What Is Tertiary Treatment of Wastewater?

What tertiary treatment means

Tertiary treatment is the treatment stage applied after secondary biological treatment when additional polishing is needed, typically to remove nutrients such as nitrogen and phosphorus. In practical terms, it is the part of the wastewater treatment train that addresses what secondary treatment does not fully remove.

For operators and designers, the important point is that tertiary treatment is not a single process. It is a function: further treatment beyond the biological carbon removal stage. The selected process may vary, but the objective is usually nutrient reduction, effluent polishing, or both.

Within the EU framework, tertiary treatment sits alongside the wider treatment-stage logic set out in the Urban Wastewater Treatment Directive recast, Directive (EU) 2024/3019. That directive defines secondary treatment separately as biological treatment with secondary settlement, or another process removing biodegradable organic matter. Tertiary treatment comes after that.

How tertiary treatment fits into the treatment train

A conventional municipal works is usually arranged in stages:

• preliminary treatment: screening, grit removal and related inlet protection
• primary treatment: settling of readily settleable solids, where provided
• secondary treatment: biological removal of biodegradable organic matter
• tertiary treatment: further removal of nutrients and, in some cases, additional suspended matter
• quaternary treatment: micropollutant removal, where required

The exact train depends on plant size, discharge consent, climate, catchment sensitivity and whether the works discharges to a river, lake, estuary or coastal water. However, tertiary treatment generally follows a secondary process that already meets the secondary effluent standards.

For context on treatment-stage terminology, see /treatment-stages.

What tertiary treatment removes

Tertiary treatment is mainly associated with nutrient removal.

Nitrogen removal

Nitrogen in wastewater can be present as ammonia, nitrate and organic nitrogen. If discharged in significant amounts, it can contribute to oxygen depletion and ecological imbalance in receiving waters.

Tertiary nitrogen removal commonly aims to reduce total nitrogen through process steps such as:

• nitrification, where ammonia is biologically oxidised to nitrate
• denitrification, where nitrate is biologically reduced to nitrogen gas
• step-feed or other process configurations that improve contact between carbon and nitrate

The precise design depends on the selected biological process and the influent characteristics. In many plants, the tertiary objective is best understood as achieving lower residual nitrogen than secondary treatment alone can reliably deliver.

Phosphorus removal

Phosphorus is another key driver for tertiary treatment, particularly in waters vulnerable to eutrophication. Excess phosphorus can accelerate algal growth and degrade water quality.

Phosphorus removal may be achieved by:

• biological phosphorus removal in suitably designed systems
• chemical precipitation using metal salts
• a combined biological and chemical approach
• filtration or polishing steps where necessary to capture residual solids-bound phosphorus

As with nitrogen, the selected method is site-specific. The process must be matched to the influent composition, the desired effluent quality and the operating strategy of the works.

Common tertiary treatment processes

There is no single universal tertiary process. The following are common options, often used alone or in combination.

Biological nutrient removal

Many modern works integrate nutrient removal into the biological process train rather than treating it as a separate end-of-pipe stage. Anoxic and aerobic zones, internal recycle streams and sludge age control can all be used to promote nitrogen and phosphorus removal.

This can be considered tertiary in function even when it is built into the secondary process, because the operational objective is nutrient polishing beyond basic carbon removal.

Chemical phosphorus removal

Chemical dosing is a common and robust method for phosphorus reduction. It is often used when biological removal alone is not sufficient, or as a polishing step to improve compliance and process stability.

Typical design and operating considerations include:

• dose point selection
• mixing and floc formation
• impacts on sludge production
• downstream solids separation capacity

The suitability of chemical removal depends on plant-specific effluent targets and sludge handling constraints.

Filtration and solids polishing

Where residual suspended solids carry phosphorus or other pollutants, filtration can improve effluent quality. Media filtration, cloth filtration and similar polishing steps may be applied after secondary settlement or after nutrient removal.

This is particularly relevant where permit requirements are tight or where variable hydraulic loading makes final clarification performance less predictable.

