EU Wastewater Directive 2035: Secondary Treatment Requirements

EU Wastewater Directive 2035 requirements: what changes for secondary treatment

The recast Urban Wastewater Treatment Directive, Directive (EU) 2024/3019, changes the baseline for municipal wastewater treatment across the EU. For operators, municipalities and consulting engineers, the key 2035 milestone is the extension of secondary treatment obligations to smaller agglomerations. In practical terms, this means more plants will need robust biological treatment and a reliable secondary settlement stage, or another process that achieves equivalent removal of biodegradable organic matter.

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 Member States must transpose it into national law by 31 July 2027, and the old Directive 91/271/EEC is repealed with effect from 1 August 2027. National implementation will therefore vary in timing and detail, but the EU-wide direction is clear: secondary treatment remains the central compliance step for municipal wastewater, and the 2035 extension is a major planning trigger.

For context on the wider framework, see the EU Directive overview.

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. That definition matters because it reflects function rather than a single process train.

In practical plant terms, secondary treatment may be delivered by:

• conventional activated sludge systems,
• oxidation ditches,
• sequencing batch reactors,
• trickling filters with appropriate downstream solids separation,
• moving bed biofilm reactor systems,
• other biological processes that achieve biodegradable organic matter removal.

The essential point is performance: the process must remove the biodegradable load and provide an effluent suitable for discharge under the applicable framework.

The 2035 threshold: who must have secondary treatment

Under the recast directive, secondary (biological) treatment is mandatory for agglomerations of 2,000 p.e. and above. That requirement is extended to agglomerations of 1,000 p.e. and above by 31 December 2035.

For planning purposes, the 2035 date is important because it broadens the compliance base substantially. Plants that previously sat below the older thresholds may now fall within scope, particularly where urban growth, sewer extensions or revised p.e. assessments bring more load into the catchment.

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. This is the reference for sizing and regulatory classification, so load estimation should be checked carefully rather than relying on population alone.

Transitional and exemption arrangements

The directive includes transitional and exemption arrangements, including for certain coastal areas and for Bulgaria, Croatia and Romania. These provisions mean that the legal pathway to 2035 compliance is not identical across all locations.

From an engineering standpoint, the right approach is to:

• confirm the agglomeration p.e. basis used by the competent authority,
• check whether transitional or exemption provisions apply,
• verify how national transposition has framed the EU requirement,
• document the existing treatment performance against the future obligation.

Secondary effluent standards to design against

Annex I of the recast directive sets the secondary-treatment effluent standards. These are the basic benchmarks for biological treatment performance:

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

These are not design set-points in themselves, but they are the compliance targets that the process must reliably meet under the relevant conditions.

What the standards mean operationally

Each parameter points to a different part of plant performance:

• BOD5 reflects removal of biodegradable organics and overall biological process efficiency.
• COD indicates the broader oxidisable load, including fractions not fully captured by BOD5.
• TSS is closely linked to clarification, sludge settleability and solids carryover.

A plant can have satisfactory biological oxidation but still struggle on TSS if secondary settlement is unstable. Similarly, a plant with adequate solids separation may fail the organic limits if aeration, sludge age or recirculation are not well controlled.

Engineering implications for existing and new plants

The 2035 requirement is not just a legal threshold; it is a design and process-control question. Secondary treatment must be sized and operated for variable inflow, seasonal changes, diurnal peaks and wet-weather impacts where relevant.

Typical engineering considerations include:

Influent characterisation

Use site data where available. If data are limited, assess:

• average and peak flows,
• biodegradable organic load,
• industrial or trade contributions,
• storm infiltration and inflow,
• seasonal population variation.

A p.e. value based only on census data may understate or overstate the true organic load.

Biological process stability

Common design parameters such as sludge age, dissolved oxygen, mixed liquor concentration and return sludge ratio are site-specific and should be established from process modelling, pilot data or operational records. The correct values depend on treatment objective, temperature, hydraulics and the selected process configuration.

Indicatively, engineers should examine:

• oxygen transfer capacity and aeration reliability,
• sludge settleability and bulking risk,
• process resilience at low load and shock load,
• internal recycles and hydraulic retention,
• sludge production and handling capacity.

Secondary settlement

Because the directive definition explicitly refers to biological treatment with a secondary settlement stage, or another process that removes biodegradable organic matter, solids separation is central. Settlement performance should be checked for:

• peak solids loading,
• sludge blanket control,
• scum and pin floc behaviour,
• sludge return and wasting arrangements,
• decant or effluent withdrawal hydraulics.

Asset condition and upgrade pathways

For existing works, the most common questions are whether current assets can be optimised, retrofitted or replaced. Options may include:

• aeration refurbishment,
• additional or improved secondary clarification,
• process intensification,
• enhanced control and instrumentation,
• new biological treatment lines,
• flow splitting or equalisation measures.

The best option is rarely universal. It depends on the existing hydraulic profile, land availability, discharge consent context and future load growth.

How 2035 fits with the wider directive timetable

The 2035 secondary-treatment milestone sits within a longer regulatory sequence:

• Directive (EU) 2024/3019 entered into force on 1 January 2025.
• Member States must transpose it by 31 July 2027.
• Directive 91/271/EEC is repealed with effect from 1 August 2027.
• Secondary treatment extends to agglomerations of 1,000 p.e. and above by 31 December 2035.
• Tertiary treatment and quaternary treatment requirements follow later milestones.
• Energy neutrality for the urban wastewater treatment sector is to be achieved by 2045.

For secondary-treatment projects, the practical implication is that planning should not stop at the minimum current compliance position. Plants brought into scope by 2035 may later need further upgrades for nutrient removal or micropollutant treatment, depending on size and location. That does not mean every plant should be overdesigned now, but it does mean that spatial, hydraulic and electrical allowances should be considered early.

What operators and engineers should do now

If your plant may fall within the 2035 scope, the sensible next steps are straightforward:

• confirm the agglomeration p.e. basis and catchment definition,
• review current treatment against the Annex I secondary standards,
• check whether your site already has effective biological treatment and reliable secondary settlement,
• identify hydraulic bottlenecks and solids-handling constraints,
• assess whether any transitional or exemption arrangement applies,
• align investment planning with national transposition timing.

Where the existing process is marginal, the key risk is not only non-compliance but operational fragility. Plants that meet annual averages may still fail during wet weather, cold conditions, high load periods or poor sludge settleability events.

If you need a structured way to assess process adequacy, constraints and upgrade options, Start the Engineering Assessment.

How this applies to your plant

The 2035 requirement is a prompt to test whether your secondary treatment is genuinely robust, not just nominally present. For some plants, the answer will be optimisation and operational control. For others, it will be a retrofit or a new biological treatment stage.

To judge the right path, focus on four questions:

• Does the plant meet the secondary-treatment definition in practice?
• Can it reliably meet BOD5, COD and TSS standards under real operating conditions?
• Is the p.e. basis accurate, and does the site fall within any transitional or exemption arrangement?
• Is the asset base flexible enough for later nutrient or energy-related requirements?

If you are planning upgrades, use the directive timetable as the framework, but base the engineering decision on site data. The fastest route to clarity is to review the current process, hydraulic limits and future compliance risks together in a structured assessment.