Chemical Phosphorus Removal for Tertiary Treatment
Precipitation with iron or aluminium salts — the workhorse of phosphorus compliance.
Chemical phosphorus precipitation is the most widely used method for achieving the stringent TP limits set by the revised EU Directive (typically less than 1.0 mg/L, or less than 0.5 mg/L for large plants). It works by dosing a metal salt — usually ferric chloride (FeCl₃) or poly-aluminium chloride (PAC) — to convert dissolved phosphorus into insoluble particles that are then captured by a downstream filter. The technology is simple, proven, and can be added to virtually any existing plant as a standalone dosing station with minimal civil works. It is the essential complement to any tertiary filtration system targeting phosphorus removal.
Key Numbers
Common Reagents
Ferric Chloride (FeCl₃)
+ Most widely used; reliable; works across pH range
Note: Produces iron-coloured sludge; may stain equipment
Aluminium Sulphate (Alum)
+ Lighter sludge colour; effective at pH 5.5–7.5
Note: Narrower effective pH range; residual aluminium limits
Poly-Aluminium Chloride (PAC)
+ Pre-polymerised — faster floc formation; lower dose; less sludge
Note: Higher unit cost per kg Al; best for polishing applications
Ferric Sulphate (Fe₂(SO₄)₃)
+ Alternative to FeCl₃ where chloride limits apply
Note: Less common; may need longer floc maturation
Engineering Insight: Dose Optimisation
Dosing Point Comparison
Pre-secondary clarifier (simultaneous)
P removed in existing sludge; no extra infrastructure
Best for: Plants with moderate TP targets (1–2 mg/L)
Post-secondary / pre-tertiary filter
Dissolved P precipitated + captured on tertiary filter
Best for: Plants targeting TP less than 0.5 mg/L; most common retrofit approach
Two-point dosing (both locations)
Primary dose in biology + polishing dose ahead of filter
Best for: Plants with very high P loads or stringent efficiency requirements (≥90 %)
Regulatory Context
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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.