What is quaternary treatment and how it affects urban WWTPs

What is quaternary treatment

Quaternary treatment is a new stage in the urban wastewater treatment process, placed after tertiary treatment. Its goal is to remove micropollutants that conventional processes cannot retain: microplastics, textile microfibres, pharmaceutical residues, and other emerging compounds.

Until now, WWTPs worked with three treatment levels — primary (physical), secondary (biological), and tertiary (disinfection). Quaternary is now becoming mandatory to guarantee the quality of water returned to the environment.

Why it's needed to remove microplastics

Microplastics — particles smaller than 5 mm — pass through conventional filters and biological systems due to their size and low density. Up to 90% of the microplastics in incoming water reach the natural environment without specific treatment. Main sources: microfibres from washing synthetic fabrics, tyre wear, cosmetics, and packaging degradation.

EU Directive 2024/3019: what it requires

EU Directive 2024/3019 requires progressive implementation of quaternary treatment in all WWTPs serving more than 150,000 population equivalents before 2045, with intermediate milestones (20% by 2033, 60% by 2036, 80% by 2039). Full details, thresholds and extended producer responsibility are explained in the EU Directive 2024/3019 compliance guide.

Available technologies for quaternary treatment

The main options are activated carbon, ozonation, advanced membrane filtration and magnetic capture. Each has very different profiles of efficiency, operating cost and waste generated — we analyse them in detail in the quaternary treatment technologies comparison.

What WWTP operators should do now

• Audit the current load of micropollutants and microplastics.
• Evaluate technologies compatible with the existing process.
• Plan multi-year investments aligned with the 2033-2039 milestones.
• Run pilot tests with selected technology providers.