What Happens After 2035? EU Secondary Treatment Outlook

What changes after 2035?

For many wastewater treatment plants, 2035 is not the end point. It is the point at which the EU baseline for secondary treatment becomes broader, while pressure also increases from nutrient removal, micropollutants and energy performance requirements.

Under the recast Urban Wastewater Treatment Directive, secondary treatment remains the core biological barrier for biodegradable organic matter. It is already mandatory for agglomerations of 2,000 p.e. and above, and that obligation extends to agglomerations of 1,000 p.e. and above by 31 December 2035, subject to transitional and exemption arrangements. For operators, that means the period after 2035 is mainly about three things:

• ensuring the biological process is robust enough for future loading and consent conditions
• planning upgrades where secondary treatment is currently absent or marginal
• aligning longer-term compliance with nutrient, micropollutant and energy requirements

If you need a structured view of how the directive affects your site, start with the EU Directive overview and then Start the Engineering Assessment.

The post-2035 baseline: secondary treatment is wider, not looser

The directive defines secondary treatment as treatment that generally involves biological treatment with a secondary settlement stage, or another process that removes biodegradable organic matter. In practical terms, that means activated sludge systems, biological filters, lagoons with suitable performance, and other biological configurations can all be part of compliance if they achieve the required function and effluent quality.

The important point is that post-2035 does not mean a different biological principle. It means a wider obligation.

What remains central

Secondary treatment will still be judged on whether the plant can consistently remove biodegradable organic load and deliver the Annex I effluent standards:

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

Those figures are the legal anchor for the secondary-treatment stage. A plant that only achieves them intermittently is at risk when flows, loads, weather and infiltration all vary across the year.

What becomes more important after 2035

After 2035, operators will increasingly need to look beyond the nominal process diagram and ask whether the plant can do the following reliably:

• absorb peak wet-weather flows without washing out biomass
• retain solids through the secondary settler
• manage shock loads from industrial or seasonal discharge patterns
• maintain process stability at low temperatures and variable influent strength
• avoid losing compliance when one unit is out of service

These are not new process concepts, but they become more critical as more agglomerations are brought into scope and as enforcement attention shifts from basic provision to performance.

What plants may need to change

The exact upgrade path depends on the existing process, available footprint, influent variability and whether the plant is already designed with spare capacity. Typical retrofit themes are well known to practitioners, but they remain site-specific.

1) Improve biological capacity

Where the limiting factor is oxygen transfer, biomass inventory or reactor volume, the solution may involve process intensification rather than a full rebuild. Common options include:

• aeration upgrades
• better process control
• adding an anoxic or selector zone where needed for stability
• reconfiguration of reactor compartments
• enhanced sludge recycle control

Indicative design values such as dissolved oxygen, SRT, MLSS or F/M ratio should always be set on the basis of the actual influent, temperature profile and process objective. There is no universal number that suits every site.

2) Protect the secondary settlement stage

Many compliance problems originate in solids separation rather than biological conversion itself. If the final clarifier is overloaded, the plant may fail BOD, COD and TSS targets even when the biological reactor is performing acceptably.

Common issues include:

• high surface overflow rates during storms
• poor sludge blanket control
• filamentous bulking or pin floc
• inadequate sludge return capacity
• hydraulic short-circuiting

For older works, the post-2035 question is often whether the existing clarifiers can still support the required effluent standard after upstream changes or tighter flow conditions.

3) Add resilience, not just nominal capacity

A plant can meet average-day loading and still be vulnerable. After 2035, the more resilient asset is usually the one that can maintain performance across a wider envelope of operating conditions.

This often means designing or retrofitting for:

• redundancy in critical equipment
• reliable standby blowers and return sludge pumping
• better instrumentation and online control
• improved storm response strategy
• clearer maintenance windows with less dependence on manual intervention

4) Check the limits of the existing footprint

For many municipal works, the main constraint is space. Secondary treatment upgrades are often limited by the ability to expand tanks, add clarifiers, or reroute flows within the current site boundary.

Where footprint is constrained, operators may need to consider:

• compact process reconfiguration
• intensification of existing units
• phased upgrades while maintaining treatment
• targeted bottleneck removal rather than whole-plant replacement

These choices should be tested against both hydraulic capacity and long-term operating stability.

How 2035 fits with the longer EU timetable

It is important not to treat 2035 as the final compliance milestone.

The recast directive also sets later obligations for treatment beyond conventional secondary treatment:

• 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

For plant operators, this matters because secondary treatment investments should not be made in isolation. A short-term fix that leaves no route to nutrient removal, micropollutant treatment or energy optimisation may become expensive later.

Practical implications for operators and municipalities

The post-2035 environment is mainly a planning challenge.

For operators

Focus on the process risks that affect secondary performance first:

• clarifier capacity and solids carryover
• aeration and oxygen transfer constraints
• sludge handling and return rates
• wet-weather hydraulic loading
• process monitoring and alarm response

If the plant already struggles to meet BOD5, COD or TSS under variable conditions, it is unlikely to have much margin for future compliance pressure.

For municipalities

The central questions are strategic:

• Does every agglomeration in the service area have a clear route to compliant secondary treatment?
• Are the smallest works, especially those approaching the 1,000 p.e. threshold, included in the upgrade plan?
• Are capital works sequenced so that future tertiary or quaternary upgrades do not require avoidable rework?
• Is there a realistic plan for operation, maintenance and process control after commissioning?

Because national transposition and implementation timing can vary, local obligations should always be checked against the national framework once the directive is transposed.

How this applies to your plant

If your site is already compliant today, the right question is whether it will remain compliant under a wider scope, higher hydraulic stress and future upgrade pressures after 2035. If your site is not yet compliant, the priority is to identify whether the bottleneck is biological conversion, settlement, hydraulics or operational control.

A good assessment should test:

• current and projected p.e. loading
• hydraulic profile in dry and wet weather
• BOD5, COD and TSS performance margin
• clarifier and sludge return capacity
• scope for staged upgrades within the existing footprint
• compatibility with later nutrient, micropollutant and energy objectives

If you want a structured way to review these issues, Start the Engineering Assessment. It will help identify whether your plant needs operational optimisation, a targeted retrofit, or a longer-term process upgrade pathway.

In short, after 2035 the question is not whether secondary treatment still matters. It does. The real question is whether the plant has enough process margin to remain compliant while the regulatory framework keeps moving upward around it.