Ammonia oxidizer adaptation to low dissolved oxygen concentrations for biological nutrient removal – a review on oxygen affinity, dual-substrate limitation, and decay

Abstract

Low dissolved oxygen (DO) operation in suspended growth biological nutrient removal (BNR) systems has become increasingly important for energy-efficient and intensified treatment processes. Novel strategies like continuous low DO, ammonia-based aeration control (ABAC), partial-denitrification anammox (PdNA), and partial-nitritation anammox (PNA) are becoming more commonplace and can drive processes to operate in DO regimes far below conventional norms. Yet nitrification, particularly the first step (ammonia oxidation), must be maintained. This review provides the first comprehensive overview of experimental data on ammonia oxidizer adaptation to low DO, along with analysis on how and why ammonia oxidizer oxygen kinetics respond to these low DO environments. This review examines the available evidence that stable ammonia oxidation at low DO is possible and usually corresponds to a reduction in the estimated ammonia oxidation Monod oxygen half-saturation coefficient (K_O2). These changes can arise from intrinsic shifts in microbial community structure (particularly increased comammox Nitrospira (CMX) abundance), from extant factors like floc and microcolony size, or both. Despite evidence for these shifts, the drivers, limits, and durability of kinetic adaptation remain open questions. Also reviewed is the potential interaction of oxygen and substrate affinity, which is poorly understood and infrequently studied, especially under the non-starvation ammonia conditions common in advanced processes. This review also highlights a need to better study ammonia oxidizer decay and decay rate adaptation to low DO. Despite being critical to determining ammonia oxidizer population and thus nitrification capacity, decay kinetics under low DO remain largely unmeasured, mischaracterized, or assumed constant. Existing methods for determining decay are not well suited to capture or characterize the impacts of low DO operation. These findings suggest a necessary shift in practice: ammonia oxidation kinetic parameters, especially K_O2, should not be treated as fixed parameters. SRT, DO, substrate concentration, and community structure all interact to drive nitrification performance. This review seeks to provide a comprehensive and systematic review of existing experimental results within each of these domains, with perspective on future research needs and implications for design and operation.