EU Urban Wastewater Treatment Directive 2024/3019 Explained

What Directive (EU) 2024/3019 changes

Directive (EU) 2024/3019 is the recast Urban Wastewater Treatment Directive. For operators, municipalities and consulting engineers, the practical point is simple: the EU framework for urban wastewater collection and treatment has been updated, and the biological treatment obligations are being widened over time.

The 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 directive into national law by 31 July 2027. Implementation on the ground will still depend on national legislation, asset programmes and local catchment conditions, but the EU-wide obligations are now clear.

For a broader overview of the directive framework, see the EU Directive page.

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, that normally means an activated sludge process, biofilm process, or another biological route that achieves removal of readily biodegradable organic load. The directive does not prescribe one process configuration. The compliance question is whether the treatment achieves the required reduction and effluent quality for the receiving agglomeration and discharge context.

Common biological process types include:

• conventional activated sludge
• oxidation ditch systems
• sequential batch reactors
• moving bed biofilm reactors
• trickling filters and other fixed-film systems

The choice of process remains site-specific. In practice, selection depends on influent strength and variability, available footprint, hydraulic balance, sludge handling capacity, receiving water sensitivity and operator capability.

The main population-equivalent thresholds

The directive extends the reach of secondary treatment in stages.

Mandatory coverage from 2,000 p.e.

Secondary biological treatment is mandatory for agglomerations of 2,000 p.e. and above.

Extension to 1,000 p.e. by 2035

That obligation is extended to agglomerations of 1,000 p.e. and above by 31 December 2035, with transitional and exemption arrangements, including for certain coastal areas and for Bulgaria, Croatia and Romania.

For asset planners, that means the lower end of the municipal wastewater market is now part of the long-term compliance horizon. Smaller plants that previously sat outside a secondary-treatment obligation may now require feasibility work, land appraisal, network studies or phased upgrades.

What p.e. means

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 definition matters because p.e. is a load-based measure, not just a headcount. Industrial contributions, infiltration, and seasonality can affect the effective loading on a works, so the apparent resident population may not match the compliance size basis.

Secondary-treatment effluent standards

Annex I of the recast directive sets the secondary-treatment effluent standards as follows:

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

These are the key parameters to check when reviewing a biological treatment process and its secondary settlement performance.

What this means in operational terms

The standards point to three linked functions:

• biodegradation of soluble organics, reflected in BOD5 and COD
• solids separation, reflected in TSS
• stable clarification and sludge settleability, which underpin both of the above

From an operating standpoint, a plant that is meeting the organic removal target but losing solids through the final settlement stage may still fail compliance. Likewise, a works with good settlement but poor biological activity will not achieve the required BOD5 and COD reductions.

Typical design and operating values for biological treatment are site-specific, but engineers commonly review:

• mixed liquor suspended solids
• dissolved oxygen setpoints
• sludge age or solids retention time
• food-to-microorganism balance
• sludge volume index and settlement behaviour
• hydraulic retention and peak wet-weather response

These are not fixed regulatory numbers. They are process controls used to achieve reliable treatment under the actual influent and climate conditions of the site.

How the directive changes long-term planning

The recast directive is not only about immediate compliance. It also changes the planning context for the next two decades.

Tertiary treatment context

For context, 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.

This is not the core focus of secondary-treatment design, but it affects upstream biological process selection. A plant that may later need nutrient removal should not be designed only for today’s carbon removal if that would block future retrofit.

Quaternary treatment context

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 is funded largely through extended producer responsibility for the pharmaceutical and cosmetics sectors.

Again, this is downstream context rather than a secondary-treatment duty. Still, it influences site layout, energy demand, hydraulic profile and future upgrade corridors.

Energy neutrality by 2045

The urban wastewater treatment sector is to achieve energy neutrality by 2045.

For biological treatment assets, that makes process efficiency more important than ever. Lower aeration demand, improved sludge handling, better primary solids capture where applicable, and digester or biogas integration can all support the broader energy balance. The directive does not set a single plant-level design recipe, so the engineering response must be tailored to each works.

Practical engineering implications for operators

For operators and designers, the directive should trigger a structured review of the treatment line rather than a narrow compliance check.

1. Confirm the agglomeration basis

Start by confirming:

• the current p.e. basis used for the agglomeration
• whether industrial loads or seasonal peaks materially change the load basis
• whether the works falls above or below the relevant thresholds now and in future planning scenarios

2. Review biological process performance

Check whether the existing process can consistently meet the Annex I secondary-treatment standards under:

• dry weather
• storm flow conditions
• low temperature periods
• shock loads and variable influent strength

The key question is not whether the plant can pass a single grab sample, but whether it can reliably maintain process stability across the year.

3. Assess clarifier and sludge handling capacity

Secondary treatment depends on effective solids separation. Review:

• secondary clarifier loading and hydraulic capacity
• return activated sludge and waste sludge handling
• bulking or pin floc risk
• sludge thickening and dewatering constraints

These elements often determine whether a biological process remains compliant during peak flows or higher load periods.

4. Plan for future retrofits

Where future tertiary or quaternary requirements are plausible, reserve space and hydraulic headroom early. It is usually more efficient to make medium-term allowances now than to force a constrained retrofit later.

5. Link compliance to catchment planning

National transposition will vary, but the underlying EU direction is clear: more agglomerations will fall under secondary treatment, and more plants will face nutrient and energy requirements over time. That means network condition, infiltration/inflow control and growth forecasts matter as much as the reactor process itself.

How this applies to your plant

If your plant is currently designed around carbon removal alone, or if it serves an agglomeration near the 1,000 p.e. or 2,000 p.e. thresholds, now is the time to check the long-term treatment strategy.

A practical review should confirm:

• the current and forecast p.e. basis
• compliance with the Annex I secondary-treatment standards
• clarifier, aeration and sludge handling limits
• whether future nutrient or micropollutant treatment would fit on site
• whether energy balance and process efficiency support the 2045 direction of travel

If you want to turn the directive into an action list for your works, Start the Engineering Assessment.