Bauhs, Kayla, Bauhs

Bauhs, Kayla

Kayla Bauhs is a speaker at WEFTEC 2026.

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About Bauhs, Kayla

Kayla Bauhs is a speaker at WEFTEC 2026.

Bauhs, Kayla

Kayla Bauhs is a speaker at WEFTEC 2026.

Titles from this speaker

A Journey of Upgrades and Innovations to Achieve Capacity Improvements at Metro Water Services' Central WRF

Abstract

Introduction The Central WRF has undergone significant upgrades to improve its treatment efficiency and capacity. With a rated capacity of 125 MGD and peak secondary flow of 350 MGD, the facility's treatment process includes preliminary treatment, PCs, aeration tanks (ATs), SCs, and UV disinfection. A key upgrade was the conversion of the aeration tanks from draft tube aerators to fine bubble diffused aeration and from two-pass, complete-mix reactors, to a plug-flow A/O process. This upgrade came with several unique challenges due to the sidewater depth of the tanks exceeding 30 ft. The draft tube aerators were inefficient at mixing resulting in pockets of low DO or anoxic conditions and thus reduced nitrification efficiency and promoted low DO filamentous bulking and foaming. The deep tanks were also followed by SCs with a sidewater depth of 16-ft. These conditions would result in nitrogen gas dissolving into the process water in the deep ATs that would evolve in the SCs causing poor settling and floating sludge blankets. The design criteria for diffused aeration at such depths were also not available from manufacturers or in the limited literature. The deepest diffuser submergence that manufactures could factory test their diffusers at was 23 ft. Therefore, assumptions of SOTE had to be made during design that accounted for potentially lower SOTE values than expected due to the air-side oxygen depletion during bubble transit. The assumed values were validated after startup and commissioning by performing off-gas testing. Methods Relevant tank improvements in each of the 8 ATs included anaerobic selector zones, baffle walls, a new aeration system, surface wasting stations, and degassing zones. The new aeration system included blowers, control valves and airflow meters, instrumentation, and fine bubble diffusers. The efficiency of fine-pore diffusers was calculated in clean water according to the ASCE (2006) standard and in wastewater via off-gas testing (ASCE, 2018). Plug-flow was achieved by installing RFP baffle walls in a serpentine manner and divided into 5 aeration zones as shown in Fig. 1. Since the PC effluent and RAS are added separately to the anaerobic zones, an influent chimney was used to direct the influent to the bottom of the anaerobic zone where it would come into close contact with the incoming RAS flow as shown in Fig. 2. To degas the mixed liquor before the SCs, a novel solution was implemented that included mid-depth coarse bubble aeration at the end of the aerobic zones as shown in Fig. 2. This system is critical at peak flows because the degassing that occurs in the aerated effluent channels between the ATs and SCs is significantly reduced at such short retention times. To help control and reduce nuisance foaming, surface wasting stations were installed in each AT as shown in Fig. 3. As the mixed liquor flows past the wasting station, a surface baffle directs any foam to the wasting station and is wasted from the system. Results and Conclusions The anaerobic selector zone has been instrumental in improving sludge settleability. Selector zones create an environment with a high F/M to out-select filamentous organisms, favoring floc-formers with high substrate uptake rates. The first two upgraded ATs were put in service in Jun 2022, resulting in an immediate drop in SVI as demonstrated in Fig. 4. The SVI continued to stabilize as additional pairs of upgraded ATs were put in service in Sep and Nov 2022. SVI averaged 235 +/- 92 mL/g for Oct 2021 — May 2022, and subsequently averaged 91 +/- 41 mL/g for June 2022 — Dec 2022. The SVI has also started to track with changes in F/M: when F/M is greater than 1, SVI averages 69 +/- 18 mL/g. This suggests that selector zone operation is critical for SVI stability. The new surface wasting stations also contributed to improved settleability by removing nuisance organisms accumulated in surface scum that cause foaming and bulking. Rapid reductions in the scum layer were observed after the first upgraded ATs were put in service. A comparison of the surface scum of the SCs before and after upgrades are provided in Fig. 5. As shown in Fig. 6, the average effluent ammonia concentration was approximately 2.5 mg /L (Jan 2021 — May 2022) and after the initiation of the upgrades fell to consistently below the detection limit of 0.4 mg/L (>95% of samples). As shown in the plot, the effluent ammonia has not peaked above 2 mg/L even though the plant operates at an aggressive SRT of 6-8 days year-round. Off gas testing of three ATs was carried out to assess the change in performance with different diffuser age (2, 6, and 10 months of operation). The off-gas testing results are compared to the design assumptions and clean water testing in Table 1. The assumed alpha and fouling factors used to correct the clean water performance for field conditions aligned well with the values measured in the AT with 2-month-old diffusers. The fouling factors decreased significantly for the 6- and 10-month-old diffusers. This information supported the recommendation that the diffusers should be inspected and cleaned at least annually to maintain adequate efficiency. In conclusion, the upgrades at the Central WRF, including the replacement of the draft tube aerators with a fine bubble aeration system, have significantly improved plant performance. The full paper and presentation will contain expanded discussions and all technical detail

