Process Overview
Stages of Wastewater Treatment
Wastewater is treated in successive stages, each removing what the previous one leaves behind: primary (physical), secondary (biological), tertiary (nutrients) and quaternary (micropollutants).
The Four Stages
1. Primary Treatment
Physical removal of settleable and floating solids by screening, grit removal and primary sedimentation.
Explore2. Secondary (Biological) Treatment
Microorganisms remove dissolved and suspended organic matter, meeting BOD₅ and COD standards.
Explore3. Tertiary Treatment
Removal of nutrients — nitrogen and phosphorus — plus further polishing of the effluent.
Explore4. Quaternary Treatment
Advanced removal of micropollutants such as pharmaceutical residues, e.g. by ozonation or activated carbon.
ExploreSecondary Treatment Standards (EU)
Secondary treatment is the biological core of the process. Under Directive (EU) 2024/3019 it must meet the following discharge standards for urban wastewater:
BOD₅
Or 70–90% reduction
COD
75% reduction
Total Suspended Solids
Typical secondary standard
Scope & Deadlines Under the Recast Directive
Secondary treatment extended to agglomerations of 1,000 p.e. and above by 31 December 2035.
Tertiary (nutrient) removal for plants of 150,000 p.e. and above by 2039, and 10,000 p.e. and above in sensitive areas by 2045.
Quaternary treatment for micropollutants at 150,000 p.e. and above by 2045, with interim milestones in 2033 and 2039.
An energy-neutrality objective for the urban wastewater sector by 2045.
Go Deeper
Engineering Note
Frequently Asked Questions
Primary treatment removes settleable and floating solids physically. Secondary (biological) treatment uses microorganisms to remove dissolved and suspended organic matter, meeting BOD₅ and COD standards. Tertiary treatment adds nutrient removal (nitrogen and phosphorus) and further polishing. A fourth, quaternary stage targets micropollutants.
Under Directive (EU) 2024/3019, secondary treatment must achieve BOD₅ of 25 mg/L (or 70–90% reduction), COD of 125 mg/L (75% reduction) and total suspended solids of around 35 mg/L. These are the core biological-treatment discharge standards for urban wastewater.
The recast directive extends nutrient (tertiary) removal to larger agglomerations — for example plants of 150,000 p.e. and above by 2039 and 10,000 p.e. and above in sensitive areas by 2045 — and introduces quaternary treatment for micropollutants at 150,000 p.e. and above by 2045, with interim milestones. Secondary treatment itself is extended down to agglomerations of 1,000 p.e. by 31 December 2035.
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