Abstract

ATTA, KPT et al. Chemical phosphate removal from Hartbeespoort Dam water, South Africa. Water SA [online]. 2020, vol.46, n.4, pp.610-614. ISSN 1816-7950.  http://dx.doi.org/10.17159/wsa/2020.v46.i4.9074.

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.

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