Charles Bott

Low–dissolved oxygen (DO) biological nutrient removal is an attractive option for energy and cost reduction, but it is not common practice as critical outcomes remain uncertain. Open questions include whether ammonia‐oxidizer oxygen kinetics are fixed or can adapt to operating conditions; whether low DO increases simultaneous nitrification and denitrification (SND); whether good settling can be maintained; and whether N ₂ O emissions rise or fall at low DO. In this study, a continuously fed pilot anaerobic/oxic (A/O) process was run with 6.5‐ to 8.5‐day solids retention time (SRT), stable temperature, and constant influent flow, and the operating DO was decreased from 2.0 to 0.2 mg O ₂ /L over multiple months while maintaining complete nitrification. In situ ammonia uptake rate, ammonia oxidizer oxygen affinity (K _O ) (via two independent batch methods), quantitative PCR (qPCR)‐based community composition, floc size, settling, and SND were monitored throughout the transition to low DO. N ₂ O was measured before and after the low DO transition. Ammonia oxidizer K _O decreased significantly with reduced operating DO, driven primarily by a shift toward comammox Nitrospira (CMX), particularly Clade A, which comprised > 80% of the nitrifier community at the lowest operating DO. SND, measured as total inorganic nitrogen (TIN) removal across the aerobic zone, did not increase at lower DO within this well‐mixed A/O configuration. Settling performance remained acceptable across setpoints. The low‐DO sampling campaign had lower N ₂ O emissions than the high‐DO campaign, coincident with CMX enrichment. Total supplied airflow was reduced by 44% at 0.2 relative to 2.0 mg O ₂ /L, indicating reduced aeration demand.

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.

In the present study, a novel pilot ozone contactor configuration was employed using hydrogen peroxide (H ₂ O ₂ ) and multiple ozone diffusion zones in an over-under contactor to reduce bromate formation in three wastewater effluents.

A Digital Twin built around Activated Sludge Model No. 1 was deployed at a full-scale water resource recovery facility. Its design included a waste rate recommender system based on automatic scenario analyses, where influent loads and waste rates are varied to determine their impact on nitrification. At the same frequency as these scenario analyses, scheduled auto-calibrations allow for nitrifier maximum specific growth rate (μmax-NITO) soft sensing, the only kinetic parameter shown to require adjustment if the objective is aeration tank effluent ammonia forecasting accuracy. By integrating temperature forecasting over the next three sludge ages, this Digital Twin approach creates opportunities for advancing waste rate decisions in anticipation of seasonal temperature changes, optimizing ammonia control authority under varying influent loads, and furnishing valuable insights for future capital projects requiring nitrifier kinetic understanding and modelling.

Wastewater alkalinity enhancement is a promising approach for ocean alkalinity enhancement due to its potential to deliver strong bases with minimum secondary precipitation and its potential use of the global network of wastewater treatment plants (WWTPs). WWTPs are also significant sources of CO₂ due to organic matter oxidation, and integrating alkalinity addition into treatment processes may both reduce in-plant CO₂ emissions and increase downstream CO₂ uptake. This study presents a modeling framework that combines a modern activated sludge model-based WWTP simulator with an integrated hydrodynamic-biogeochemical-carbonate chemistry model of coastal oceans. We evaluate the effects of adding alkalinity either upstream (UpAdd) of the biological treatment stage or downstream at the discharge location (DnAdd) on WWTP carbon emission reduction and marine CO₂ removal. The carbon emission from WWTPs decreases with increasing alkalinity dosage in UpAdd and can be eliminated at a dosage level that is feasible to implement. However, carbon uptake in the surrounding oceanic water is much reduced due to elevated dissolved inorganic carbon in the discharge water. DnAdd does not affect CO₂ emissions from WWTPs but enhances carbon uptake in the ocean, with the net oceanic uptake of atmospheric CO₂ increasing with increasing dosage level. Across all tested dosage levels, total CO₂ removal, including emission reduction at the WWTPs and enhanced carbon uptake in the ocean, is 30% greater in UpAdd than in DnAdd. WWTP treatment tanks have much higher pCO₂ than in the ocean, and aeration of process tanks enhances the gas transfer. The upstream alkalinity addition leads to sharp declines in pCO₂ in the treatment tanks and large reductions in carbon emission from the WWTPs. These results have implications for developing strategies to reduce global carbon emission and enhance oceanic carbon burial using WWTPs as a delivery mechanism.