Reglering av IFAS-processen på Nykvarnsverket : Utvärdering av kväverening och implementering av en överordnad styrstrategi

Sammanfattning

In 2023, Tekniska verken AB implemented an IFAS-process at Nykvarnsverket in Linköping, which has since been manually controlled through adjustments of oxygen setpoint and flow rate. IFAS (Integrated Fixed-film Activated Sludge) is a biological nitrogen removal process in which the reactor contains both activated sludge and suspended biofilm carriers. This work concerns evaluation of the IFAS process and implementation of automatic control, with the aim of replacing the current manual operation. Through analysis of operational data and step-response experiments, the process dynamics were investigated together with a suitable control strategy for full-scale operation. The results showed that approximately 55 % of the total nitrification takes place in the IFAS reactor, which motivates active control of the process. It was also found that both the oxygen setpoint and the water flow rate influence the ammonia reduction. However, the study was limited to control of the oxygen setpoint as the energy efficiency of the different alternatives could not be reliably determined. The step response experiments indicated that the process exhibits slow dynamics, and they could not be used to tune a PI controller. Therefore, a discrete-time purely integrating controller was selected as a supervisory ammonia controller within an ABAC structure (Ammonia-Based Aeration Control). The controller was implemented at full scale and was found to reduce the need for manual supervision. After evaluating two different values of the integrator gain, a more conservative control strategy was deemed appropriate. A longer evaluation period is required to properly assess the long-term effects of the control strategy on nitrogen removal, energy consumption, and external carbon dosing. Keywords: IFAS, ABAC, automatic control, biological nitrogen removal, nitrification