EU Wastewater Directive 2045: What Secondary Treatment Plants Need

EU Wastewater Directive 2045 requirements in context

The recast Urban Wastewater Treatment Directive, Directive (EU) 2024/3019, resets the regulatory framework for urban wastewater treatment across the EU. For secondary treatment, the message is clear: biological treatment remains the core compliance step, and more agglomerations will fall within scope over time.

For operators, municipalities and consulting engineers, the practical question is not only what the directive requires, but how those requirements affect process selection, capacity, stability and future upgrade planning. This article focuses on secondary treatment and the implications up to 2045.

The directive is already in force. It entered into force on 1 January 2025, with transposition into national law due by 31 July 2027. The old Directive 91/271/EEC is repealed with effect from 1 August 2027. National implementation will vary, but the EU-wide framework is now set.

What counts as secondary treatment under the directive

The recast directive defines secondary treatment as treatment that generally involves biological treatment with a secondary settlement stage, or another process that removes biodegradable organic matter.

In practical terms, this includes established biological systems such as:

• activated sludge with final clarification
• oxidation ditches
• trickling filters and other attached-growth systems where biodegradable organic matter is removed effectively
• integrated biological treatment processes that achieve equivalent removal of biodegradable load

The wording is important because it is outcome-based as well as process-based. The key issue is not the exact configuration, but whether the plant reliably removes biodegradable organic matter and meets the applicable effluent standards.

Who must have secondary treatment, and by when

Secondary biological treatment is mandatory for agglomerations of 2,000 p.e. and above. The obligation extends to agglomerations of 1,000 p.e. and above by 31 December 2035.

A population equivalent, or p.e., is defined as the organic biodegradable load having a five-day biochemical oxygen demand (BOD5) of 60 g of oxygen per day. That definition matters because the size threshold is load-based, not simply a headcount.

There are transitional and exemption arrangements, including for certain coastal areas and for Bulgaria, Croatia and Romania. For any project or compliance review, the relevant national transposition rules and site-specific status must be checked carefully.

From an engineering standpoint, this phased extension means more small and medium plants will need to demonstrate stable biological treatment performance, even where current systems were originally designed only for basic treatment or limited seasonal operation.

Annex I effluent standards for secondary treatment

For secondary-treatment effluent, Annex I sets the following standards:

• BOD5: 25 mg/L, or 70-90% reduction
• COD: 125 mg/L, or 75% reduction
• TSS: 35 mg/L

These are the core performance values to keep in mind when reviewing existing plants, sizing new process trains or planning rehabilitation work.

A few practical points follow from these limits:

• BOD5 compliance depends on stable biological oxidation and adequate solids separation.
• COD performance is influenced by influent characteristics, biological conversion efficiency and carry-over of particulate organics.
• TSS is often a sign of clarification performance and sludge settleability, not just aeration adequacy.

In other words, good secondary treatment is not just about the aeration tank. Final settlement, sludge handling, recirculation control and process stability all affect the final effluent.

What the 2045 timeline means for plant design and operation

The EU directive introduces a longer-term pathway rather than a single deadline. For secondary treatment, the immediate regulatory emphasis is on the expansion of scope to smaller agglomerations by 2035, but plant owners should already think ahead to the wider 2045 framework.

By 2045, the sector is also expected to operate in a more demanding overall environment because:

• tertiary treatment becomes mandatory for plants of 150,000 p.e. and above by 2039
• tertiary treatment is required for plants of 10,000 p.e. and above in areas at risk of eutrophication by 2045
• quaternary treatment for micropollutant removal applies to plants of 150,000 p.e. and above by 2045, with interim milestones in 2033 and 2039
• energy neutrality for the urban wastewater treatment sector is to be achieved by 2045

Although those later obligations are not secondary treatment requirements in themselves, they affect how secondary plants are designed and upgraded. In many cases, the secondary stage will need to provide a stronger and more stable base for downstream nutrient or micropollutant treatment.

Typical engineering implications

Indicative, site-specific design and operational considerations often include:

• preserving sufficient process volume for hydraulic and load variability
• maintaining stable solids retention and sludge separation
• allowing for future integration of tertiary or quaternary stages
• keeping energy use under review, especially for aeration-intensive systems
• ensuring resilience against wet-weather flows and shock loads

These are not universal design numbers, but they are common issues that should be assessed early in any upgrade study.

Common weak points in secondary treatment compliance

Plants that struggle with secondary-treatment compliance often have one or more of the following issues:

• inadequate aeration capacity or poor oxygen transfer
• insufficient biomass inventory or unstable sludge age
• poor sludge settleability leading to solids carry-over
• hydraulic overloading during storm events or infiltration peaks
• limited redundancy or inflexible process control
• ageing settlement structures and deteriorating internal hydraulics

In many older plants, the process may still achieve acceptable average performance but fail during high-load periods, cold weather or wet-weather events. That is often where compliance risk appears first.

The recast directive’s broader scope means that plants previously considered marginal may now need a structured performance review rather than only reactive maintenance.

How to assess whether your plant is ready

A practical assessment should combine regulatory review with process engineering review. Typical questions include:

• Does the plant fall within current or future p.e. thresholds?
• Can the existing biological process reliably meet the Annex I BOD5, COD and TSS standards?
• Is the final settlement stage robust enough for present and future loads?
• Are there seasonal, diurnal or wet-weather peaks that reduce compliance margin?
• Is there enough space, hydraulic capacity and process flexibility for future nutrient or micropollutant treatment?
• What modifications are needed to maintain compliance through 2035 and beyond?

If you are planning rehabilitation, capacity expansion or a new works study, it is worth testing the plant against the directive pathway now rather than waiting for national transposition milestones.

For a structured starting point, Start the Engineering Assessment.

You can also review the wider directive framework here: /eu-directive.

How this applies to your plant

The most important secondary-treatment message from Directive (EU) 2024/3019 is that biological treatment remains the foundation of compliance, but the scope of that obligation is widening. If your plant serves 2,000 p.e. or more, secondary treatment is already required. If it serves 1,000 p.e. or more, the obligation applies by 31 December 2035, subject to the directive’s transitional and exemption provisions.

For an existing plant, the immediate task is to confirm whether the current process can reliably achieve the Annex I standards of BOD5 25 mg/L, COD 125 mg/L and TSS 35 mg/L under all expected operating conditions. For a planned project, the task is to choose a process that can remain compliant as the regulatory framework expands towards 2045.

A focused engineering assessment should therefore cover:

• current loading and future growth
• hydraulic robustness and settlement performance
• process stability and operator control
• compatibility with future tertiary or quaternary upgrades
• energy intensity and operational resilience

If you are preparing a feasibility study, asset management plan or upgrade concept, use the directive pathway as the basis for the review. The best time to identify gaps is before national deadlines and procurement decisions narrow your options.