Tertiary Treatment Requirements
The discharge limits and removal efficiencies your plant must achieve — in plain terms.
Under the EU Urban Wastewater Directive (2024/3019), tertiary treatment must bring total nitrogen down to 10 mg/L (or achieve at least 80% removal) and total phosphorus to 1.0 mg/L for plants between 10,000 and 150,000 PE, or 0.5 mg/L for plants above 150,000 PE. Suspended solids are typically polished to around 10 mg/L. Compliance is judged on annual load-weighted averages rather than single samples, which shapes how a plant is designed and operated.
Discharge Limits at a Glance
What the Numbers Mean for Design
Meeting these limits reliably — not just on a good day, but as an annual average — drives several engineering decisions:
- Nitrogen: reaching 10 mg/L total nitrogen usually depends on stable nitrification and denitrification, sometimes supported by an additional polishing step.
- Phosphorus: low phosphorus targets almost always require chemical precipitation combined with effective solids separation, since much of the residual phosphorus is particle-bound.
- Suspended solids: a final filtration stage protects the phosphorus result and stabilises effluent quality during load peaks.
- Margins: because compliance is averaged over the year, designs need headroom for storm flows, temperature swings and maintenance downtime.
Engineering Insight: The Phosphorus–Solids Link
Translating Limits into a Plan
Figures are EU baseline values
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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.