moreover, the stress distributions in cylindrical steel tanks were compared depending on the shape of the chamber top. The research results allowed us to identify the design errors of the closed fermentation chambers. It finds out that the connection of the cylindrical part of the tank with the conical roof requires reinforcement. A tank repair technology using Belzona composites was proposed to modify the joint geometry. A simulation was performed, taking into account the proposed repair technology. Its results showed a reduction of stresses compared to the state before repair by 19%.