Does My WWTP Need Secondary Treatment?

Short answer

If your wastewater treatment plant serves an agglomeration that falls within the scope of the EU Urban Wastewater Treatment framework, then secondary treatment is often not optional. Under Directive (EU) 2024/3019, secondary biological treatment is mandatory for agglomerations of 2,000 p.e. and above, with the obligation extended to agglomerations of 1,000 p.e. and above by 31 December 2035, subject to transitional and exemption arrangements.

For plant owners and operators, the practical question is not only whether the plant must have secondary treatment, but whether the existing biological stage still has enough hydraulic and organic capacity to perform reliably. If it does not, the plant is likely facing an upgrade issue rather than a simple operating issue.

What secondary treatment means

The recast 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 practice, this usually means one of the following:

• Activated sludge systems with secondary clarification
• Trickling filters or other attached-growth biological systems
• Sequencing batch reactors and similar biological processes
• Other approved biological processes designed to reduce biodegradable load

The key point is removal of biodegradable organic matter, not merely screening or primary settlement. Primary treatment can reduce solids and settleable material, but it does not normally deliver the level of carbon removal expected from secondary treatment.

When the requirement applies

Under the verified EU framework:

• Secondary biological treatment is mandatory for agglomerations of 2,000 p.e. and above
• The obligation is extended to agglomerations of 1,000 p.e. and above by 31 December 2035
• Transitional and exemption arrangements may apply, including for certain coastal areas and for Bulgaria, Croatia and Romania

A population equivalent matters because the directive uses p.e. to express treatment obligations. A p.e. is the organic biodegradable load having a five-day biochemical oxygen demand of 60 g of oxygen per day.

For an operator, this means that capacity is assessed against the load the plant receives, not only the nominal tank size or original permit. Growth in domestic connections, infiltration, inflow, trade contributions, and seasonal variation can all push a plant towards or beyond the design basis.

Signs that your plant may now need secondary treatment

Even where a plant already has some form of biological stage, the following are common triggers for reassessment:

• The plant is approaching or exceeding its permitted organic load
• Final effluent quality is deteriorating, especially on BOD and suspended solids
• Settled sludge quality is unstable or clarifier performance is poor
• Aeration capacity is consistently at or near limit
• Peak wet-weather flows are bypassing or suppressing biological performance
• The existing process was designed for a lower population equivalent than current conditions

What secondary treatment must achieve

The directive’s Annex I secondary-treatment effluent standards are:

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

These values set the general performance expectation for secondary treatment under the EU framework. In engineering terms, they indicate the plant must provide stable biological oxidation and adequate solids separation.

That is important because many capacity problems are not caused by biology alone. Secondary treatment depends on the interaction of:

• Influent characteristics
• Hydraulic retention and sludge age
• Aeration system performance
• Mixed liquor settleability
• Secondary clarifier capacity
• Internal recycles and return sludge control

If one of these elements is under-sized or degraded, the plant may fail to meet effluent standards even if the biological process itself is theoretically sound.

Common capacity constraints in secondary treatment

For plants facing insufficient capacity, the limiting factor is often one of the following.

1) Organic loading exceeds design intent

If influent BOD5 is higher than the plant was designed for, the biomass may not have enough time or oxygen transfer capacity to remove the load consistently. This is especially relevant where the original process was built for lower population growth or where trade waste has increased over time.

2) Hydraulic overload affects clarification

Secondary clarification is often the first component to show stress during wet weather or peak flow events. Hydraulic overload can shorten solids separation time, carry biomass into the effluent, and create apparent process failure even when aeration is adequate.

3) Aeration system is limiting

Biological oxidation requires oxygen transfer. Common operating indicators of aeration limitation include:

• Persistent high ammonia in systems that should be nitrifying
• Rising oxygen demand with little process reserve
• Large blower operating hours or frequent maximum output
• Poor mixed liquor condition during peak load periods

Typical design and operational targets for dissolved oxygen, sludge age, MLSS and F/M ratio are site-specific and should be treated as indicative rather than universal. The correct values depend on the process type, temperature, influent composition and effluent objectives.

4) Solids handling is constraining biology

If sludge wasting, return sludge or secondary settlement is unstable, the biological stage may not retain enough active biomass. That can lead to poor BOD removal, poor settleability and unstable effluent solids.

5) The plant has aged beyond its original process window

Many plants still operate with equipment and process assumptions from the original design era. Over time, changes in loading, infiltration, storm response and maintenance condition can reduce actual capacity below nominal capacity.

How to assess whether your plant needs an upgrade

A useful engineering assessment starts with a simple question: is the plant non-compliant because it lacks secondary treatment, or because its existing secondary treatment no longer has sufficient capacity?

A structured review should cover:

• Current and projected p.e. loading
• Influent BOD5, COD and TSS profile
• Hydraulic profile in dry and wet weather
• Existing treatment process configuration
• Clarifier and aeration performance
• Sludge production, wasting and return rates
• Effluent compliance history
• Available footprint and civil constraints
• Future regulatory obligations under the recast directive

If the answer is unclear, start a site-specific review rather than relying on nominal design data alone. A plant can be technically “secondary treated” on paper and still fail in practice if process capacity is exhausted.

You can Start the Engineering Assessment to check whether your plant needs a secondary treatment upgrade and where the limiting constraint is likely to be.

What an upgrade usually involves

The right upgrade depends on the constraint, but common measures include:

• Increasing aeration capacity or oxygen transfer efficiency
• Adding biological volume or reconfiguring process zones
• Expanding or refurbishing secondary clarification
• Improving sludge return and wasting control
• Separating storm flows or reducing hydraulic peaks
• Converting to a more robust biological process where space is limited

Not every plant needs a complete rebuild. In many cases, the most effective solution is to remove the bottleneck that is preventing the existing biological stage from performing to its design intent.

Secondary treatment versus tertiary and quaternary treatment

Secondary treatment removes biodegradable organic matter. Tertiary treatment is a further stage for nitrogen and phosphorus removal, and quaternary treatment addresses micropollutants. Those later stages are relevant for some plants, but they should not distract from the immediate question of whether the core biological stage can still achieve basic carbon removal and solids separation.

For many overloaded plants, secondary treatment capacity is the first and most urgent issue.

How this applies to your plant

If your WWTP serves 2,000 p.e. or more, or is moving towards that threshold, secondary treatment is already part of the regulatory baseline under the EU framework. If your plant is below that today but projected growth takes it towards 1,000 p.e. and above by 31 December 2035, it should be reviewed in the context of the recast directive and national transposition.

From an engineering perspective, the key issue is whether your biological stage can still absorb present and future load without losing effluent quality. The most common warning signs are rising BOD, COD or TSS in the final effluent, poor clarification, limited aeration reserve and wet-weather instability.

If you need a practical way to frame the decision, use the assessment to separate regulatory obligation from process capacity. That is usually the fastest route to deciding whether you need operational optimisation, targeted refurbishment, or a full secondary treatment upgrade.