Date of Award

Summer 8-2023

Document Type


Degree Name

Master of Science (MS)


Civil & Environmental Engineering


Environmental Engineering

Committee Director

Gary Schafran

Committee Member

Charles B. Bott

Committee Member

Mujde Erten-Unal

Committee Member

Xixi Wang


Low dissolved oxygen (DO) operation is one of the innovative technologies in biological nutrient removal (BNR) processes that is being investigated due to the potential to reduce aeration requirements, reduce a treatment plant’s carbon footprint, and to lower costs associated with chemical addition to promote denitrification. These benefits are achieved through development of micro-zones, within aerated basins where simultaneous nitrification and denitrification (SND) can occur. In this study, an activated sludge BNR pilot was run in an anerobic/aerobic (A/O) configuration at a controlled temperature and constant flow to observe microorganism-driven transformations of nitrogen and phosphorus at varying low DO conditions. This operation allowed insight into the influence of low DO conditions on nitrogen and phosphorus transformations and the ability to quantify parameters in kinetic equations used to model the process. The DO concentrations in the aerobic reactors were gradually decreased after reaching a desired steady state concentration (setpoint), and batch tests using respirometry and substrate utilization methods were conducted at each DO setpoint. It was observed that even at a DO concentration of 0.4 mg/L, 100% nitrification and enhanced biological phosphorus removal (EBPR) could be achieved, with effluent NH3 concentration of 0.06 ± 0.02 mg-N/L and effluent orthophosphate (OP) concentration of 0.16 ± 0.05 mg-P/L. Therefore, stable nitrification and EBPR were possible under low DO conditions. It was found that ammonia oxidizing bacteria (AOB), nitrite oxidizing bacteria (NOB) and polyphosphate accumulating organisms (PAOs) adapted to gradual decrease in DO setpoints. This adaptation was manifested by a concurrent decrease in the oxygen half saturation coefficient (Ko) values, while maximum removal rates remained relatively constant. The heterotrophic biomass also adapted to gradual decrease in DO increasing their oxygen affinity. As a result, no net increase in SND efficiency was observed during the adaptation process and the effluent NO3-N remained constant at 10 mg/L. The oxygen kinetic parameters determined from this research can be further utilized for modeling purposes and can facilitate the design of the transition of high DO full-scale process to low DO.


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