Recent upgrades to the deep aeration tanks at the MWS Central WRF have improved sludge settleability, nitrification reliability, and reduced nuisance foam. Key improvements included anaerobic selector zones, baffle walls to reduce short-circuiting, a new aeration system, surface wasting stations, and degassing zones. To degas the mixed liquor before the clarifiers, an innovate solution was implemented that included mid-depth coarse bubble aeration at the end of the aerobic zones.

SpeakerMiller, Mark

Presentation time

16:30:00

17:00:00

Session time

15:30:00

17:00:00

SessionOptimizing Design for Increased Capacity

Session number228

Session locationRoom 349

TopicFacility Operations and Maintenance, Intermediate Level, Municipal Wastewater Treatment Design

Author(s)

Miller, Mark, Jimenez, Jose, Bauhs, Kayla, Yarosz, Douglas, Rosso, Diego

Author(s)M. Miller1, J.A. Jimenez2, K. Bauhs3, D. Yarosz3, D. Rosso4

Author affiliation(s)1Brown and Caldwell, NC, 2Brown and Caldwell, FL, 3Brown and Caldwell, TN, 4University of California, Irvine, CA

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2024

DOI10.2175/193864718825159630

Volume / Issue

Content sourceWEFTEC

Word count17

Advancing Low-Energy Biological Nutrient Removal through Low Dissolved Oxygen Operation: Is Biological Selection the Key?

