WWTP Upgrade Timeline 2026–2045 under EU Directive 2024/3019

Introduction

Directive (EU) 2024/3019 sets a long upgrade horizon for urban wastewater treatment in the EU, but the path is not linear. For plant owners and operators, the practical question is not only what the final 2045 position looks like, but what must be planned first, what can be sequenced, and where design decisions made now will constrain later compliance.

The directive entered into force on 1 January 2025. It repeals the old Directive 91/271/EEC with effect from 1 August 2027, and Member States must transpose it into national law by 31 July 2027. National implementation will therefore evolve over time, but the EU-wide direction is clear: secondary treatment remains the baseline, nutrient removal expands over time, micropollutant removal is introduced for larger plants, and energy neutrality becomes a sector objective by 2045.

This article sets out the upgrade timeline from 2026 to 2045, with a focus on secondary biological treatment and the implications for WWTP planning.

2026–2027: Convert the directive into a plant programme

The first practical milestone is administrative and strategic rather than process-specific. During 2026 and 2027, plant owners should move from a compliance reading of the directive to an engineering programme.

What the directive already fixes

Under the recast directive:

Secondary treatment is defined as treatment that generally involves biological treatment with a secondary settlement stage, or another process that removes biodegradable organic matter.
Secondary biological treatment is mandatory for agglomerations of 2,000 p.e. and above.
The obligation is extended to agglomerations of 1,000 p.e. and above by 31 December 2035, subject to transitional and exemption arrangements, including for certain coastal areas and for Bulgaria, Croatia and Romania.
Annex I secondary-treatment effluent standards are BOD5 25 mg/L, COD 125 mg/L and TSS 35 mg/L.

A population equivalent is the organic biodegradable load having a five-day biochemical oxygen demand of 60 g of oxygen per day.

What operators should do now

At this stage, the key tasks are to identify where the present process stands relative to the future baseline and to avoid dead-end investments. Typical actions include:

confirm current agglomeration size in p.e. and likely growth to 2035, 2039 and 2045;
check whether the existing process already meets the Annex I secondary-treatment standards consistently;
review hydraulic capacity, sludge handling and settlement performance, because these often limit upgrade options before biological capacity does;
assess whether the current layout can accommodate future nutrient or quaternary treatment without major civil rework;
establish whether the site has enough footprint, power distribution and sludge processing resilience for phased expansion.

For plants still operating close to the lower end of the secondary-treatment threshold, the question is not only whether biological treatment is needed now, but whether the chosen configuration can be extended later. A compact process may appear cost-effective in the short term, yet be difficult to adapt to nutrient removal, aeration upgrades or tertiary solids separation.

For a regulatory overview, see EU Directive context.

2027–2035: Secure secondary treatment and build flexibility

The period to 31 July 2027 is the transposition window, and the period to 31 December 2035 is the main secondary-treatment expansion horizon for smaller agglomerations.

The compliance focus

By this stage, utilities should ensure that the plant can do three things reliably:

achieve biological removal of biodegradable organic matter;
maintain secondary settlement or an equivalent solids separation stage where applicable;
meet the Annex I effluent values on a routine basis, not only during favourable conditions.

In engineering terms, this usually means tightening control over aeration, sludge age, return activated sludge management and clarification stability. The exact operating values are site-specific, but a typical design review will consider whether the process has sufficient buffer against load variation, storm inflow and seasonal temperature changes.

Common upgrade pathways

Secondary-treatment upgrades between 2027 and 2035 are likely to fall into a few broad categories:

Existing activated sludge optimisation: improve aeration, recycle control, and secondary clarification performance.
Biological capacity increase: add tank volume, parallel lines or additional aeration capability where load growth is expected.
Process conversion or intensification: move to a configuration with better solids separation or greater resilience where footprint is constrained.
Pre-treatment and flow management improvements: reduce shock loading and protect the biological stage.

Typical design choices remain highly site-specific. Indicative ranges for parameters such as dissolved oxygen, mixed liquor suspended solids, food-to-microorganism ratio and solids retention time must be derived from the actual influent, climate and treatment objective. They should not be copied from one plant to another without review.

Why this phase matters

If the secondary process is under-designed now, later nutrient and micropollutant requirements become much harder to retrofit. A plant that cannot consistently meet secondary standards will struggle to justify later investment in tertiary or quaternary stages. Conversely, if the secondary line is stable and flexible, later additions can often be integrated around it.

