EU Wastewater Directive 2030: Secondary Treatment Requirements

What the EU Wastewater Directive means for 2030

The EU Urban Wastewater Treatment Directive, recast as Directive (EU) 2024/3019, sets the framework that will shape municipal wastewater compliance through 2030 and beyond. For operators and engineers, the most immediate issue is secondary treatment: where it is required, what effluent standards apply, and how existing works should be assessed against future obligations.

Although the directive covers more than secondary treatment, this article focuses on the biological treatment stage and the practical implications for wastewater treatment plants, municipalities and consulting engineers. If you want the broader directive context, see the EU Directive overview.

Timeline and regulatory context

The recast 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 the new rules into national law by 31 July 2027.

For plant planning, this timing matters because the legal framework is already in force at EU level, while national implementation will vary by Member State. In practice, operators should expect local transposition measures, permitting updates and compliance programmes to be developed during the transition period.

A key point for 2030 planning is that the directive does not treat secondary treatment as a legacy issue. It remains the baseline biological treatment requirement for many agglomerations, and the population thresholds will expand over time.

What counts as secondary treatment

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.

In engineering terms, this includes the familiar activated sludge family and other biological systems that achieve removal of biodegradable load, provided the treatment objective and performance are met. The definition is intentionally process-neutral, which allows flexibility in technology selection while keeping the compliance focus on effluent quality.

Typical process forms

Common secondary treatment approaches include:

• conventional activated sludge
• extended aeration
• oxidation ditches
• sequencing batch reactors
• moving bed or attached-growth biological systems where they achieve the required biodegradable organic matter removal

The directive itself does not prescribe a single design route. That is useful for utilities because influent characteristics, footprint, seasonal loads, stormwater interactions and sludge management constraints often determine the most robust solution.

Secondary treatment obligations and p.e. 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.

This is the core 2030 planning issue for many small and medium-sized settlements: a plant that is compliant today may still need to be reviewed for the next phase of obligations. The lower threshold will bring more agglomerations into scope, although the directive also includes 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 p.e. matters in engineering practice

The p.e. concept is not just a legal label. It is the basis for comparing influent load, process capacity and treatment obligation. When assessing an existing plant, engineers should verify:

• the actual connected population and industrial contributions
• infiltration and inflow effects on hydraulic loading
• seasonal tourism or intermittent load spikes
• whether current treatment performance reflects stable biological removal or reliance on dilution

Where the site is close to a threshold, the compliance question should be based on documented load assessment rather than assumptions drawn from nominal design capacity alone.

Annex I secondary-treatment effluent standards

For secondary treatment, 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 are the key parameters that define whether the biological stage is delivering the required performance. In practice, compliance is a function of process stability as well as final settlement and sludge separation. A plant with adequate reactor volume but poor secondary clarification may still fail to meet the standards consistently.

What this means operationally

For operators, the standards point to several common control priorities:

• maintain a stable biomass inventory
• avoid washout during hydraulic peaks
• control sludge blanket levels in secondary settlement
• manage filamentous growth and bulking risk
• ensure aeration and mixing are sufficient for the influent load

The directive sets the outcome; it does not mandate a particular oxygen setpoint, sludge age or mixed liquor concentration. Those are site-specific design and optimisation variables that should be selected based on influent strength, temperature, process type and downstream requirements.

How to assess whether an existing plant is ready

An existing works should be reviewed from both compliance and process-resilience perspectives. A plant that nominally achieves secondary treatment may still need modification if the current configuration is marginal under wet-weather conditions, seasonal load variation or growth in connected load.

A practical engineering assessment should cover:

• influent load characterisation against p.e. capacity
• current effluent performance for BOD5, COD and TSS
• hydraulic capacity in dry and wet weather
• condition and performance of primary, biological and secondary settlement units
• aeration system condition and control philosophy
• sludge production, return sludge and wasting arrangements
• odour, energy and resilience considerations
• headroom for future tightening or expansion

For sites facing compliance uncertainty, the most efficient next step is usually a structured plant review rather than immediate assumptions about process replacement. You can Start the Engineering Assessment to evaluate current performance and upgrade options.

Relationship to tertiary and quaternary treatment

The 2030 conversation often extends beyond secondary treatment because future EU obligations also include nutrient removal and micropollutant removal at larger plants. However, those requirements are distinct from the baseline biological obligation discussed here.

For context only:

• 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 most operators, the immediate engineering question is still the condition of the secondary stage: if the biological process is not stable, any later treatment train becomes more difficult to design and operate.

Practical design and upgrade considerations

The directive is technology-neutral, but compliance depends on sound process design. When reviewing or upgrading a biological treatment stage, engineers typically focus on the following indicative, site-specific factors:

Biological process stability

• adequate reactor volume and contact time for the load profile
• sufficient dissolved oxygen transfer capacity
• appropriate sludge age and biomass retention
• resilience to shock loads and toxic events

Secondary settlement and solids separation

• clarifier loading under dry-weather and peak-flow conditions
• sludge blanket control and return activated sludge stability
• scum and solids capture performance
• hydraulic short-circuiting risks

Sludge and energy implications

• waste sludge handling and downstream treatment capacity
• aeration energy demand and control efficiency
• integration with digesters or other sludge treatment units
• opportunities for operational optimisation before major capital works

These are not universal design numbers; they are assessment themes. Final sizing and setpoints must be developed from site data, process modelling and operational constraints.

How this applies to your plant

If your agglomeration is already above 2,000 p.e., secondary treatment should be treated as a current compliance obligation, not a future possibility. If you are between 1,000 and 2,000 p.e., the 31 December 2035 extension means the plant should still be considered in medium-term planning, especially where growth, tourism or infiltration may push the site into scope earlier than expected.

For plant owners and consultants, the right next step is to confirm:

• whether the agglomeration is within the relevant p.e. threshold
• whether existing biological treatment and secondary settlement reliably meet Annex I performance
• whether the plant has enough hydraulic and process headroom for future load growth
• whether the configuration will remain viable if tertiary or quaternary treatment is later required

If you need a structured route from regulatory requirement to process decision, Start the Engineering Assessment. It is the most direct way to compare current performance, identify bottlenecks and define a realistic upgrade strategy for secondary treatment compliance.