EU Wastewater Directive 2037: Secondary Treatment Requirements

What “2037 requirements” means in practice

Although the recast EU Urban Wastewater Treatment Directive is often discussed in terms of future compliance milestones, the key secondary-treatment changes are already set by the directive adopted as Directive (EU) 2024/3019. The directive entered into force on 1 January 2025 and repeals the old Directive 91/271/EEC with effect from 1 August 2027. Member States must transpose it into national law by 31 July 2027.

For operators and designers, the practical question is not only what the directive says, but what it means for plant process selection, hydraulic capacity, biological stability and future upgrade pathways. This article focuses on secondary, biological treatment only.

For a broader directive overview, see the internal reference page on the EU directive.

What secondary treatment is under the recast directive

The 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 deliberately technology-neutral. In engineering terms, it covers conventional activated sludge and related suspended-growth systems, and it can also include other biological processes where the principal purpose is removal of biodegradable organic load.

In practice, the plant must be able to consistently achieve the Annex I secondary-treatment standards under expected operating conditions. That means the process selection, solids separation and sludge handling chain all matter, not just the aeration basin.

Core functional elements

A secondary-treatment line typically needs:

a reliable biological reaction stage for BOD removal
adequate oxygen transfer and process control
effective solids-liquid separation, commonly a final settlement stage
sludge return and wasting arrangements that preserve biomass age and settleability
wet-weather and peak-flow management aligned with the process capacity

The exact design approach is site-specific. Indicative design values such as sludge age, dissolved oxygen, MLSS and F/M ratio depend on influent characteristics, temperature, load variability, industrial inputs and compliance margin.

Which agglomerations must provide secondary treatment

The recast directive makes secondary biological treatment 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.

A population equivalent, or p.e., is defined as the organic biodegradable load having a five-day biochemical oxygen demand of 60 g of oxygen per day.

Why the p.e. definition matters

Because p.e. is linked to organic biodegradable load rather than just headcount, the compliance boundary depends on:

resident population
industrial or commercial organic load contributions
seasonal variation, especially in tourist areas
infiltration and inflow effects that change hydraulic loading but not p.e. directly

For asset owners, this means a plant’s design basis may not match its current or future regulatory category. That is particularly relevant where population growth, tourism, or trade effluents have changed the loading profile since the original works were commissioned.

Annex I secondary-treatment effluent standards

The directive’s Annex I sets the secondary-treatment effluent standards. These are the reference numbers that matter for compliance monitoring and process design.

Effluent standards

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

These are secondary-treatment standards, not tertiary or quaternary requirements. They are the baseline expectations for biological treatment performance.

Design implications

To meet these standards consistently, the plant must control both biological conversion and clarification performance. In many secondary-treatment systems, compliance risk is driven by:

insufficient biomass retention under variable loads
poor settleability or bulking sludge
inadequate oxygen transfer at high loading
storm-driven solids washout
undersized or hydraulically overloaded final settlement capacity
unstable upstream flows causing shock loading

Where the biology is sound but solids separation is weak, BOD and TSS compliance can fail together. Conversely, a well-performing clarifier cannot compensate for under-aeration or chronic process overloading.

Practical monitoring points

Operators commonly track:

influent and effluent BOD5, COD and TSS
mixed liquor suspended solids and sludge volume index
dissolved oxygen in aeration zones
return activated sludge and waste sludge rates
ammonia only as a process health indicator where relevant, even though it is not a secondary standard
storm overflow frequency and wet-weather bypass risks

These parameters are indicative process controls, not universal targets. The correct operating window depends on process configuration and local influent characteristics.

How secondary treatment fits into the wider directive

Secondary treatment is the foundation of the new framework, but it is not the full compliance picture.

The directive also introduces later obligations for nutrient removal and micropollutant removal. Briefly, tertiary treatment for nitrogen and phosphorus removal is required for plants of 150,000 p.e. and above by 2039, and for plants of 10,000 p.e. and above in areas at risk of eutrophication by 2045. Quaternary treatment for micropollutant removal is required for plants of 150,000 p.e. and above by 2045, with interim milestones in 2033 and 2039, and financing largely through extended producer responsibility for the pharmaceutical and cosmetics sectors.

For most plants, the design implication is that secondary treatment should not be treated as a dead-end investment. It should be assessed as the hydraulic, biological and civil foundation for future process upgrades.

Engineering questions to ask now

Can the current biological stage reliably meet BOD, COD and TSS limits under summer peak and wet-weather conditions?
Is there enough process volume and clarification capacity for future load growth?
Can the works be extended to nutrient removal without wholesale reconstruction?
Are sludge handling and side-stream returns compatible with longer-term biological upgrades?
Does the site have operational resilience for tighter compliance expectations over the next decade?

Typical engineering considerations for upgrades and new builds

Secondary treatment schemes vary, but the following issues are common when aligning a plant with the directive.

1. Hydraulic capacity and peak factors

The biological stage may be sized adequately for average dry-weather flow but still fail under storm-driven hydraulic peaks. Clarifier overflow rates, sludge washout and short-circuiting are frequent constraints. Any upgrade review should distinguish between average load and peak hydraulic stress.

2. Process selection

Conventional activated sludge remains a common choice because it is well understood and adaptable. Other biological processes may also meet the directive’s definition of secondary treatment if they remove biodegradable organic matter and provide the required performance.

Selection should consider:

footprint availability
load variability
energy demand
operator skill requirements
ease of future nutrient removal retrofit
resilience under low-temperature or shock-loading conditions

3. Solids separation

Because the directive’s standards include TSS, the settlement stage is not ancillary; it is part of compliance. Final clarifier design, sludge blanket control and return activated sludge management are central to achieving stable performance.

4. Sludge age and biomass stability

Indicative sludge age ranges differ by process and temperature, but the principle is constant: the biomass inventory must be sufficient to maintain treatment stability without excessive solids escape. Too short a biomass residence time can weaken nitrification potential and sludge settleability; too long can increase oxygen demand and sludge age-related operational issues.

5. Integration with future compliance

If nutrient removal is likely later, the secondary process should preserve the option to add anoxic zones, internal recycles, carbon management and more advanced control strategies. A plant that meets today’s BOD and TSS targets but blocks future retrofit can create avoidable lifecycle cost and operational disruption.

What operators and consultants should review now

Even where national transposition is still being completed, asset owners should start a gap assessment against the directive’s biological treatment obligations.

A practical review usually includes:

current agglomeration size in p.e. and projected growth
whether the works are already required to provide secondary treatment
current effluent compliance against BOD5, COD and TSS
process robustness during wet weather and seasonal peaks
settlement performance and solids losses
available footprint for expansion or refurbishment
sludge management and energy demand implications
likely future nutrient or micropollutant upgrade routes

For a structured plant-level review, Start the Engineering Assessment.

How this applies to your plant

If your works serves 2,000 p.e. or more, secondary biological treatment is already part of the EU compliance framework. If the catchment is smaller today but projected to exceed 1,000 p.e. before the end of 2035, the same planning logic applies now because biological treatment, clarification capacity and process resilience cannot be designed at the last minute.

The immediate engineering task is to confirm three things:

whether the plant is within the directive’s current or future secondary-treatment scope
whether the existing biological and settlement stages can meet the Annex I standards consistently
whether the site can be adapted later for nutrient removal without major reconstruction

That is the right starting point for operators, municipalities and consultants. If you need to turn the regulatory framework into a practical scope of works, begin with the Engineering Assessment and use it to map current performance, upgrade pathways and implementation risk.