Biological phosphorus removal has come of age after having been introduced with the treatment plants for the city of Johannesburg, South Africa, in 1974. There is a large number of plants in the world today and a great variety of basin configurations is used. Generally, we see excellent performance with the right wastewater characteristics and the correct design and operation, but there are still some issues not fully understood that have led to the perception that biological phosphorus removal is unreliable and unpredictable. These issues include nitrates in the feed, excessive return activated sludge recycle rates, secondary release of phosphorus and toxicity. This paper focuses on some of the issues that still frustrate designers and plant operators. There are also many claims for excellent removal by plants that appear not to conform to the established requirements for biological phosphorus removal. There is normally an explanation if the full picture is considered. The conclusion is that biological phosphorus removal is predictable and reliable if the process is properly understood.

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