Full-scale tests carried out for the extension of a 700 000 PE treatment plant are used to show the consequences of advanced denitrification requirements. Three nitrate recirculation strategies were tested: feedback control via the nitrate content at the denitrification zone outflow, control via the redox potential and adaptive control of the recirculated nitrate load in accordance with the input of biodegradable materials, estimated on the basis of a BOD-MS measurement (3 minute BOD).
In a situation in which advanced, process-stable primary denitrification has to be carried out in a restricted space, especially in large treatment plants, controlled nitrate recirculation oriented on the requirement for biodegradables is a feasible method. If denitrification is limited by lack of biodegradable substrate, supplements in line with denitrification requirements will be needed. The necessary control can be implemented via a measurement of BOD-M3 in the influent and measurement of nitrate content in the effluent from the activated sludge tank.
Sequential removal of selenate, nitrate and sulfate and recovery of elemental selenium in a multi-stage bioreactor process with redox potential feedback control
