The use of fiber-reinforced polymer (FRP) composite materials proves to be an effective solution for structures requiring high chemical resistance, such as those in wastewater treatment facilities. This paper presents a case study involving the design and implementation of a stand-alone FRP stair structure, developed to provide access to a concrete inspection tank. The design was tailored to accommodate the geometric constraints of the existing concrete structure, including protrusions and spatial limitations due to the proximity of an area used by maintenance vehicles. Located in Emilia-Romagna, Italy—an area significantly affected by the 2012 earthquake—the structure was designed in compliance with the stricter seismic regulations introduced thereafter. Structural calculations addressed vertical maintenance loads, wind pressure, and seismic forces, in accordance with Italian standard D.M. 17 January 2018 (Technical Standards for Construction). The stair structure reaches a total height of 5.29 m, distributed across three ramps, and has a footprint of 4.26 × 4.93 m. The final landing includes a cantilevered section, enabling safe access to the tank without direct contact. The entire structure, including the grating, is constructed from FRP materials, with only the base connections and a few joints executed in stainless steel. This case highlights the advantages of FRP in harsh environments and under demanding structural requirements.