Secondary Treatment Under the New EU Wastewater Directive

Secondary treatment in the new EU framework

The recast Urban Wastewater Treatment Directive, Directive (EU) 2024/3019, changes the compliance context for municipal wastewater treatment across the European Union. For operators and designers, the most immediate question is not the legal text alone, but what it means for the biological stage of a plant: what must be treated, to what standard, and by when.

In practical terms, secondary treatment remains the core biological removal step for biodegradable organic matter. Under the directive, it generally involves biological treatment with a secondary settlement stage, or another process that achieves the removal of biodegradable organic matter. That definition is broad enough to cover conventional activated sludge, biofilm-based systems, and other process configurations, provided the required performance is achieved.

For background on the wider legal framework, see the EU directive overview page: /eu-directive.

What the directive means by secondary treatment

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.

From an engineering perspective, this means the focus is on treatment performance rather than one prescribed process. The essential outcome is the reduction of biodegradable pollution, usually expressed through BOD5 and related indicators such as COD and suspended solids.

Typical process routes include:

• conventional activated sludge with final clarification
• oxidation ditch systems
• sequencing batch reactors
• attached-growth systems where the overall process delivers equivalent biodegradable organic matter removal
• hybrid configurations combining suspended-growth and biofilm treatment

The key point is that the plant must be able to sustain performance under real influent variability, not only under average dry-weather conditions.

Scope and timing: who is covered and when

The recast directive establishes a staged expansion of secondary treatment obligations.

Population equivalent thresholds

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.

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.

Transposition and repeal timing

The directive entered into force on 1 January 2025.

Member States must transpose it into national law by 31 July 2027. The old Directive 91/271/EEC is repealed with effect from 1 August 2027.

For plant owners, this means the legal basis for implementation will shift at national level during the transposition period. The directive sets the EU framework, but the detailed application, administrative steps and any transitional arrangements will be handled through national law.

Transitional and exemption arrangements

The directive includes transitional and exemption arrangements, including for certain coastal areas and for Bulgaria, Croatia and Romania. Because these provisions depend on the legal and geographical context of each case, operators should verify how their specific agglomeration is treated under national implementation.

Annex I effluent standards for secondary treatment

For conventional secondary treatment, the directive’s Annex I sets the following effluent standards:

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

These values are compliance targets for the treated effluent. In practice, they should be considered alongside influent quality, flow variability, weather-driven dilution, industrial contributions and the stability of the biological and solids separation stages.

Why these parameters matter operationally

The standards reflect the three main dimensions of secondary treatment performance:

• BOD5 shows how effectively biodegradable organic matter is removed.
• COD captures a broader measure of oxidisable material and is useful for checking the overall treatment burden.
• TSS indicates solids carryover and the quality of clarification or solids separation.

A plant may meet one parameter consistently while struggling with another. For example, a process can achieve good organic removal but still fail TSS due to poor settling, hydraulic overload, filamentous bulking, storm inflow, or inadequate return sludge control. For that reason, compliance assessment should examine the process as a system, not a single tank or unit process.

Engineering issues that drive compliance

The directive does not prescribe a single design, but it does set a clear performance expectation. That puts the engineering burden on the plant owner and designer to make sure the process can deliver compliant operation across the full range of expected conditions.

Biological capacity and load variability

Secondary treatment capacity is governed by the balance between influent load, oxygen transfer, biomass concentration, sludge age and solids separation performance. Plants that were originally sized for stable domestic sewage may now face higher and more variable loads due to population growth, infiltration/inflow, trade effluent and seasonal changes.

Indicative design and operating values such as mixed liquor suspended solids, dissolved oxygen, food-to-microorganism ratio and solids retention time are site-specific. They should be reviewed against actual influent data, process configuration and required effluent quality rather than assumed from generic rules.

Secondary settlement and solids washout

Because the directive’s definition generally includes a secondary settlement stage, clarification performance becomes a central issue for many plants. Even where the biological stage is adequate, poor sludge settleability or hydraulic overloading can drive suspended solids exceedance and, in turn, compromise BOD5 compliance.

Common operational checks include:

• final clarifier solids loading and hydraulic loading
• sludge blanket control
• return activated sludge rate stability
• sludge settleability and blanket rise behaviour
• peak wet-weather performance

Shock loads and wet-weather performance

Many secondary treatment plants are challenged less by average conditions than by short periods of hydraulic or organic shock loading. Wet-weather events can reduce hydraulic residence time, resuspend solids and increase washout risk. Industrial discharges can create toxicity, oxygen demand spikes or process inhibition.

A compliance review should therefore include both steady-state and upset-condition behaviour. If the plant only meets the standard in dry weather, it may still be operationally vulnerable.

What operators should review now

Although national implementation timelines vary, the engineering implications are immediate. Plants should assess whether the current process can reliably achieve the Annex I standards and whether the works have enough operational flexibility for future load growth.

A practical review should cover:

• current agglomeration size in p.e. and projected growth
• actual effluent performance against BOD5, COD and TSS
• performance during wet weather and peak loading
• sludge settlement behaviour and clarifier condition
• aeration capacity and distribution
• redundancy of critical assets such as blowers, pumps and final settlement units
• influent variability, infiltration/inflow and trade waste effects
• instrumentation, control philosophy and operator response procedures

Where plants are already close to their compliance limits, the most cost-effective action is often to identify the actual bottleneck before considering process expansion. That may be aeration, secondary settlement, upstream screening and grit removal, sludge wasting control, or simply insufficient hydraulic capacity.

Relationship to tertiary, quaternary and energy targets

This article focuses on secondary treatment, but the recast directive places it within a broader tightening of urban wastewater performance.

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. The directive also sets an energy neutrality objective for the urban wastewater treatment sector by 2045.

For many works, this means secondary treatment reviews should not be carried out in isolation. A plant that is being upgraded for nutrient removal or energy efficiency should still verify that the biological and solids separation stages are robust enough to support the longer-term compliance pathway.

How this applies to your plant

If your plant serves an agglomeration of 2,000 p.e. or more, secondary treatment is already within scope under the new EU framework. If the catchment is between 1,000 and 2,000 p.e., the obligation may apply by 31 December 2035, subject to the directive’s transitional and exemption arrangements.

The most useful next step is a structured engineering check of:

• current compliance against the Annex I secondary standards
• process headroom for load and flow variation
• secondary settlement performance
• future growth against p.e. thresholds
• readiness for national transposition requirements

If you need a practical way to start, Start the Engineering Assessment. A focused review will show whether your existing biological process can meet the new directive’s performance expectations, or whether you need targeted upgrades to aeration, clarification, sludge handling or hydraulic capacity.

For further context on the directive itself, refer to /eu-directive.