Extended aeration plants achieve almost complete nitrification, due to their mean solids retention time of more than two weeks. Intelligent control of nitrification and denitrification processes is called for, due to high energy-costs for additional oxygenation, uncontrolled denitrification processes in the final settling tanks (which impair the effluent quality) and lastly, high effluent nitrogen concentrations. Long experience is available from the application of an on-line nitrate monitor; interpretation of this gives detailed information about nitrification and denitrification processes and kinetics under real operating conditions, depending on temperature, MLSS, BOD and total nitrogen mass flow rates. The poor availability and reliability of the nitrate monitor as a control element led to the development of computer aided control of nitrogen removal processes. A common computer system has been completed with electronic filters, A/D converters and relay interfaces to control the functions of a new fine bubble diffusor system and three air blowers. The software is able to calculate the length of aerobic and anoxic intervals in order to realize a saw-tooth like graph of NOx concentration under various conditions. It also surveys the DO level during aeration periods.

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