What does the fourth treatment stage mean? — New European Union regulation from 2025
From 1 January 2025, the revised EU Urban Wastewater Treatment Directive entered into force, introducing new requirements in the field of wastewater management to strengthen the protection of human health and the environment. One of its key elements is the gradual introduction of the so-called “fourth treatment stage”.
What are the existing three treatment stages?
Conventional wastewater treatment consists of three main stages:
1. Mechanical treatment – removal of coarse solids (e.g. screens, grit and grease traps);
2. Biological treatment – degradation of organic matter using microorganisms;
3. Chemical treatment – reduction of nutrients (nitrogen and phosphorus) by chemical processes.
Limitations of the three-stage treatment
Traditional treatment processes are not capable of completely removing micro-scale contaminants, such as dissolved pharmaceutical residues, pesticide active substances, compounds originating from cosmetics, or certain persistent organic micropollutants, including microplastics.
Why is a fourth treatment stage needed?
The EU aims to ensure the removal from municipal wastewater of pollutants that cannot be sufficiently eliminated by conventional treatment methods, yet may be harmful to aquatic ecosystems, accumulate in the environment over time, enter the food chain, and pose risks to human health.
This is particularly critical in areas where the receiving water body—such as a stream, river or lake—is sensitive or already under environmental pressure.
What does the “fourth treatment stage” involve?
The fourth stage is an additional advanced treatment step requiring the application of specific technologies, such as:
- specialised filtration systems (e.g. activated carbon filtration)
- oxidation processes
- advanced membrane technologies
Which are specifically designed for the removal of micropollutants.
Where and when is it required?
According to the Directive, wastewater treatment plants with a capacity exceeding 150,000 population equivalents (PE) must be equipped with a fourth treatment stage as follows:
- by the end of 2033: 20% of plants,
- by the end of 2039: 60% of plants,
- by the end of 2045: 100% of plants.
Technological solutions for the fourth treatment stage
1. Activated carbon technologies (BAT solution)
One of the most widely applied and accepted solutions in Europe. Variants:
PAC – powdered activated carbon, a relatively low-risk technology that can be well integrated into existing plants: dosed into treated wastewater, subsequently removed by sedimentation or filtration.
GAC – granular activated carbon used in fixed-bed or moving-bed filters.
Advantages:
Broad-spectrum micropollutant removal; good compatibility with existing plants; relatively low technological risk.
Disadvantages:
Costs of carbon replacement and regeneration; additional sludge production (in the case of PAC).
2. Ozonation (O₃)
Ozone chemically degrades micropollutant molecules.
Characteristics:
Applied after biological treatment; often combined with a downstream activated carbon step.
Advantages:
Highly effective for many pharmaceutical compounds; fast reaction kinetics.
Disadvantages:
Formation of transformation by-products; requires precise control and monitoring; energy-intensive.
3. Combined technologies (typical EU solution)
These increase removal reliability and reduce the risk of problematic by-products.
In practice, many new systems are two-stage solutions, such as:
Ozonation + activated carbon (PAC);
Activated carbon (PAC) + sand or membrane filtration.
4. Membrane technologies
Due to their disadvantages, these are applied only to a limited extent, mainly for specific industrial wastewater streams.
Types:
Nanofiltration (NF); reverse osmosis (RO).
Advantage:
Very high removal efficiency.
Disadvantages:
Extremely high investment and operating costs; management of the concentrate (retentate) is problematic.
5. Advanced Oxidation Processes (AOP)
UV + H₂O₂ (hydrogen peroxide):
In these systems, UV radiation does not act alone but activates hydrogen peroxide, forming one of the most effective advanced oxidation processes in wastewater treatment.
UV + O₃ (ozone):
In another configuration, ozone (O₃) is decomposed under UV light, generating hydroxyl radicals (•OH). These are typically applied as supplementary solutions in special cases, mainly for industrial wastewater.
The UV/ozone system is particularly effective for: pharmaceutical residues; hormone-active compounds; pesticides; industrial organic micropollutants; colour- and odour-causing substances.