Abstract

Introduction In the activated sludge process (ASP), dissolved oxygen (DO) is an important factor for aeration cost, nitrification efficiency, nutrient removal, and N2O emission. Even though, the term 'low DO' has been used in a somewhat subjective manner and can vary depending on the context and specific requirements, the overall objective of this study is to pave the way for full-scale implementation of low DO BNR through scientific investigation to systematically answer questions on the impact of DO concentrations on nitrification, denitrification, BPR, microbial ecology, and greenhouse gas emissions. This abstract presents results developed as part of the WRF No. 5083 titled 'Advancing Low-Energy Biological Nitrogen and Phosphorus Removal.' Methodology To quantify the impact and benefits of low DO operation in the capacity and performance of existing ASP, several participating facilities that were operating at low DO and high DO conditions were studied as part of this project. Long-term operational data from the facilities was assessed and compared with the baseline information collected during the literature review. Mixed liquor samples from the participating utilities were collected and assessed through lab-scale studies. Low dissolved oxygen control and batch test procedures were developed as part of the project and used to investigate the impact of low DO on biokinetic rates. This information provided a unique comparison and tabulation of important biokinetic parameters to better understand the implications of low DO BNR on plant capacity and performance. In addition, this provides guidelines for the selection of proper important biokinetic coefficients for process modeling. Results and Discussion Bench-scale, and full-scale experiments were conducted to investigate the impact of DO concentrations on nitrification, denitrification, BPR, microbial ecology, and N2O emissions with the goal to advance low DO BNR. The following biokinetic insights were collected as part of this study. Nitrification: Specific nitrification rates for ASP operating with long-term exposure to low DO conditions were approximately 60-80% of the nitrification rates at high DO as presented in Figure 1. This study concluded that long-term low DO operation results in a shift in the nitrifier community structure, with comammox, and in some case ammonia oxidizing archaea, being selected over ammonia oxidizing bacteria. Experimental results confirm that these communities selected under stable low DO operation have a higher oxygen affinity than AOB and Nitrobacter-related NOB. TIN Removal: The maximum TIN removal rates were found at DO concentrations ranging from 0.2 to 0.5 mg/L. At this DO range, the nitrification rate equals the denitrification rate, which would therefore lead to complete SND in non-carbon limiting conditions as depicted in Figure 2. At DO concentrations of less than 0.2 mg/L, the TINRR is inhibited by nitrification and at DO concentrations higher than 0.5 mg/L, TINRR is limited by denitrification. This observation may change from plant to plant since there are multiple parameters that could affect the results including floc size and porosity, mixing intensity, and microbial population. Biological Phosphorus Removal: Biological phosphorus removal is not impacted by low DO operation. Based on the experimental results from 12 full-scale plants, the low DO phosphorus uptake rates measured in this study were higher compared to the P uptake rates for high DO/ conventional systems. Phosphorus uptake rates slows down significantly when the DO is 0.1 mg/L suggesting that the KDO for PAO is lower around 0.1 mg/L. N2O Emissions: In this work, pilot and bench-scale testing demonstrated that long-term exposure of biomass to low DO does not result in an increase in total N2O emissions (Fig. 3a). Even though this study did not investigate the specific N2O production mechanisms and pathways associated with low DO BNR, the work suggests that the N2O emissions are lower than conventional high DO systems since the relative abundance of AOB and Nitrobacter-related NOB are relatively low (Fig. 3b) limiting the N2O produced via NH2OH oxidation pathway and nitrifier denitrification by AOB. In the low DO BNR systems, Nitrospira-related NOB and comammox were dominant nitrifiers suggesting they do not have the same reduction pathways of NO2 and NO to N2O and therefore produce less N2O comparable to AOB. Conclusion DO is an important parameter influencing aeration cost, nitrification efficiency, nitrogen and phosphorus removal efficiency, treatment plant capacity and N2O emission. As presented in this study, there are several WRRF that utilize low DO operation in the ASP achieving effluent water quality with respect to N and P and require less energy than conventional ASP. The DO setpoints in such plants vary depending on loading conditions, aeration controls, and operator's preferences. Therefore, selecting a single DO setpoint to represent low DO BNR is difficult. Based on the experimental results obtained during this study, low DO can be defined as the bulk liquid DO concentration or range in the aerobic zones where simultaneous N and P removal reactions occur, often ranging from 0.2 mg/L to 0.5 mg/L. Results from this study demonstrate that long-term exposure to biomass at these DO concentrations allow for biological selection of microorganisms with very high oxygen affinities.

In the ASP, DO is an important factor for cost, nitrification efficiency, and N2O emission. The objective of this study was to investigate the impact of DO concentrations on nitrification, denitrification, BPR, microbial ecology, and N2O emissions. Based on the experimental, low DO can be defined as the DO concentration or range in the aerobic zones where SND reactions occur, often ranging from 0.2 to 0.5 mg/L allowing for biological selection of microorganisms with very high oxygen affinities.

