The paper describes the upgrading of an existing trickling filter (TF) plant by adding a new activated sludge (AS) plant in order to remove nutrients from the combined effluent of both plants. In spite of the low COD/TKN ratio, good nitrogen and phosphate removals were obtained in the three-stage Bardenpho (AS) plant. This is at variance with the University of Capetown mathematical model which predicted that nutrient removal in this plant would not be possible due to predicted recycle of nitrates to the anaerobic basin. The results showed that the average effluent phosphate was below 1 mg P/ℓ while the ammonia and nitrates could be controlled to values well below the limits of the South African General Standard. Possible reasons for the anomaly are given.

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