Phosphorus Remover — Frequently Asked Questions

Because biological treatment removes only a fraction of the phosphorus load. Conventional activated sludge takes up phosphorus for cell growth, which accounts for a modest proportion; enhanced biological removal can do considerably better but is sensitive to influent composition and upsets. Consents of 1 mg/l or below, and increasingly 0.5 or 0.1, are beneath what biology alone reliably delivers.

It depends on the objective. Dosing upstream of primary settlement removes load early and reduces the burden on the biological stage. Dosing at the final stage gives the tightest control of effluent phosphorus. Many works dose at both points, using the primary dose for bulk removal and a trim dose at the end for consent compliance.

Sludge. Precipitating phosphorus as a metal salt produces additional chemical sludge that must be dewatered and disposed of, and the cost of that can be significant. Iron and aluminium dosing also consumes alkalinity and can depress pH, which may need correction. Dose optimisation against the actual consent, rather than overdosing for safety, is where the economics are won.

It depends on the works. Iron salts are generally cheaper and perform well but can colour the effluent and affect downstream processes. Aluminium salts produce a lighter floc and less colour. Existing chemical handling, sludge disposal route and any interaction with other dosing all influence the decision, and jar testing on the actual effluent is the reliable basis.