SpeakerJimenez, Jose

Presentation time

13:40:00

13:50:00

Session time

13:30:00

15:00:00

SessionMicrobial Ecology Selection Under Low DO Concentration

Session number409

Session locationRoom 253

TopicAdvanced Level, Energy Production, Conservation, and Management, Municipal Wastewater Treatment Design, Nutrients, Research and Innovation

Author(s)

Jimenez, Jose, Miller, Mark, Bauhs, Kayla, Sturm, Belinda, Wittman, Megan, Dold, Peter, Fevig, Stephanie

Author(s)J.A. Jimenez1, M. Miller2, K. Bauhs3, B. Sturm4, M. Wittman4, P. Dold5, S.M. Fevig6

Author affiliation(s)1Brown and Caldwell, FL, 2Brown and Caldwell, NC, 3Brown and Caldwell, TN, 4University of Kansas, KS, 4, 5Envirosim Associates Ltd., ON, 6South Platte Renew, CO

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2024

DOI10.2175/193864718825159576

Volume / Issue

Content sourceWEFTEC

Word count16

Description: Denitrification, Hydrolysis and Internal Store Carbon in Low Dissolved Oxygen...

Denitrification, Hydrolysis and Internal Store Carbon in Low Dissolved Oxygen Biological Nutrient Removal

Abstract

Introduction

Dissolved oxygen (DO) is a key factor in the activated sludge process, influencing aeration costs, nitrogen/phosphorus removal, and N2O emissions. Research shows nitrifiers and denitrifiers can coexist and function in low DO zones, enabling 'simultaneous nitrification-denitrification' (SND). This highlights microbial adaptation to suboptimal nitrogen removal conditions. While progress has been made on nitrification and microbial selection at low DO (WRF Project No. 5083, 2025), low DO denitrification during SND remains underexplored. Understanding its mechanisms and influencing factors is crucial for optimizing low DO BNR systems, enhancing nitrogen removal, reducing costs, and improving carbon management.

Among the critical processes in BNR, denitrification (DN) -the biological conversion of nitrogen oxide (nitrate [NO3], nitrite [NO2-]) to nitrogen gas (N2)-plays a pivotal role in removing nitrogen from wastewater. Conventionally, DN is considered an anoxic process, occurring in environments with low or negligible DO concentrations. However, studies have challenged this paradigm by demonstrating the potential for DN under low DO conditions, opening new avenues for optimizing BNR systems using aeration controls.

The interplay between DN, DO conditions and electron donor are particularly relevant in energy-efficient wastewater treatment designs, which aim to reduce aeration energy without compromising treatment efficacy. This paper explores the dynamics of DN in low DO environments within BNR systems providing insights into hydrolysis and internally store carbon utilization. Key factors such as microbial community structure, substrate availability, removal pathways and reactor design are analyzed to understand their roles in enabling DN under such conditions. The findings aim to inform the development of optimized BNR strategies that leverage low DO DN during SND to improve efficiency, reduce energy consumption, and address the growing need for sustainable wastewater treatment solutions.

Materials and Methods
To evaluate the effects of oxygen on denitrification kinetics, microbial communities, and substrate availability, including hydrolysis, several full-scale and pilot-scale facilities operating under low DO conditions were analyzed. Long-term operational data from these facilities were compared with baseline information from a literature review. Mixed liquor samples were collected for laboratory-scale studies, and microbial ecology was examined using Quantitative PCR (qPCR) on DNA extracts. These findings enhanced understanding of the role of oxygen in DN within SND systems.

Results
The performance of full-scale low DO BNR plants was evaluated using one-year average data from facilities with DO levels ≤0.6 mg/L. These plants, with varied configurations, climates, and capacities, showed TIN removal from 65—95% with influent COD:N ratios of 7.5—13. SND contributed ~50% of TIN removal, depending on configuration. Complete nitrification (effluent NH4+ <0.5 mgN/L) was achieved. High selector F/M and rbCOD bleed-through enhanced DN under low DO conditions, as observed by profiling and lab tests not included in the abstract.

