Secondary Treatment Deadline 2037: What It Means for EU Plants

What the secondary treatment deadline means

The phrase “secondary treatment deadline 2037” is often used loosely in discussions about EU wastewater compliance. In practice, the recast Urban Wastewater Treatment Directive (Directive (EU) 2024/3019) sets the legal framework for when secondary treatment must be provided, how it is defined, and what performance it must achieve.

For operators and engineers, the key point is not a single plant-wide date, but a phased compliance obligation based on agglomeration size and national implementation. The directive entered into force on 1 January 2025. Member States must transpose it into national law by 31 July 2027, and the old Directive 91/271/EEC is repealed with effect from 1 August 2027.

If you need to check how this applies to a specific plant or sewer catchment, it is sensible to Start the Engineering Assessment early, because the implication of the rule depends on population equivalent, existing treatment line configuration and planned upgrades.

What counts as secondary treatment

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.

That definition is important because it recognises both conventional activated sludge systems and other biological processes that achieve the same core objective: removal of biodegradable organic load from settled wastewater.

In practical terms, secondary treatment is the process stage that delivers the main reduction in BOD5 and a large share of suspended solids removal. It is typically implemented through one of the following process families:

• activated sludge systems
• biofilm processes
• sequencing batch systems
• other biological processes designed for biodegradable organic matter removal

The directive does not require a single technology. It requires performance and compliance with the relevant treatment category.

Who must provide secondary treatment and by when

The directive makes secondary (biological) treatment mandatory for agglomerations of 2,000 p.e. and above. It also extends the obligation to agglomerations of 1,000 p.e. and above by 31 December 2035.

A few practical implications follow from this:

• Plants serving agglomerations at or above 2,000 p.e. are already within the secondary treatment obligation under the recast framework.
• Smaller agglomerations are brought into scope later, with the 1,000 p.e. threshold applying by 31 December 2035.
• Transitional and exemption arrangements exist, including for certain coastal areas and for Bulgaria, Croatia and Romania.

So, when people refer to a “2037 deadline”, they are usually conflating different implementation milestones, national transposition timing, and local compliance programmes. The directive itself uses the dates above; country-level delivery schedules will depend on transposition, local planning and any applicable transitional arrangements.

For municipalities and consultants, the first task is to confirm which agglomerations are in scope, what their current p.e. is, and whether the existing plant already meets the required biological treatment standard.

Effluent standards for secondary treatment

The recast directive includes Annex I standards for secondary-treatment effluent. These are the core performance benchmarks used to judge whether a plant’s secondary treatment is adequate.

The standards are:

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

These figures are the relevant EU-wide benchmarks for secondary treatment under the recast directive. In design and operations work, they should be treated as compliance criteria, not as routine target values for every plant under every condition.

In practice, whether a plant consistently meets them depends on factors such as:

• hydraulic and organic loading variation
• industrial contribution to the sewer
• stormwater inflow and infiltration
• sludge settleability and solids separation performance
• process control stability
• seasonal temperature effects

A plant may technically have “secondary treatment” in place but still fail to meet Annex I performance if the biological stage or secondary settlement stage is overloaded, unstable or poorly controlled.

What this means for existing plants

For an existing wastewater treatment plant, the directive creates three broad questions.

1. Is the agglomeration in scope?

The first check is population equivalent. The directive defines p.e. as the organic biodegradable load having a five-day biochemical oxygen demand (BOD5) of 60 g of oxygen per day.

That definition matters because compliance thresholds are tied to p.e., not simply resident population. Industrial inputs, tourism peaks and infiltration can change the effective loading basis for treatment design and compliance.

2. Does the plant already meet the definition and performance standard?

A plant with conventional secondary treatment may still need review if:

• the aeration system is under capacity
• secondary clarifiers are hydraulically stressed
• solids capture is unreliable
• process control is manual or outdated
• the biological stage is not stable at low temperatures or variable flow

Where the plant uses another process that removes biodegradable organic matter, the compliance case should still be built around actual performance data and confirmed effluent quality.

3. Is there a wider upgrade trajectory?

Secondary treatment is not the end of the compliance pathway. The same directive sets later obligations for:

• tertiary treatment for nitrogen and phosphorus removal
• quaternary treatment for micropollutant removal
• energy neutrality by 2045

That means a plant size or process choice that is acceptable for secondary treatment today may not be the best long-term option if future nutrient removal or advanced treatment will be required. For many sites, the sensible approach is to assess whether the secondary treatment upgrade should preserve space, hydraulic headroom and process flexibility for later stages.

Design and operational considerations

For engineering review, the most useful question is not “does the plant have secondary treatment?” but “can the line reliably achieve secondary-treatment standards under real operating conditions?”

Typical design and operational considerations include:

• Process configuration: conventional activated sludge, extended aeration, biofilm systems or hybrid arrangements may all be suitable, depending on influent character and site constraints.
• Secondary settlement: clarification capacity, sludge blanket control and solids capture are often the limiting factors in performance.
• Aeration and oxygen transfer: the biological stage must be stable enough to remove biodegradable organics across seasonal load variation.
• Sludge age and biomass retention: process stability often depends on maintaining an appropriate biological state for the influent profile.
• Monitoring and control: reliable influent, process and effluent data are essential for demonstrating compliance.
• Storm resilience: combined sewer influence and hydraulic peaks can affect settlement performance even where the biological process itself is adequate.

These are indicative engineering points, not universal design numbers. Actual sizing, control strategy and process margins must be site-specific.

Why the 2037 framing matters in practice

The reason this topic attracts attention is that compliance timing affects procurement, planning permission, capital budgeting and asset strategy. Even where the legal deadline is not exactly 2037, the practical planning horizon can extend to that period once transposition, local implementation, and staged capital delivery are considered.

This is especially relevant for:

• municipalities with ageing biological treatment assets
• plants currently operating close to capacity
• catchments with population growth or seasonal load variation
• sites that may later need nutrient removal or quaternary treatment

In other words, the secondary treatment obligation should be assessed alongside the future compliance path, not in isolation.

The internal EU framework is already set at a high level in the directive overview on our EU Directive page. For project-level interpretation, however, the plant-level data and catchment context remain decisive.

How this applies to your plant

If you operate, own or design a wastewater treatment plant, the practical next step is to test whether your site is ready for the secondary-treatment obligation and whether it is being planned with later requirements in mind.

A sound review should cover:

• current agglomeration p.e.
• existing treatment configuration
• effluent performance against the Annex I secondary-treatment standards
• hydraulic and organic headroom
• available space for future upgrade stages
• likely need for nutrient or advanced treatment later in the compliance pathway

If you are unsure whether your plant is in scope, or whether the current process can reliably meet the required standards, Start the Engineering Assessment. A structured assessment is the fastest way to identify whether you need operational optimisation, a process upgrade or a phased capital programme.

Secondary treatment is the foundation of EU urban wastewater compliance. The recast directive makes that foundation more explicit, more measurable and more widely applicable over time. The earlier a plant’s status is checked, the easier it is to plan a compliant and technically robust upgrade path.