What Wastewater Plants Need to Do Before 2035

Why 2035 matters for wastewater plants

The EU Urban Wastewater Treatment Directive (Directive (EU) 2024/3019) sets a clear direction for the next decade: more agglomerations will need secondary biological treatment, and existing plants will need to prove that their processes are robust enough to meet the required effluent performance.

For operators and engineers, the immediate task is not to design for every later-stage requirement at once. The priority before 2035 is to confirm that the plant can reliably deliver compliant secondary treatment, understand where current assets fall short, and build a realistic upgrade path.

The recast 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 Member States must transpose it into national law by 31 July 2027, and the old Directive 91/271/EEC is repealed with effect from 1 August 2027.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 evolve over time, but the EU-wide framework is already set.

For a broader overview of the directive framework, see /eu-directive.

What the directive requires before 2035

The main secondary-treatment milestone before 2035 is coverage. Secondary biological treatment is mandatory for agglomerations of 2,000 p.e. and above, and this obligation is extended to agglomerations of 1,000 p.e. and above by 31 December 2035.

That extension is important because many smaller plants that were previously outside the full scope of secondary treatment will need to move to a biological treatment basis, subject to transitional and exemption arrangements. The directive also includes specific arrangements for certain coastal areas and for Bulgaria, Croatia and Romania.

In practical terms, this means plant owners should treat 2035 as a planning deadline, not a construction deadline. Procurement, permitting, civil works, process commissioning and operational stabilisation all take time. Sites that wait until the last moment are likely to face avoidable schedule and resilience risks.

What counts as secondary treatment

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 is broad enough to include several common process configurations, provided they consistently remove biodegradable load:

• Activated sludge systems with secondary clarification
• Aerated lagoon or oxidation-based biological treatment where appropriate
• Biofilm-based systems, depending on site design and performance
• Other biological processes that achieve the required organic matter removal

The key point is performance. The plant must be able to remove biodegradable organic matter reliably, not just during favourable weather or low loading conditions.

The effluent standards plants need to meet

Annex I of the recast directive sets the secondary-treatment effluent standards that operators and designers should use as the compliance reference point:

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

These are not design values in themselves; they are the performance standards the plant must achieve. How a site gets there depends on influent characteristics, flow variability, temperature, sludge age, aeration capacity, clarification performance and other process-specific factors.

When reviewing an existing plant, it is useful to ask three questions:

1. Can the process consistently meet the effluent standard under peak and adverse conditions?
2. Are there bottlenecks in aeration, sludge settlement, return sludge handling or hydraulics?
3. Is the current asset base still appropriate for future load growth and wider compliance obligations?

What operators should review now

Before 2035, most plants will benefit from a structured engineering assessment focused on the biological process train and its limits. Typical review items include the following.

1) Influent load and flow variability

Confirm the true organic and hydraulic load entering the plant. Apparent compliance problems often arise from changes in peaking, infiltration/inflow, trade effluent, or seasonal population effects rather than from the biological process alone.

2) Process capacity and stability

Check whether the biological stage has enough capacity to handle the actual load without chronic overload. Indicative process parameters such as SRT, DO, MLSS and F/M ratio should be reviewed as site-specific indicators rather than universal targets. The same applies to aeration intensity and mixing performance.

3) Secondary settlement performance

If the plant relies on secondary clarification, assess sludge settleability, solids capture, hydraulic loading, sludge blanket control and return activated sludge performance. A biological process that removes carbon well can still fail compliance if solids carryover is excessive.

4) Storm and peak flow handling

Many plants are limited by wet-weather hydraulics rather than by steady-state biology. Check bypass risk, inlet works, internal recirculation limits, tank freeboard and the ability to maintain process stability under variable flow.

5) Sludge production and handling

More robust biological treatment usually changes sludge production patterns and thickening/dewatering demand. Review whether the downstream sludge line can support the process without creating hidden bottlenecks.

6) Instrumentation and control

Reliable compliance depends on control, not just tanks and blowers. Online DO control, ammonia feedback where available, flow-paced return sludge, and alarms for abnormal solids loss can improve stability, but the control philosophy must suit the actual plant configuration.

How to plan upgrades without overbuilding

The directive also points towards future tightening beyond secondary treatment, including tertiary treatment, quaternary treatment and energy neutrality. However, before 2035 the most immediate obligation for many sites is still to secure robust secondary treatment and prepare for broader coverage.

That means upgrades should be staged.

A sensible sequence is:

• Confirm current compliance against the secondary-treatment standard
• Remove the main hydraulic or biological bottlenecks first
• Protect settlement and solids retention
• Reserve space and hydraulic flexibility where future nutrient or micropollutant treatment is plausible
• Align capital works with the local transposition timetable

For plants that will later need nutrient removal or quaternary treatment, it is usually wise to protect future expandability in the layout, even if the later process is not yet installed. That can reduce the risk of expensive rework.

The directive also sets longer-term requirements for tertiary and quaternary treatment, and an energy-neutral sector by 2045. Those topics are not the 2035 deadline itself, but they are relevant when deciding whether a near-term rehabilitation should be a simple refurbishment or a more strategic site masterplan.

What this means for different plant types

Small plants moving into scope

Plants serving smaller agglomerations should first determine whether the 1,000 p.e. threshold will bring them into scope by 2035. If so, the main challenge is often choosing a biological process that is simple, robust and affordable to operate.

Existing secondary-treatment plants

Plants already providing biological treatment should focus on reliability. The question is not only whether the plant passes a current sample, but whether it can sustain compliance across seasons, load variation and maintenance events.

Plants nearing capacity

Where flows or loads are rising, the most urgent issue is process headroom. If a plant is already close to the limit, the 2035 compliance timeline may be tight even if current effluent looks acceptable.

How this applies to your plant

If your plant is affected by the 2035 expansion of secondary-treatment obligations, the key question is whether the current process train can reliably meet the Annex I standards and support future regulatory changes without major disruption.

A plant-specific engineering review should cover process performance, hydraulic capacity, sludge settlement, control philosophy and room for future expansion. That review is the right basis for deciding whether you need operational optimisation, targeted refurbishment or a full upgrade path.

If you are planning next steps, Start the Engineering Assessment to identify the most practical compliance route for your site.