To study DO's effect on DN rates, mixed liquor from a low DO (<0.5 mg/L) plant performing SND was tested in batch reactors at varying DO concentrations (Figure 2). Results show maximum NUR under anoxic conditions, with rapid declines as DO rises. At 0.5 mg/L DO, NUR decreases by ~50%, and no DN occurs at 1.0 mg/L, likely due to fully aerobic flocs and oxidized carbon. The DN's oxygen half-saturation inhibition coefficient (Kio) averaged 0.19 ± 0.08 mg/L across facilities. This is an important process modeling parameter to simulate SND.

The role of soluble substrate (Ss) in DN is well-studied, but less is known about the role of the carbon dynamics on DN in low DO systems. Particulate substrate (Xs) biodegradation begins with hydrolysis, a critical step in DN during SND. In wastewater treatment, hydrolysis involves extracellular enzymes in microbial flocs converting Xs to Ss, enabling bacterial growth and carbon availability for denitrification. This process, slower than heterotrophic growth, is the rate-limiting step for organic compound degradation and DN in SND. DN rates were tested in two NUR batch tests under anoxic and low DO conditions using settled wastewater and acetate (Figures 3). In anoxic tests, rapid NO3 reduction with sCOD removal was followed by slower NO3 reduction attributed to hydrolysis, with specific sCOD removal of 4.8 mg sCOD/mgNO3. In low DO tests, three NUR slopes were observed: rapid NUR using sCOD, slower NUR using stored COD, and the slowest NUR via hydrolysis. The higher sCOD removal (e.g., 6.2 mg sCOD/mgNO3) was likely due to aerobic oxidation for cell growth. Process model predictions were fitted to the measured NUR and the hydrolysis rates were 2.9 d-1 (anoxic) and 3.8 d-1 (low DO).

Conclusions
SND has the potential to maximize TIN removal with influent carbon while reducing energy and chemical dependency. Denitrification in low DO systems is a critical step in SND with many factors in play. Understanding internal stored carbon and hydrolysis are key aspects of denitrification, especially in low DO systems. This study establishes important kinetic parameters including hydrolysis rate that can be used to simulate denitrification in low DO systems.

This paper was presented at WEFTEC 2025, held September 27-October 1, 2025 in Chicago, Illinois.

Presentation time

08:50:00

09:00:00

Session time

08:30:00

10:00:00

SessionMicrobial Selection and Performance Under Low DO Conditions

Session locationMcCormick Place, Chicago, Illinois, USA

TopicLiquid Stream Treatment - Nutrient Removal and Recovery

Author(s)

Jimenez, Jose, Bauhs, Kayla, Al-Omari, Ahmed, Garrido, Manel, Sturm, Belinda, Miller, Mark

Author(s)J. Jimenez1, K. Bauhs1, A. Al-Omari1, M. Garrido1, B. Sturm, M. Miller1

Author affiliation(s)Brown and Caldwell1, Jacobs2

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Sep 2025

DOI10.2175/193864718825159962

Volume / Issue

Content sourceWEFTEC

Word count14

Description: Distinguishing AOB and Comammox Nitrifier Kinetics to Effectively Nitrify at Low...

Distinguishing AOB and Comammox Nitrifier Kinetics to Effectively Nitrify at Low Dissolved Oxygen Conditions

