A brief survey of activated sludge systems for nutrient removal from wastewaters iss given and their characteristic features are discussed. Four modifications of the nutrient removal activated sludge process were tested in pilot-plant units with a wastewater from the town of Pilsen. The re-aeration of the returned sludge proved to be advantageous for achieving high nitrogen removal. For the design of a new wastewater treatment plant for the town of Pilsen, the activated sludge process, consisting of the anaerobic-anoxic-oxic sequence with the reaeration of the returned sludge, was used. The newly designed technological line of the so-called R-AN-D-N Process is very flexible thanks to the wastewater inlet in both the anaerobic and the anoxic zone and to the possibility of introducing the internal recycle in either the anoxic or the anaerobic zone.

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