Small wastewater treatment plants often lack advanced digitalisation tools. In many rural facilities, limited instrumentation and fragmented documentation make it difficult to visualise processes, train operators, or communicate plant operation to stakeholders. Within the iMERMAID project, ITCL developed an immersive Digital Twin (DT) of the San Esteban de Litera Wastewater Treatment Plant, located in San Esteban de Litera. A detailed 3D model of the facility was created on Unity and deployed on Meta Quest 3 virtual reality glasses. The solution allows users to walk through the plant in an immersive environment, explore each treatment stage, and understand the layout and operation of the infrastructure in an intuitive way. Due to limited on-site sensorisation, the DT is currently focused on spatial, structural and educational value rather than real-time data integration. However, the platform is designed to be scalable and ready for future integration of monitoring systems. This immersive DT enhances communication, training and dissemination capacities, transforming a small rural WWTP into an innovative demonstrator of digitalisation in the water sector. Furthermore, it forms a reference case for immersive digitalisation of small wastewater treatment plants. Future steps include the integration of process models and real-time data once additional sensorisation is implemented, enabling advanced monitoring, simulation and decision-support functionalities.