Primary, Secondary and Tertiary Treatment Explained

Introduction

Primary, secondary and tertiary treatment are the main stages used to remove solids, biodegradable organics and, where required, nutrients from municipal wastewater. For operators and designers, the key point is that these stages are not independent blocks. Each one changes the load on the next, and the overall treatment train must be matched to the actual influent, the receiving water and the compliance objective.

Under the recast EU Urban Wastewater Treatment Directive, secondary treatment remains the core biological requirement for most municipal plants, with more advanced treatment added where population, environmental sensitivity or future obligations demand it. If you are reviewing an existing works or planning an upgrade, it helps to understand what each stage does and what it does not do. A useful overview of the wider treatment train is available on the Treatment Stages page.

What primary treatment does

Primary treatment is mainly a physical separation stage. Its purpose is to remove settleable solids, floatables and part of the suspended load before biological treatment. In many conventional works, this is done in primary settlement tanks, sometimes with screening and grit removal upstream.

Typical functions of primary treatment include:

• reducing the solids load to aeration or other biological units
• lowering downstream sludge production in the biological stage
• smoothing hydraulic and solids loading to improve process stability
• protecting pumps, blowers, diffusers and downstream equipment from unnecessary wear

Primary treatment does not remove dissolved biodegradable pollution to the same extent as secondary treatment. It therefore cannot by itself meet the main compliance objective for municipal wastewater in most cases. Its role is preparatory: it creates a more manageable influent for the biology that follows.

From an operational perspective, the main issues are sludge settlement characteristics, solids capture efficiency and hydraulic short-circuiting. Where primary settlement is poorly performing, the biological stage receives a higher and more variable organic load, which can affect oxygen demand, sludge age and clarifier performance.

What secondary treatment does

Secondary treatment is the biological stage. The recast directive defines it as treatment that generally involves biological treatment with a secondary settlement stage, or another process that removes biodegradable organic matter. In practice, this is the heart of municipal wastewater treatment because it targets the biodegradable organics measured through indicators such as BOD5.

The directive requires secondary (biological) treatment 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.

The main objective of secondary treatment is to convert dissolved and fine particulate biodegradable pollution into biomass and stable end products. Common process families include:

• activated sludge systems
• biofilm processes
• sequencing batch reactors
• oxidation ditches
• other biological processes that achieve removal of biodegradable organic matter

Although process layouts differ, the design intent is the same: provide sufficient contact time, oxygen transfer and biomass retention to achieve the required organic removal. Secondary settlement, where used, separates treated water from the biomass and returns part of the sludge to maintain process concentration.

Typical design considerations

Secondary treatment must be sized against both average and peak conditions. In practice, engineers look at:

• influent BOD load and flow variation
• temperature effects on biological activity
• sludge age or solids retention time, where relevant
• dissolved oxygen control and aeration capacity
• mixed liquor concentration and settleability
• return activated sludge and waste sludge rates

These are site-specific engineering variables rather than fixed universal values. A process that works well in one catchment may underperform in another if industrial contributions, infiltration, storm response or temperature differ materially.

Regulatory performance expectations

For secondary treatment, the annexed effluent standards in the recast directive are:

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

These figures describe the expected performance of the secondary stage and are central to compliance planning. In practice, operators should treat them as design and operating targets that depend on the selected process, influent variability and downstream settling performance.

What tertiary treatment adds

Tertiary treatment is an additional stage applied after secondary treatment when more stringent removal is needed. Its main role is nutrient control, especially nitrogen and phosphorus, to protect sensitive receiving waters and reduce eutrophication risk.

The recast directive sets phased obligations for tertiary treatment:

• plants of 150,000 p.e. and above by 2039
• plants of 10,000 p.e. and above in areas at risk of eutrophication by 2045

Tertiary treatment is therefore not a substitute for secondary treatment; it builds on it. The biological core must already be stable and compliant before nutrient removal is added.

Typical tertiary functions may include:

• enhanced biological nutrient removal within the main biological stage
• filtration or polishing to improve solids and associated phosphorus removal
• chemical phosphorus precipitation, where required by the site strategy
• nitrogen removal through process configuration and anoxic/oxic management

Because tertiary treatment depends heavily on the receiving water and local sensitivity, the exact process selection is a site-specific engineering decision. National transposition and implementation also vary, so the project team should confirm the applicable legal route and timing in the relevant Member State.

How the stages work together in practice

The three stages should be viewed as a sequence, not as separate compliance silos.

Primary treatment upstream protects biology

If primary removal is effective, the downstream biological stage sees a more stable load. That can help with aeration demand, sludge handling and clarifier operation. Where primary treatment is limited or absent, the secondary stage must absorb more organic matter and more variability.

Secondary treatment is the compliance backbone

For most municipal works, secondary treatment is the stage that determines whether the plant meets the main effluent standard. It is also the stage most affected by process upsets, storm dilution, toxic shocks and mechanical downtime. Good control of oxygen transfer, sludge return and sludge wasting is often the difference between steady compliance and repeated excursions.

Tertiary treatment is added where the receiving water demands it

Where nutrient removal is required, tertiary treatment reduces the residual impact of the effluent after the organic load has already been treated. It is best thought of as a refinement step. If the biology is unstable, tertiary units often have to compensate for problems that should have been solved earlier in the train.

Design and operational implications for WWTPs

For operators and engineers, the practical questions are usually not academic. They are about capacity, reliability and future compliance.

Key issues to review include:

• whether the existing primary settlement is still adequate for current and projected loads
• whether the biological stage has sufficient aeration, volume and sludge retention to remain stable under wet weather and peak organic loading
• whether secondary clarification is limiting effluent TSS and biomass retention
• whether nutrient removal will be required later under the directive timeline
• whether the plant footprint, hydraulics and process control philosophy can support staged upgrades

A common failure mode in older works is that primary and secondary units were designed for a different influent profile than the one now being received. Growth, infiltration, industrial discharges and changing storm patterns can all alter the balance. In those cases, the right answer may be optimisation, partial refurbishment or a phased upgrade rather than immediate full replacement.

How this applies to your plant

If you are assessing an existing works, start by asking three questions:

1. Is the primary stage removing enough solids to protect the downstream biology?
2. Is the secondary stage meeting the organic effluent targets reliably across normal and wet-weather conditions?
3. Is tertiary treatment likely to become necessary under the EU timetable for your agglomeration size and receiving water?

The right next step is a structured engineering review of flows, loads, process performance and compliance risk. You can Start the Engineering Assessment to evaluate the current treatment train and identify whether optimisation, upgrade or expansion is the most appropriate route.

If you are planning a new plant or a major refurbishment, use the treatment stages together rather than in isolation. The best design is the one that matches primary solids capture, stable secondary biology and any future tertiary needs to the real conditions at the site, not just to nominal design assumptions.