• As SIAAP’s historic facility and Europe’s largest treatment site, Seine Aval serves a crucial role in the Paris region’s sanitation, treating wastewater from 6 million inhabitants daily.
• The site achieves a significant milestone in its modernization with the inauguration of a new primary settling unit, delivering more agile, efficient and energy-saving wastewater treatment while reducing nuisances.
• Constructed by Veolia, this new facility can treat up to 34 m3/s of wastewater — equivalent to more than 50 Olympic swimming pools — making it Europe’s largest treatment capacity installation.

SIAAP (Greater Paris Interdepartmental Sanitation Authority) and Veolia, global leader in ecological transformation, inaugurated a new primary settling unit at the Seine Aval plant (78) on Tuesday, March 18 2025. The ceremony was attended by Estelle Brachlianoff, Veolia Group CEO, François-Marie Didier, SIAAP President, and Anne Le Guennec, Veolia Senior Executive Vice President of Worldwide Water Technologies, along with elected officials and state representatives.

In the Paris region, efficient wastewater treatment is particularly critical due to the high population density and substantial volumes of wastewater requiring treatment. With millions of inhabitants, treatment plants must operate efficiently to prevent system saturation and minimize pollution in waterways such as the Seine and Marne rivers. Additionally, since the region frequently experiences heavy rainfall, the primary settling system is designed to handle flow variations by treating up to 34 m3/s — equivalent to more than 50 Olympic pools per hour. This helps preserve water quality and ensure sustainable sanitation for the region’s residents.

As part of commitments made to comply with European water directives, the new installation complements existing systems and enables better wastewater treatment by eliminating fine particles, reducing carbon and phosphorus content, and optimizing the water purification process.

This inauguration marks a significant milestone in the extensive modernization project of Seine Aval, initiated in 2009. As SIAAP’s historic facility and Europe’s largest treatment site, it plays a vital role in the region’s sanitation, treating wastewater from 6 million inhabitants daily.

A new installation with exceptional performance

Primary settling represents a critical phase in wastewater treatment. Following the pretreatment stage, it focuses on removing suspended matter and phosphorus ahead of biological treatment. This essential step conditions the water for subsequent purification processes.

The new primary settling unit, which complements existing installations, stands as Europe’s largest such facility and marks a significant breakthrough in wastewater treatment for the Paris region. Completed after three years of work, the project required an investment of 511 million euros, with 498 million allocated to the Veolia-led consortium.

It relies on several major innovations to:

• Better distribute effluents based on flow rates and pollution loads.
• Improve the quality of water discharged into the Seine river through advanced technologies including lamellar settlers that remove ultra-fine waste particles and treat dissolved phosphorus and carbon pollution.
• Reduce odor nuisances for residents through fully covered installations equipped with an odor treatment system.
   

A model of energy and environmental efficiency

The new primary settling reduces its environmental impact while maximizing its performance:

• A heat recovery system has been installed on treated water (Energido™).
• The settlers benefit from an elaborate washing system ensuring better sludge quality enabling optimized biogas production.
• Energy and reagent consumption (Predifloc™) is optimized through predictive and real-time control using latest-generation digital tools.
   

Furthermore, biodiversity protection is enhanced through preservation measures for local fauna and flora.

This new primary settling unit demonstrates that industrial performance, environmental protection and residents’ quality of life can be successfully harmonized, paving the way for a new generation of water treatment facilities.

“Since 2019, the Seine Aval site, Europe’s largest treatment plant, has undergone continuous modernization. SIAAP’s primary focus remains improving industrial safety and treatment performance across its facilities. The new primary settling unit, built by Veolia and its partners, fully achieves these goals. It will provide SIAAP with a new facility that concentrates technologies perfectly suited to the challenges of its activity and mission in service of water and the environment,” states François-Marie Didier, SIAAP President.

“We are very proud to inaugurate today, alongside SIAAP, this new primary settling unit at the Seine-Aval site,” adds Estelle Brachlianoff, Veolia CEO. “Through this landmark project, Veolia reaffirms its commitment to operational excellence and innovation serving territories and their inhabitants. This partnership, leveraging our know-how and expertise, not only meets growing regulatory requirements but also the legitimate expectations of elected officials and residents regarding environmental performance. It perfectly embodies the vision we carry within our GreenUp strategic program: bringing cutting-edge solutions to address current and future societal challenges.”

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ABOUT VEOLIA

Veolia group aims to become the benchmark company for ecological transformation. Present on five continents with 215,000 employees, the Group designs and deploys useful, practical solutions for the management of water, waste and energy that are contributing to a radical turnaround of the current situation. Through its three complementary activities, Veolia helps to develop access to resources, to preserve available resources and to renew them. In 2024, the Veolia group provided 111 million inhabitants with drinking water and 98 million with sanitation, produced 42 million megawatt hours of energy and treated 65 million tonnes of waste. Veolia Environnement (Paris Euronext: VIE) achieved consolidated revenue of 44.7 billion euros in 2024. www.veolia.com