What is installed between two pipe flanges to ensure a sealed connection?

The correct answer is a gasket, which is specifically designed to create a seal between two mating surfaces, such as pipe flanges. Gaskets are typically made from compressible materials to fill any gaps or irregularities present in the flange surfaces, thereby preventing leaks of fluids or gases. This sealing capability is crucial in sewage treatment processes where the containment of liquids and gases is essential for safety and efficiency.

Using gaskets of appropriate material and thickness ensures that the connection can withstand the pressure and temperature of the substances being transported while preventing any environmental contaminants from escaping. In contrast, other options like sheaves, bosses, and fillets have different applications and do not serve the purpose of sealing connections between flanges.