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Nachklärbecken einer kommunalen Kläranlage mit klarem Ablaufwasser bei der tertiären Abwasserreinigung

Tertiary Treatment for Wastewater Treatment Plants

The authoritative engineering guide to third-stage wastewater treatment.

Tertiary treatment is the advanced stage of wastewater processing that follows primary settling and secondary biological treatment. It targets residual nutrients (nitrogen, phosphorus), suspended solids, and increasingly micropollutants to meet stringent discharge requirements. Under the revised EU Urban Wastewater Directive (2024/3019), tertiary treatment will become mandatory for all plants above 10,000 PE by 2039.

Key Numbers

0
mg/L
Total Nitrogen limit (EU)
0.0
mg/L
Total Phosphorus limit (large plants)
0
mg/L
TSS limit after tertiary treatment
0%
%
Minimum nutrient removal efficiency

Technical Overview

Tertiary treatment encompasses all processes that follow conventional secondary biological treatment. The primary objectives are:

  • Nutrient removal: Reducing total nitrogen and total phosphorus to levels that prevent eutrophication in receiving water bodies.
  • Solids polishing: Removing residual suspended solids that carry particulate phosphorus and BOD.
  • Pathogen reduction: Additional disinfection where required for water reuse or bathing water standards.
  • Micropollutant removal: Increasingly required as a quaternary treatment step under the same regulatory framework.

Engineering Insight: Process Integration

Tertiary treatment is most cost-effective when integrated into the overall plant hydraulic profile. Gravity-fed systems reduce energy costs by 30-50% compared to pumped configurations. Early hydraulic assessment is critical for retrofit projects.

Engineering Process

Assess
Design
Validate
Engineer
Manufacture
Implement

Technology Comparison

Disc Filtration

high

Rotating disc filters with woven or pile cloth media. Compact footprint, low energy consumption.

Footprint: Very small
Energy: Very low
Advantages
  • Very compact footprint
  • Low energy (0.01-0.03 kWh/m³)
  • Simple O&M
  • Modular design
Limitations
  • Limited to TSS/particulate P
  • No dissolved nutrient removal
  • Media replacement cycles

Deep-Bed Sand Filtration

high

Gravity-fed continuous backwash sand filters. Proven technology for TSS and particulate P removal.

Footprint: Medium
Energy: Low
Advantages
  • Proven technology
  • Chemical dosing possible
  • Good TSS removal
  • Long media life
Limitations
  • Larger footprint
  • Backwash water management
  • Higher civil works cost

Membrane Filtration (MF/UF)

medium

Microfiltration or ultrafiltration membranes providing absolute barrier for solids and pathogens.

Footprint: Small
Energy: Medium-High
Advantages
  • Absolute barrier
  • Consistent effluent quality
  • Pathogen removal
  • Compact
Limitations
  • Higher energy demand
  • Membrane fouling
  • Chemical cleaning required
  • Higher OPEX

Dissolved Air Flotation (DAF)

medium

Flotation process using micro-bubbles to separate flocs, suspended solids, and oils.

Footprint: Medium
Energy: Medium
Advantages
  • Effective with coagulants
  • Handles algae well
  • Good for P removal
  • Flexible operation
Limitations
  • Chemical consumption
  • Sludge production
  • Energy for air saturation
  • Larger than disc filters

For Existing Plants

When retrofitting tertiary treatment into an existing plant, consider: available footprint, hydraulic head availability, existing chemical dosing infrastructure, and sludge handling capacity. A thorough site assessment is the first step — use our Plant Assessment Tool to get a preliminary engineering evaluation.

EU Urban Wastewater Directive (2024/3019)

Articles 7 and 8 establish mandatory tertiary treatment for nutrient removal at all urban wastewater treatment plants above 10,000 PE. Compliance deadlines range from 2033 to 2039 depending on plant size. National implementation may impose stricter requirements.
Source: Directive (EU) 2024/3019

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Frequently Asked Questions

Related Content

Nitrogen Removal

Strategies for meeting TN discharge limits

Phosphorus Removal

Chemical and biological P removal

TSS / Filtration

Filtration technologies for solids removal

Retrofit Solutions

Upgrading existing infrastructure

EU Requirements

Directive 2024/3019 compliance guide

Technology Selection

How to choose the right process

Last reviewed: August 2026. Regulatory requirements may vary by national implementation. This content is for informational purposes and does not constitute legal or engineering advice.