Phosphorus removal in a waste-stabilization pond containing limestone rock filters

This study analyzed samples taken along internal transects through a small waste-stabilization pond system in New Zealand that had rock filters installed between the final maturation cells. The aim of the study was to determine the respective importance of the ponds and rock filters for phosphorus (P) removal since routine monitoring from the preceding 2 months had indicated effluent P levels below 2 g/m³ for influent levels around 9 g/m³. Despite the filters being constructed from a reactive sorbent (i.e., limestone) it was found that phosphorus removal was mainly occurring in the ponds. A solubility analysis suggested that phosphorus removal may have been due to moderate calcium hardness levels of around 60 g/m³ as Ca²⁺, while an analysis of sludge samples in the system suggested that the ratio of calcium to phosphorus in the sludge was consistent with the precipitation of phosphorus as calcium hydroxyapatite.Key words: waste-stabilization ponds, phosphorus removal, limestone rock filters.