Abstract

Background
Low dissolved oxygen (DO) operation (0.3 — 0.6 mg/L) enables biological nutrient removal (BNR) facilities to reduce energy and costs associated with aeration while achieving nitrogen and phosphorus removal. Although operator concerns about nitrification at low DO exist, our previous work shows that efficient nitrification is possible at low DO because low DO operation leads to biological selection of nitrification communities with high oxygen affinities1. Nitrification often occurs in a two-step process by autotrophic ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB). However, nitrifiers are a diverse community, with ammonia oxidizing archaea (AOA) and complete ammonia oxidizing bacteria (comammox, CMX) contributing to ammonia removal (Figure 1). Nitrifiers at low DO concentrations adapt with lower apparent half saturation constants (KDO) 1. It is unclear if these lower observed KDO values are due to selection of different nitrifiers or another mechanism. Initial qPCR results showed that the efficient nitrification at low DO coincides with an increase in the relative abundance of comammox. Yet current methods to determine the nitrification kinetics do not distinguish between active nitrifiers. The KDO of specific nitrifiers like comammox have yet to be reported 2. Additionally, investigating comammox kinetics is important for low DO operation, as comammox abundance may coincide with a decrease in nitrous oxide (N2O) emissions because comammox lacks nitric oxide reductases (NOR) that produce N2O through the nitrifier denitrification pathway3. In this study, the KDO for individual nitrifying communities were determined across low DO, mid DO, and high DO operating regimes, in addition to capturing the associated N2O generation. This will provide a better understanding of low DO nitrification, enabling more confidence in designing plants to operate at low DO. Objective: Determine KDO values for individual nitrifying organisms, including AOB and Comammox, to investigate if adaptations to low DO concentrations are driven by the selection of different nitrifiers.

Methods
A 200-L pilot-scale reactor was operated as an A2/O process, detailed in our previous work1. Three experimental phases were completed to facilitate varying nitrifier diversity based on changes to bulk DO: Phase 1 DO = 0.3 mg/L (Low DO), Phase 2 DO = 0.8 mg/L (Mid DO), Phase 3 DO = 2.0 mg/L (High DO). After three SRTs at the operating DO, steady state was established for the reactors. Reactor chemical performance was tracked weekly to evaluate nutrient removal, and DNA analysis was also performed weekly to evaluate microbial changes in the nitrifying communities.

The KDO and maximum ammonia removal rates were determined for ammonia oxidation, while monitoring N2O production with Unisense N2O probes. Mixed liquor samples from the pilot plant were controlled in batch tests at a specified DO setpoint (i.e., 0.2, 0.5, 0.7 and 2 mg O2/L). At the beginning of the test, 30 mg/L of NH3-N was added, and all nitrogen species were measured for 1-2 hours. Inhibitory chemicals were added to isolate nitrifier communities: Chlorate inhibits comammox, and 1-octyne inhibits AOB (Table 1) 6,7,8.9. Initial OUR testing determined the dosage of inhibitory chemicals to add.

Results
Nitrification was well maintained in all phases, with average effluent ammonia concentrations of 0.37 ± 0.71 mg/L at low DO, 0.05 ± 0.02 mg/L at mid DO, and 0.19 ± 0.11 mg/L at high DO. Low liquid phase N2O was usually observed in the reactor but peaks in N2O were observed when DO was outside of the desired operating band for an extended period. Monod curves were fitted for each experiment within each reactor operating phase to determine the KDO and the maximum activity rate. When considering the entire nitrifying community, or when no biomass was inhibited, lower KDO were observed at lower bulk DO conditions (Figure 2). Specifically, for the same test when all microbial community contributions to nitrification are considered, biomass acclimated to low DO had a KDO of 0.25 mg/L, compared to a high DO KDO of 1.21 mg/L, confirming the expectation that nitrifiers in lower DO systems adapt with lower observed KDO values (Table 2).

Initial qPCR results showed that the relative abundance of comammox was the highest at low DO (10.9%), but AOB and canonical NOB remained as the dominant nitrifiers in the system. All qPCR results will be included in the conference presentation. Inhibition testing confirmed lower KDO values for comammox at low and mid DO, demonstrating higher oxygen affinity and enhanced nitrification (Figure 3). Additionally, for all DO operation modes, the CMX KDO is lower than the overall KDO and the nitritation KDO, as well as the maximum growth rate. The CMX KDO for low DO and mid DO sludge ranged from 0.15 to 0.23 mg/L and since the comammox abundance appears to increase with the reduction in DO, the presence of comammox seems to help explain the overall KDO reduction with the low DO sludge1.

