The Vaal River Barrage reservoir is an important source of raw water for Rand Water which supplies at least 10 million people with drinking water daily. Return effluents from waste water treatment works, industries and informal urban settlements in the catchment has resulted in algal problems in the raw water and treatment process.
In order to attempt to reduce the effect of eutrophication of South African inland water quality the Department of Water Affairs implemented in 1985 a 1 mg/l ortho-phosphate standard for waste water treatment works effluents.
The amount of phosphate originating from the waste treatment works decreased significantly over the 10 years after the standard was implemented even though 40% of the time the works do not comply to the standard. This has resulted in no significant change in the chlorophyll values at Rand Water Intake No. 1. The algal species composition has changed with blue-greens becoming more dominant which has resulted in a higher incidence of algal related problems. The stabilizing of the chlorophyll values is not only as a result of lower ortho-phosphate levels but also due to residence times, turbidity and conductivity.
To further reduce the algal concentrations at Rand Water Intake No. 1 the 1 mg/l ortho-phosphate standard will have to be enforced, the standard possibly reduced and diffuse sources managed on a catchment basis.