In its first full year of operation, London’s Thames Tideway Tunnel has significantly curtailed sewage overflow into the River Thames, according to Thames Water. The £5 billion, 16-mile “super sewer” was designed to address longstanding issues of sewage discharge during heavy rainfall, which has traditionally posed health risks and environmental concerns.

Since the tunnel became fully operational, the number of days when elevated health risks due to sewage overflow were recorded dropped sharply from 269 in 2023 to just 38 in 2025. Thames Water attributes this improvement to the tunnel’s capacity to capture and divert excess sewage that would otherwise have been released into the river system.

The project aims to alleviate the problem of combined sewer overflows, which occur when heavy rain overwhelms the existing sewer infrastructure. By intercepting and transporting excess wastewater to treatment facilities, the Thames Tideway Tunnel helps prevent pollutants, including sewage-derived litter, from entering the river. Thames Water reports that approximately 84 tonnes of such litter were kept out of the Thames as a result of the tunnel’s operations during this period.

The tunnel, running beneath London’s riverbed, is considered a major infrastructure upgrade for the city’s wastewater management system. Its implementation addresses both environmental protection and public health concerns by reducing pollution levels and improving water quality in the River Thames.

This significant reduction in sewage overflow aligns with broader efforts to enhance urban water management and sustainability in the face of climate change and population growth. The successful operation of the Thames Tideway Tunnel marks a milestone for London's infrastructure resilience and environmental stewardship.