What Secondary Treatment Means Under EU Wastewater Law

Introduction

Under EU law, secondary treatment is the core biological treatment stage in municipal wastewater treatment. It is the point at which biodegradable organic matter is removed to a level that meets the Urban Wastewater Treatment Directive’s requirements for treated effluent.

For operators, municipalities and consulting engineers, the practical question is not only what the term means, but where it applies, what performance is expected, and how it fits into the wider compliance pathway under Directive (EU) 2024/3019.

The short answer is that secondary treatment generally means biological treatment with a secondary settlement stage, or another process that removes biodegradable organic matter. The directive also sets clear population-equivalent thresholds and effluent standards that shape plant design, operation and upgrade strategy.

For a broader overview of the directive framework, see /eu-directive.

The EU legal meaning of secondary treatment

The recast Urban Wastewater Treatment 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 engineering terms, that means the emphasis is on removing the dissolved and colloidal organic load that drives BOD5 and COD in municipal wastewater. In most plants, this is achieved through conventional activated sludge, oxidation ditch, trickling filters, moving bed biological reactors, or other biological processes, followed by clarification or an equivalent solid-liquid separation step.

The legal definition is deliberately functional rather than tied to one process configuration. That matters because the directive does not require a specific reactor type. It requires a treatment outcome: removal of biodegradable organic matter to the specified standard.

What this means in practice

A process can qualify as secondary treatment if it achieves the biological removal function required by the directive. Typical arrangements include:

suspended-growth systems with secondary clarification;
attached-growth systems with an equivalent separation stage;
hybrid biological systems where the process demonstrably removes biodegradable organic matter.

The key point is that the treatment line must deliver compliant effluent, not merely biological activity in isolation.

Where secondary treatment is mandatory

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.

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

The p.e. concept is central because it links the legal threshold to organic load rather than simply headcount. For design and compliance purposes, this means the treated load may differ significantly from the resident population alone, depending on industrial inputs, infiltration, seasonal variation and tourism.

For engineers, the p.e. threshold is therefore not a formality. It is the basis for deciding:

whether secondary treatment is required;
the design basis for treatment capacity;
the degree of hydraulic and organic resilience needed;
the appropriate upgrade pathway for ageing plants.

Effluent standards for secondary treatment

Annex I of the directive sets the secondary-treatment effluent standards. These are:

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

These standards define the level of performance expected at the outlet of secondary treatment. In practice, meeting them consistently requires control of aeration, sludge age, clarification performance, sludge return, and hydraulic loading.

Engineering implications

Although the directive states the numerical effluent values, the process route to compliance is site-specific. Common design and operational considerations include:

sufficient biological capacity for the organic load;
stable oxygen transfer and aeration control;
sludge settleability and clarification performance;
resilience to peak flows and storm events;
upstream screening and primary treatment where applicable;
sludge handling that supports stable process operation.

Indicative design values such as SRT, DO, MLSS and F/M are process- and site-specific. They should be selected through process design, modelling and operational review rather than treated as universal constants.

Meeting the standards reliably

Plants that are close to the threshold, subject to variable influent, or operating with limited redundancy will often need a detailed process assessment. In many cases, the limiting factor is not the biological reaction alone but solids separation and hydraulic stability.

That is why compliance should be assessed across the full secondary treatment line, not just the reactor.

How secondary treatment fits within the wider directive

The recast directive is not limited to secondary treatment. It also introduces staged obligations for nutrient removal, micropollutant removal and energy neutrality. For secondary-treatment plants, this matters because many assets will need to be designed or upgraded with future steps in mind.

Tertiary treatment

Tertiary treatment for nitrogen and phosphorus removal is required:

for plants of 150,000 p.e. and above by 2039;
for plants of 10,000 p.e. and above in areas at risk of eutrophication by 2045.

This is relevant because a plant that is compliant at the secondary level may still require future nutrient-removal upgrades.

Quaternary treatment

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. Funding is to be largely supported through extended producer responsibility for the pharmaceutical and cosmetics sectors.

For secondary treatment planning, the key issue is interface design: available footprint, hydraulic profile, process flexibility and the ability to integrate future treatment stages without major reconstruction.

Energy neutrality

The directive also sets a sector-wide objective of energy neutrality by 2045. This does not change the legal meaning of secondary treatment, but it affects how treatment lines are evaluated. Energy-intensive aeration systems, pumping arrangements and sludge handling strategies may need review when upgrades are planned.

What operators and engineers should check

If you are responsible for a municipal plant or a design brief, the first step is to establish whether the plant is already covered by the secondary-treatment obligation, or will be covered by the 2035 extension.

A practical review should check:

agglomeration size in p.e.;
current biological process type and performance;
outlet BOD5, COD and TSS compliance history;
secondary clarification or equivalent solids separation performance;
hydraulic capacity under wet-weather conditions;
sludge age, oxygen transfer and process stability;
whether the plant can accommodate later nutrient or micropollutant upgrades.

If you are starting a compliance or upgrade study, use the engineering review to confirm the actual treatment bottlenecks and whether the plant line meets the legal definition as well as the performance standard. You can Start the Engineering Assessment.

Common misunderstandings

Secondary treatment is not just aeration

Aeration alone does not satisfy the directive. The treatment must remove biodegradable organic matter and, in the usual case, include a secondary settlement stage or an equivalent separation process.

The p.e. threshold is not the same as population

A plant serving fewer residents can still exceed a p.e. threshold because of industrial loading, infiltration or seasonal peaks. Compliance assessments should therefore be load-based, not only census-based.

Meeting one parameter is not enough

The effluent standard is multi-parameter. A plant that meets BOD5 but fails TSS or COD is not fully compliant.

How this applies to your plant

If your site discharges municipal wastewater in the EU, the first question is whether it falls within the secondary-treatment obligation now or under the 2035 extension. The second question is whether the current treatment line can meet the Annex I effluent standards consistently under real operating conditions.

For ageing plants, the most common issues are not limited to reactor performance. They often include clarification limits, hydraulic overloading, aeration inefficiency and insufficient allowance for future nutrient-removal upgrades.

A structured engineering review will help you determine:

whether the plant meets the legal definition of secondary treatment;
whether the current process can reliably meet BOD5, COD and TSS limits;
whether the plant needs incremental optimisation or capital upgrade;
how to preserve options for future tertiary or quaternary treatment.

If you need to assess an existing plant or define an upgrade basis, use the site-specific engineering review before selecting process changes. Start with Start the Engineering Assessment.