SecondaryTreatment

Nutrient Removal

Phosphorus Removal

Biological and chemical routes to meet phosphorus discharge limits — and why the recast EU directive is expanding where they apply.

Why Remove Phosphorus?

Phosphorus is a key driver of eutrophication in surface waters. The Urban Wastewater Treatment Directive requires nutrient removal for larger agglomerations discharging to sensitive waters, and the recast Directive (EU) 2024/3019 tightens these requirements and lowers the size thresholds at which they apply. As a result, more plants must add or upgrade phosphorus removal as part of a secondary-treatment upgrade.

Two Complementary Routes

Enhanced Biological (EBPR)

An anaerobic selector zone favours phosphorus-accumulating organisms that take up phosphorus in excess in the following aerobic zone. Phosphorus leaves the system with the wasted sludge — no chemical cost, but it needs stable conditions and readily-biodegradable carbon.

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Chemical Precipitation

Dosing iron or aluminium salts precipitates phosphate as an insoluble solid removed with the sludge. Reliable and controllable, widely used alone or as polishing/back-up to EBPR, at the cost of chemicals and additional chemical sludge.

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Design Considerations

Anaerobic

EBPR Selector Zone

Required upstream contact zone

Fe / Al

Precipitant Salts

Iron or aluminium dosing

WAS

Phosphorus Exit Path

Removed with waste sludge

Engineering Note

EBPR and chemical dosing are often combined: EBPR carries the bulk load at low operating cost while a small chemical dose provides reliable polishing to meet a tight limit and covers periods of weak EBPR performance (for example when influent carbon is low). The right balance is site-specific and depends on influent characteristics, the discharge limit and existing tank configuration.
Recovered phosphorus (e.g. as struvite) is increasingly of interest as a resource, aligning with circular-economy goals.

Frequently Asked Questions

Phosphorus is removed either biologically (Enhanced Biological Phosphorus Removal, EBPR) or chemically. EBPR uses an anaerobic zone that selects phosphorus-accumulating organisms which later take up phosphorus in excess. Chemical removal precipitates phosphate by dosing metal salts (iron or aluminium). Many plants combine both for reliability.

EBPR relies on phosphorus-accumulating organisms (PAOs). In an anaerobic zone they release phosphorus while storing carbon; in the following aerobic zone they take up more phosphorus than they released, storing it in the biomass. The phosphorus then leaves the system with the wasted sludge. Stable EBPR requires a reliable anaerobic zone and adequate readily-biodegradable carbon.

Yes. The Urban Wastewater Treatment Directive requires nutrient removal, including phosphorus, for larger agglomerations discharging to waters sensitive to eutrophication. The recast Directive (EU) 2024/3019 tightens nutrient requirements and lowers the size thresholds at which they apply, increasing the number of plants that must remove phosphorus and nitrogen.

Engineering Assessment

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