The Vaal River Barrage reservoir is an important source of raw water for the Rand Water Board. Activities in the catchment of this reservoir resulted in enrichment of this water with concomitant eutrophication related water quality problems being experienced by the Rand Water Board.

In order to limit enrichment of raw water a standard of 1 mg PO4−P/ℓ for waste water treatment plant effluent was imposed and the effects of this standard on the algal concentrations at the No. 1 intake in the Vaal River Barrage reservoir was investigated.

Although only 24 per cent of waste water treatment plants in the catchment of the Vaal River Barrage reservoir complied with the phosphate standard for more than 75 per cent of the time, the amount of phosphate emanating from the treatment plants in general decreased significantly. This resulted in a lower orthophosphate concentration at the Board's No. 1 intake. The associated lower chlorophyll concentrations at No. 1 intake was, however, not only due to the lower orthophosphate concentrations, but also to the residence time and chemical/physical factors.

To limit the algal concentrations at No. 1 intake, the 1 mg/ℓ orthophosphate standard will have to be enforced strictly and diffuse source management will have to be implemented.

Compliance, eutrophication, water purification, orthophosphate, algae, phosphate standard, waste water treatment
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© IWA Publishing 1992
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