Combined tertiary trains

Many works use a combination of processes, for example:

• biological nitrogen removal plus chemical phosphorus dosing
• secondary treatment followed by tertiary filtration
• nutrient removal followed by further polishing before discharge

The best configuration depends on the treatment objective, available footprint, sludge management approach and existing assets.

What the EU directive requires

Under Directive (EU) 2024/3019, tertiary treatment is part of the progressive tightening of wastewater treatment requirements in the EU.

The verified directive facts relevant to tertiary treatment are:

• tertiary treatment for nitrogen and phosphorus removal is required for plants of 150,000 p.e. and above by 2039
• it is also required for plants of 10,000 p.e. and above in areas at risk of eutrophication by 2045

A population equivalent, or p.e., is defined as the organic biodegradable load having a five-day biochemical oxygen demand of 60 g of oxygen per day.

For operators, the key practical message is that tertiary requirements are no longer limited to the very largest plants. The directive extends nutrient-removal expectations to smaller works in sensitive catchments, subject to the staged timetable and the applicable national implementation arrangements.

The directive entered into force on 1 January 2025 and must be transposed into national law by 31 July 2027. The old Directive 91/271/EEC is repealed with effect from 1 August 2027. As always, local implementation details will depend on national transposition and permitting.

How tertiary treatment is different from secondary treatment

It is easy to conflate the two, because both may involve biological processes. The distinction is functional.

Secondary treatment is primarily for biodegradable organic matter removal and is defined in the directive as treatment that generally involves biological treatment with a secondary settlement stage, or another process that removes biodegradable organic matter. The 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

Tertiary treatment goes beyond that baseline. Its job is to reduce nutrients and other residual pollutants that remain after the secondary stage has done its main work.

In practice:

• secondary treatment stabilises the organic load
• tertiary treatment improves environmental compatibility of the discharge
• quaternary treatment, where required, targets micropollutants

That sequence matters when assessing process upgrades, because tertiary design should not be used to compensate for weak secondary performance. The upstream biological stage must remain stable before nutrient polishing can be reliably achieved.

Design and operational considerations

Although the exact process selection is site-specific, several engineering issues are common.

Hydraulic variability

Peak flows can reduce contact time and dilute readily available carbon needed for denitrification. Plants may therefore need flow balancing, selector zones, recycle control or external carbon dosing, depending on the process and permit.

Sludge age and oxygen control

Nitrification and phosphorus removal are sensitive to operating conditions. Typical design values for sludge age, dissolved oxygen and recycle ratios are highly site-specific and should be treated as indicative only. They must be set against temperature, influent load, process configuration and seasonal variation.

Carbon availability

A common constraint in nitrogen removal is the availability of biodegradable carbon. If most of that carbon has already been consumed in the secondary stage, denitrification may be limited unless process configuration or supplemental carbon is provided.

Sludge and sidestream impacts

Tertiary processes can affect sludge production, dewatering performance and sidestream nutrient loads. These effects need to be assessed as part of the wider plant balance, not only from the discharge-point perspective.

Existing assets

For retrofit projects, the practical question is often whether the site can accommodate tertiary treatment within the current footprint and process hydraulics. In some cases, intensification or process modification is more practical than a separate new treatment line.

How this applies to your plant

If your works already meets secondary standards, the next question is whether the receiving water, permit trajectory or EU timetable points towards nutrient removal. Plants discharging to sensitive waters, or serving larger agglomerations, should assess whether tertiary treatment will be needed in the medium term.

The right approach starts with a structured review of:

• current secondary effluent performance
• nutrient concentrations in the final effluent
• catchment sensitivity and eutrophication risk
• available footprint and process flexibility
• sludge handling and energy implications
• future compliance obligations under national transposition of Directive (EU) 2024/3019

If you are planning a upgrade or need to understand where tertiary treatment fits in the wider treatment train, Start the Engineering Assessment. It will help identify whether your plant can meet future nutrient requirements through optimisation, retrofit or a staged upgrade strategy.