The effectiveness of lime-based amendments and bauxite residues at removing phosphorus from piggery effluent.

Piggery effluent may contribute to the eutrophication of waterways, if it is not treated before disposal, because of high levels of phosphorus. Limes and red muds (a residue from bauxite refining) were used to remove phosphorus from piggery effluent (41 mg litre(-1) total P). Lime-based amendments were more effective than the red muds at removing phosphorus when compared at the same liquid: solid ratios. Based on laboratory data, the cost of treating effluent increased rapidly as the final required phosphorus concentration decreased to less than 4 mg litre(-1). Kiln dust was the cheapest amendment tested down to 2 mg litre(-1). Hydrated lime was able to clarify and flocculate the effluent to 1 mg litre(-1) within 60 min. The re-useability of all limes may be determined by a simple pH test. Red mud could be used to remove phosphorus when its pH was lowered to 6.0-6.5 and it is used at liquid:solid ratios <20:1.