2035–2039: Prepare for nutrient removal at larger plants

The next major threshold is tertiary treatment, meaning nitrogen and phosphorus removal.

Regulatory timing

Under the directive:

tertiary treatment is required for plants of 150,000 p.e. and above by 2039;
it is also required for plants of 10,000 p.e. and above in areas at risk of eutrophication by 2045.

This creates a two-track planning problem. Large plants need to be ready earlier, while medium plants in sensitive catchments may face a later but still substantial retrofit requirement.

Practical implications for plant design

Tertiary treatment is not simply an add-on. In many cases, it affects the entire process train:

biological zone configuration may need to support nitrification and denitrification;
internal recycle arrangements may need redesign;
phosphorus removal may require chemical dosing, process rebalancing or both;
sludge production and dewatering loads may change;
residual ammonia and nitrate control may need more robust instrumentation and process automation.

For a plant already approaching secondary capacity, nutrient removal usually means a broader review of aeration demand, basin hydraulics and final settlement capacity. The preferred route may be biological nutrient removal, chemical phosphorus removal, or a hybrid approach. The right solution depends on influent strength, variability, discharge sensitivity and available footprint.

Design timing

If a plant falls within the 150,000 p.e. category, the design work should begin well before 2039. Waiting until the deadline approaches risks locking in interim works that later need to be replaced. Where catchment sensitivity suggests future nutrient requirements, it is prudent to reserve space, pipework routes and electrical capacity during the secondary upgrade phase.

2033–2045: Plan for micropollutant removal and energy neutrality

The recast directive extends beyond conventional biological treatment. Two longer-term drivers are especially relevant for major plants: quaternary treatment and energy neutrality.

Quaternary treatment milestone

Quaternary treatment, defined here as micropollutant removal, is required for plants of 150,000 p.e. and above by 2045, with interim milestones in 2033 and 2039. Funding is largely through extended producer responsibility for the pharmaceutical and cosmetics sectors.

For operators, the important point is not the funding model itself, but the engineering consequence: the plant may need space, hydraulics and energy headroom for an additional treatment stage. Depending on the final selected process, this can affect:

headloss through the site;
residual solids management;
upstream clarification performance;
power demand and resilience;
operational complexity and staffing.

Energy neutrality by 2045

The directive also sets energy neutrality for the urban wastewater treatment sector by 2045. That makes energy a core design criterion, not an afterthought.

For biological treatment, this has several implications:

aeration systems should be assessed for efficiency and controllability;
process stability should be improved to avoid wasted energy from over-aeration;
sludge handling and digestion opportunities should be considered where relevant;
pumping, blowers and process automation should be reviewed as one integrated energy system.

The sector-wide target does not mean each individual plant must be self-sufficient at every point in time, but it does mean utilities should develop a credible pathway to reduce net energy demand and increase on-site recovery where feasible.

A practical 2026–2045 sequencing strategy

A sensible upgrade programme usually follows the same order:

Stabilise secondary treatment so the plant consistently meets the Annex I baseline.
Protect future expansion space for nutrient and quaternary stages.
Upgrade aeration, controls and settlement to reduce operating risk and prepare for tighter process control.
Design nutrient removal early where the plant is large enough, or where catchment sensitivity makes future retrofit likely.
Assess energy neutrality alongside process changes, not as a separate exercise.

This sequence reduces the risk of stranded assets. It also helps avoid the common mistake of treating each deadline as an isolated project.

How this applies to your plant

The right timeline depends on current capacity, process configuration, catchment sensitivity and available space. A plant that is compliant today may still need upgrades for the 2035, 2039 or 2045 horizons, especially if it serves a growing agglomeration or a sensitive receiving water.

If you are planning a secondary-treatment upgrade, the key questions are:

Does the current process consistently meet the Annex I effluent standards?
Will the plant remain below or rise above the 1,000 p.e., 10,000 p.e. or 150,000 p.e. thresholds?
Is there enough space and hydraulic capacity for nutrient removal or quaternary treatment later?
Can energy use be reduced while maintaining biological performance?
Which upgrade path avoids repeated reconstruction?

Use the engineering review to test those questions against your actual site data. Start the next step with Start the Engineering Assessment.