Some experiences concerning a retrofit design and operation for nutrients removal in small extended aeration plants are presented. In this study a new biological phosphorus removal process as well as a biological nitrogen removal process based on a sequential oxic-anoxic-oxic process is investigated. The denitrification in the first oxic zone has a high removing effect for nitrogen, and the oxidation reduction potential (ORP) value and dissolved oxygen (DO) concentration are useful indexes to maintain the optimum conditions of the first oxic zone for simultaneous denitrification. It is verified that the simultaneous denitrification can alleviate the alkalinity deficit problem against full nitrification by a stoichiometric analysis of alkalinity throughout the entire process. Additionally, a biological phosphorus removal process which uses a sludge blanket zone of secondary settling tank for phosphorus release is proposed.

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