Does My Existing WWTP Meet the New EU Requirements?

Introduction

Many existing wastewater treatment plants were designed to meet the requirements in force at the time of construction. That does not automatically mean they will satisfy the recast EU urban wastewater framework now coming into effect across Member States.

For operators and municipalities, the practical question is not simply whether the plant is operating today, but whether it will continue to meet secondary-treatment obligations, effluent standards, and future population growth. In many cases, the main constraint is not the clarifier or aeration tank alone, but overall treatment capacity, process stability, and available headroom.

The right approach is a structured review of design capacity, current load, and process performance. If you need a quick starting point, you can Start the Engineering Assessment and compare your plant against the main upgrade drivers.

What the new EU framework means for existing WWTPs

The recast Urban Wastewater Treatment 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 existing plants, the most relevant changes are not abstract legal points. They are the practical obligations around:

• who must receive secondary treatment,
• what effluent performance must be achieved,
• and when additional treatment stages become necessary.

Secondary treatment is the baseline

Secondary treatment is defined as treatment that generally involves biological treatment with a secondary settlement stage, or another process that removes biodegradable organic matter.

Under the recast directive, secondary biological treatment is mandatory for agglomerations of 2,000 p.e. and above. That obligation is extended to agglomerations of 1,000 p.e. and above by 31 December 2035, subject to transitional and exemption arrangements, including for certain coastal areas and for Bulgaria, Croatia and Romania.

For operators, this means existing small works, package plants, and older municipal plants must be checked against the future coverage threshold as well as current compliance status.

The first question: is the plant legally in scope?

Before you assess process performance, confirm whether the agglomeration served by the plant falls within the scope of the directive. The key issue is population equivalent, or p.e., which is defined as the organic biodegradable load having a five-day biochemical oxygen demand of 60 g of oxygen per day.

A plant can be physically small but still be subject to the directive if the agglomeration it serves exceeds the relevant threshold.

Practical scope check

Ask the following:

• What is the current p.e. served by the agglomeration?
• Is the served population rising due to housing growth, tourism, or industrial discharge?
• Is the plant already at or near its design hydraulic capacity?
• Does the plant achieve stable biological treatment under peak wet-weather conditions?
• Are there known bypasses, overflows, or reduced effluent quality during high load periods?

If the answer to any of these is uncertain, the plant should be reviewed as though an upgrade may be needed.

Capacity is usually the limiting factor

The most common reason an existing WWTP does not meet future requirements is insufficient capacity, not a complete lack of treatment units.

Capacity problems usually appear in one or more of these forms:

• hydraulic overload during storm events,
• organic overload from growth or changed influent strength,
• insufficient aeration capacity,
• inadequate sludge handling or settlement performance,
• limited reactor volume or solids retention time,
• poor resilience to inflow and infiltration.

A plant can still produce acceptable results on average while failing during peak periods. From an engineering perspective, that is often the point at which compliance risk begins.

What to check in an existing plant

A capacity review should examine:

• average and peak flows,
• influent BOD5, COD, and suspended solids,
• available aeration volume and oxygen transfer capability,
• clarifier loading and sludge blanket behaviour,
• return activated sludge and waste sludge handling,
• process control strategy and redundancy,
• seasonal variation in temperature and load.

These are site-specific engineering checks. There is no universal pass/fail number for all plants, because design margins, process type, and wet-weather arrangements differ.

Does the plant meet the secondary effluent standards?

For secondary treatment, the annexed effluent standards are:

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

These are the figures against which many existing works will need to be judged in practice.

Why a plant may fail even if it looks stable

A plant may appear to be performing acceptably if the operator focuses only on routine average results. However, failure often arises when:

• peak hydraulic loading shortens settling time,
• sludge age drops below the level needed for stable biology,
• aeration becomes insufficient during warm-weather load peaks,
• clarifiers wash solids out under storm conditions,
• upstream processes fluctuate and destabilise the biology.

For many older plants, the issue is not simply whether the biology works, but whether it works reliably across the full annual load profile.

Indicative engineering considerations

When reviewing secondary treatment performance, engineers commonly look at:

• mixed liquor suspended solids in relation to process stability,
• dissolved oxygen control and air distribution,
• sludge retention time as a stability indicator,
• food-to-microorganism balance,
• clarifier surface loading and sludge return efficiency.

These are indicative, site-specific parameters. They should be interpreted in the context of the actual process configuration and permit conditions.

Will you need tertiary or quaternary treatment later?

For many existing plants, the immediate question is secondary treatment. However, it is sensible to consider the next compliance layer during any upgrade review.

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. Energy neutrality for the urban wastewater treatment sector is to be achieved by 2045.

This does not mean every secondary plant needs immediate redesign for nutrient or micropollutant removal. It does mean that if a major upgrade is being considered now, the layout should be reviewed with future treatment stages in mind.

Upgrade planning should avoid dead-end investments

A short-term fix can become expensive if it blocks future expansion. For example:

• a new tank may improve current capacity but leave no space for tertiary units later,
• a blower upgrade may solve aeration shortfall but not clarifier limitation,
• a control improvement may stabilise performance but not recover lost hydraulic headroom.

An upgrade should be assessed as a whole system, not as a collection of isolated equipment replacements.

What usually triggers an upgrade decision?

An upgrade is typically needed when the plant can no longer meet one or more of the following:

• current effluent limits,
• projected future load,
• wet-weather hydraulic capacity,
• sludge settlement and solids retention performance,
• operational resilience under variable inflow,
• future compliance obligations under the recast directive.

The important point is that a plant does not need to be in visible distress before action is justified. If the margin to compliance is shrinking, a planned assessment is usually cheaper and less disruptive than waiting for repeated exceedances.

Common upgrade pathways

Depending on the process and site constraints, an existing WWTP may need:

• aeration capacity enhancement,
• additional reactor volume,
• clarifier upgrade or extra settlement capacity,
• flow balancing or storm tank improvements,
• sludge handling improvements,
• process optimisation and better automation,
• conversion to a different biological configuration.

The correct solution depends on the bottleneck identified in the assessment.

How this applies to your plant

If your WWTP was designed under older assumptions, the new EU framework may affect it in three ways:

1. the plant may now fall within a mandatory secondary-treatment scope,
2. the current process may not reliably meet the Annex I effluent standards,
3. the site may lack sufficient capacity for future growth or tighter performance expectations.

The next step is not to guess. It is to review the plant’s actual performance, load profile, and residual capacity against the new requirements.

If you are responsible for an existing works and suspect a capacity shortfall, Start the Engineering Assessment to identify whether the plant needs optimisation, expansion, or a full biological upgrade.