Chemical phosphate removal from Hartbeespoort Dam water, South Africa

Abstract

Phosphate is one of the major nutrients contributing to the increased eutrophication of lakes and natural waters. The feed water to the  Hartbeespoort Dam amounts to 650 ML/d of mainly treated sewage. Phosphate levels in the dam water need to be lowered from the current 0.2 mg/L to less than 0.05 mg/L to control eutrophication. Chemicals such as iron(III), iron(II), aluminium(III) and lime can be used to precipitate phosphate as FePO₄, Fe₃(PO₄)₂, AlPO₄ and Ca₃(PO₄)₂, respectively. OLI software was used to identify the most suitable chemical for phosphate removal. It was found to be Ca(OH)₂ as this only requires the pH to be raised to 9.5. FeCl₃, FeCl₂ and AlCl₃ were found to be unsuitable due to the required pH and/or the extent to which they could remove phosphate. For lowering of phosphate levels from 0.2 mg/L (as P), the current  concentration in the Hartbeespoort Dam water, to <0.05 mg/L (as P), the minimum concentration that is needed to support algal growth, a lime dosage of 50 mg/L is required. The cost of lime treatment will amount to 0.15 ZAR/m³. It is thus recommended that eutrophication in the  Hartbeespoort Dam be controlled by removal of phosphate through lime dosing.

Keywords: eutrophication water hyacinth cyanobacteria sediments algae lime ferric chloride aluminium chloride ferrous chloride