What Is Biological Treatment Under the EU Wastewater Directive?

Introduction

Under the EU Urban Wastewater Treatment Directive, biological treatment is the central process used to remove biodegradable organic matter from municipal wastewater. In practical terms, it is the treatment stage that converts dissolved and colloidal organic pollution into biomass and treated effluent, usually followed by separation of solids in a secondary settlement stage.

For operators, municipalities and consulting engineers, the key question is not only what biological treatment means in principle, but how it is defined in the recast directive, when it is required, and what performance level must be achieved. The answer matters for plant upgrades, compliance planning and long-term process selection.

For an overview of the directive framework, see /eu-directive. If you are assessing an existing plant or planning an upgrade, you can also Start the Engineering Assessment.

What the directive means by biological 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.

That definition is important because it is deliberately process-neutral. It does not prescribe one specific configuration. Activated sludge, aerated lagoons with solids separation, biofilm systems and other biological processes may all be relevant if they achieve the required removal of biodegradable organic load.

In engineering terms, biological treatment relies on microorganisms to consume organics in the wastewater. The process may include:

• carbon oxidation for biodegradable organic matter removal;
• biomass growth and endogenous respiration;
• oxygen transfer through aeration where an aerobic process is used;
• settling or other solids separation to retain biomass and produce a clarified effluent.

The directive is concerned with the treatment outcome. The exact process train remains a design decision based on influent characteristics, site constraints, climate, footprint, energy use, sludge handling and operational skill level.

Where biological treatment sits in the EU framework

Biological treatment is the core of secondary treatment, and secondary treatment is the baseline compliance requirement for agglomerations at or above a defined size.

Under the recast directive:

• secondary treatment is mandatory for agglomerations of 2,000 p.e. and above;
• this obligation is extended to agglomerations of 1,000 p.e. and above by 31 December 2035;
• transitional and exemption arrangements apply, 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 (BOD5) of 60 g of oxygen per day.

This definition matters because agglomeration sizing under the directive is based on load, not just resident population. Industrial contributions and seasonal variation can therefore affect whether a system falls within scope.

It is also important to distinguish between treatment levels:

• secondary treatment: removal of biodegradable organic matter;
• tertiary treatment: nutrient removal, specifically nitrogen and phosphorus;
• quaternary treatment: micropollutant removal.

This article focuses on secondary treatment only, but those later stages are relevant because they may be added to a biological plant as part of a future upgrade strategy.

Performance standards for secondary treatment

The directive sets EU-wide secondary-treatment effluent standards in Annex I. These are the reference values a plant must meet when secondary treatment is required:

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

These are compliance standards, not design targets in isolation. A plant should be designed and operated with enough process resilience to meet them under expected seasonal and hydraulic variations.

In practice, performance depends on:

• influent strength and variability;
• hydraulic loading and storm influence;
• temperature and seasonal biology;
• reactor configuration and solids retention;
• clarifier performance and sludge settleability;
• aeration control and process stability;
• sludge wasting and return activated sludge management.

For design and optimisation, the following process values are typically site-specific and should be treated as indicative rather than universal:

• dissolved oxygen setpoints;
• sludge age or solids retention time;
• mixed liquor suspended solids;
• food-to-microorganism ratio;
• hydraulic and organic loading rates;
• return and waste sludge rates.

A stable biological process is usually easier to maintain when the hydraulic and solids separation steps are matched to the biological reactor. In many plants, poor clarification rather than insufficient biodegradation is the limiting factor.

Common biological process configurations

The directive does not mandate a specific technology, so the selection of the biological process should follow the site’s actual constraints. Common secondary-treatment options include:

Activated sludge

This is the most familiar biological process in municipal wastewater treatment. Wastewater is aerated in a reactor, microorganisms degrade soluble organics, and the biomass is separated in a secondary clarifier.

Typical design considerations include:

• basin volume and reactor configuration;
• aeration equipment and control;
• sludge settleability;
• sludge return and wasting strategy;
• robustness to peak wet weather flow.

Biofilm-based systems

Processes such as trickling filters or moving bed biofilm reactors can also provide biological removal of biodegradable organics. They are often considered where footprint, operational simplicity or resilience are important.

Key considerations include:

• media or filter loading;
• oxygen transfer and ventilation;
• downstream solids capture;
• maintenance access and fouling control.

Lagoon and pond systems

In some locations, stabilisation ponds or aerated lagoons may provide secondary treatment where land is available and loading is compatible with the process.

These systems are usually more sensitive to climate and footprint, but they can be appropriate where low operational complexity is a priority.

Hybrid and upgraded systems

Many existing works use a combination of processes, for example:

• primary settlement plus activated sludge;
• fixed-film media added to an existing suspended-growth reactor;
• staged biological zones with improved control of carbon removal.

Hybridisation is often considered where an existing plant needs to improve compliance without complete reconstruction.

What operators should review when checking compliance

For an existing plant, the first step is not necessarily a new process selection, but a structured review of current capability against the directive’s requirements.

Useful questions include:

• Is the agglomeration within the scope of the directive now or in the future?
• Does the plant have reliable secondary treatment with solids separation?
• Can the process meet the Annex I BOD5, COD and TSS standards consistently?
• Are storm flows, infiltration and inflow affecting performance?
• Is the clarifier capacity sufficient for current and projected loading?
• Are aeration and sludge age control stable across seasonal conditions?
• Is there enough operational data to demonstrate compliance?

Where the answer to any of these is uncertain, a process review is warranted. That may include hydraulic assessment, biological capacity assessment, clarification review and process modelling.

A plant that appears compliant on average values may still struggle during wet weather or winter operation. For that reason, compliance should be assessed across representative load and flow conditions, not only against one-off sampling results.

How this links to future upgrades

Biological treatment is also the platform for future compliance with tighter EU requirements. Secondary treatment may later need to be complemented by:

• tertiary treatment for nutrient removal;
• quaternary treatment for micropollutant removal.

According to the recast directive, tertiary treatment will be 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 will be required for plants of 150,000 p.e. and above by 2045, with interim milestones in 2033 and 2039.

Even if those stages are not yet required at a given works, they can influence current design decisions. For example, preserving hydraulic head, allowing space for future process units, and maintaining a flexible aeration and sludge management system can reduce the cost and disruption of later upgrades.

Energy use is also relevant. The directive sets an energy-neutrality objective for the urban wastewater treatment sector by 2045. Biological treatment is typically the dominant energy consumer in secondary treatment, particularly where aeration demand is high. That makes process efficiency, oxygen control and load balancing important beyond simple compliance.

How this applies to your plant

If your plant is operating under the EU framework, biological treatment should be checked in three layers:

1. Scope: confirm whether the agglomeration falls within the directive’s current or future secondary-treatment obligation.
2. Performance: verify whether the biological process and secondary settlement stage can meet the Annex I limits for BOD5, COD and TSS.
3. Resilience: assess whether the plant remains compliant under wet weather, seasonal temperature variation and projected growth.

If the answer is uncertain, a structured engineering review is the right next step. Start with the process line, clarifier performance, aeration capacity, sludge handling and future upgrade options. Then compare the findings against the directive framework and your local transposition requirements.

To begin that review, Start the Engineering Assessment.