Abstract:
Aeration of the biological reactors for nitrogen removal contributes a major component of the annual operational costs at Watercare’s Mangere Wastewater Treatment Plant. It has been shown that by extending constant dissolved oxygen control to allow variation of the dissolved oxygen setpoint based on ammonia measurements, significant savings in aeration can be achieved whilst maintaining effluent quality. A new control strategy has been designed for the Mangere Plant using a linear feedforward controller based on ammonium measurements in the secondary influent, and a PI feedback controller based on ammonium measurements in the final aerobic zone of the reactor to control the dissolved oxygen setpoint. A performance assessment of online ammonium analysers was carried out under field conditions at the plant to ensure their reliability and accuracy before implementation. Based on this, one instrument showed superior performance at matching peaks in the ammonium concentration and was recommended for use in the control strategy. A model of the reactor at the plant was also created in order to get initial values for the control parameters to reduce the tuning needs after implementation. A qualitative understanding of the relationship between the dissolved oxygen and ammonia concentrations was also gained from the model which suggested that large reductions in the dissolved oxygen concentration are possible in all zones of the reactor. The new control strategy could then be set up in the DeltaV control architecture used by the plant, ready for implementation on site. Unfortunately implementation was not able to be completed in time for an assessment of the controller’s performance to be included in this work.
