Investigation of applicable aeration strategies for the treatment of chromium in membran bioreactor units of domestic wastewater trestment plants

This thesis investigated the effects of intermittent aeration strategies applied in membrane bioreactor (MBR) systems on hexavalent chromium (Cr⁶⁺) removal performance from synthetic domestic wastewaters. A laboratory-scale MBR system with a total volume of 4.5 L was established for this study. The system was operated under six different operational periods, maintaining a constant chemical oxygen demand (COD) concentration (400 mg/L), a fixed sludge retention time (8 days), and a constant flux (12.5 LMH). After microorganisms adapted to the environment and ensuring high organic matter removal efficiency in the system, Cr⁶⁺ was initially added to the wastewater at a concentration of 1 mg/L. This concentration was later increased to 5 mg/L in later periods. While continuous aeration was applied during the first three periods, different intermittent aeration on/off cycles (30/30, 60/60, and 60/120 min aeration/non-aeration cycles) were tested in the last three periods. Under all operational conditions, the MBR consistently maintained a high COD removal efficiency ranging from 91% to 98%, with the peak value of 98% observed specifically during the 60/120 min intermittent aeration regime. While Cr⁶⁺ removal efficiency remained limited under continuous aeration (18–35%), it significantly increased to 95% with the implementation of intermittent aeration strategies. The highest Cr⁶⁺ removal performance was observed specifically during the 60 min/120 min on/off cycle. Initial high transmembrane pressure (TMP) values in the MBR system stabilized at 120 mbar after the sludge age was adjusted to 8 days. However, with the transition to intermittent aeration, fluctuations and increases in TMP values up to 240-400 mbar were observed. In conclusion, the intermittent aeration strategy and optimization of operating parameters in MBR systems emerge as an environmentally friendly and highly efficient alternative for both chromium reduction and organic matter removal. However, further solutions are needed to minimize membrane fouling to enhance the system's economic viability and sustainability.