One of the project’s most significant milestones was the construction of the main sewer tunnel, extending 4,950 metres, using an Earth Pressure Balance (EPB) Tunnel Boring Machine (TBM). The tunnel, lined with precast concrete segmental rings and featuring an internal diameter of approximately 3.15 metres, transports wastewater from the urban area of Soria to the new treatment facility while minimizing surface disruption. Excavation progressed from south to north, taking advantage of the natural gradient to facilitate drainage throughout construction.

The tunnel presented considerable technical challenges due to the geological and hydrogeological variability encountered along the alignment. Excavation passed through limestone formations, tertiary deposits with varying grain sizes, fractured zones and sections with significant groundwater inflows. These conditions required continuous adjustment of excavation parameters and the implementation of complementary measures, including resin sealing, ground consolidation grouting and other interventions to ensure operational safety and ground stability. Despite these challenges, tunnel breakthrough was successfully achieved on 12 August 2024, followed by the connection of the North and South collectors.

The Sinova WWTP, located in Los Rábanos, has been designed to serve 135,000 population equivalent (PE) and treat an average flow of up to 24,000 m³ per day. The water treatment line includes preliminary treatment, screening, pumping, grit and grease removal, primary settling and a Sequencing Batch Reactor (SBR) biological treatment process with nutrient removal. This configuration ensures compliance with discharge requirements applicable to the protected Duero River and Tributaries SAC/SCI (LIC/ZEC), part of the Natura 2000 network.

The facility also incorporates an advanced sludge treatment line featuring anaerobic digestion and biogas-to-energy generation through cogeneration units. Additional elements include odour control systems, a photovoltaic installation, emergency power generation and an underground utility gallery. The result is a modern, efficient and sustainable infrastructure that significantly improves wastewater management in Soria and Los Rábanos, reduces environmental impact and strengthens the protection of the Duero River and its surrounding ecosystem.