On-line analyzers to monitor the effluent levels of ammonia and phosphate have been installed at all four nutrient removal activated sludge plants in Johannesburg. The data from these instruments is transferred to a SCADA system for control purposes. The on-line system is supplemented by laboratory data, which is stored on a separate computer system. A combination of this data is used for routine operational control.

Dissolved oxygen probes are installed in the aerobic reactors of all plants and are used for aeration control. Automatic aeration control has recently been installed at one of the plants. In this paper some experience with on-line monitoring is discussed in terms of its usefulness and cost savings achieved. Savings of 1 million US $ in the purchase of phosphate precipitation chemicals have been achieved in the current financial year as a result of more efficient operational control.

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