Conclusion and Significance Comammox KDO is lower than the AOB KDO at Low and Mid DO conditions, providing a selective advantage for comammox at low DO conditions and enabling nitrification. AOB may not adapt to low DO; however, due to the comammox selection at low DO, nitrification is possible.

This paper was presented at WEFTEC 2025, held September 27-October 1, 2025 in Chicago, Illinois.

Presentation time

08:40:00

08:50:00

Session time

08:30:00

10:00:00

SessionMicrobial Selection and Performance Under Low DO Conditions

Session locationMcCormick Place, Chicago, Illinois, USA

TopicResearch

Author(s)

Wittman, Megan, Sturm, Belinda, Hiripitiyage, Yasawantha, Jimenez, Jose, Miller, Mark, Bauhs, Kayla

Author(s)M. Wittman1, B. Sturm1, Y. Hiripitiyage1, J. Jimenez2, M. Miller2, K. Bauhs2

Author affiliation(s)University of Kansas1, Brown and Caldwell2

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Sep 2025

DOI10.2175/193864718825160108

Volume / Issue

Content sourceWEFTEC

Word count15

Evaluating Nitrogen Oxides Transformations at Low Dissolved Oxygen Concentrations

Abstract

This paper was presented at WEFTEC 2023 in Chicago, IL.

This study evaluates nitrogen oxides transformations at low-DO concentrations. Microbial community data coupled with biokinetics, and detailed process modeling facilitated an analysis of the fundamental mechanisms associated with SND and its interaction between biomass and intermediate nitrogen oxide species and readily biodegradable COD.

SpeakerJimenez, Jose

Presentation time

08:30:00

08:50:00

Session time

08:30:00

10:00:00

SessionLow Carbon BNR Fundamentals and Controls

Session locationRoom S502 - Level 5

TopicAdvanced Level, Facility Operations and Maintenance, Municipal Wastewater Treatment Design, Nutrients

Author(s)

Jimenez, Jose

Author(s)J. Jimenez 1; J. Boltz 2 ; M. Miller 3; K. Bauhs 5; B. Sturm 6; B. Rittmann 7; J. Jimenez 1;

Author affiliation(s)Brown and Caldwell 1; Woodard and Curran 2 ; Brown and Caldwell 3; Brown and Caldwell 5; University of Kansas 6; Arizona State University 7; Brown and Caldwell 1;

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2023

DOI10.2175/193864718825159062

Volume / Issue

Content sourceWEFTEC

Word count10

Exploring the Benefits of Internally Stored Carbon for Nutrient Removal

Abstract

Discover the future of nutrient removal during this knowledge development session. Join us as we explore how harnessing internally stored carbon (ISC) can improve BNR processes. This session will highlight innovative strategies to achieve lower nutrient discharges, cut energy usage, and maximize the value of wastewater carbon-without the high costs and drawbacks of external carbon addition. Learn how ISC, stored by bacteria as glycogen and PHAs, unlocks new pathways to boost both nitrogen and phosphorus removal and enhance sludge densification. Gain insights into the real-world benefits, challenges, and implementation strategies for ISC-driven BNR, and leave with actionable guidance to optimize your plant's performance and sustainability.

This paper was presented at WEFTEC 2026 in New Orleans, Louisiana.

Presentation time

15:30:00

17:00:00

Session time

15:30:00

17:00:00

SessionExploring the Benefits of Internally Stored Carbon for Nutrient Removal

Session locationErnest N. Morial Convention Center

TopicMunicipal Wastewater Treatment Design, Nutrients, Research and Innovation

Author(s)

Bauhs, Kayla, Jimenez, Jose, Huggins, Isabel, Coats, Erik, Sturm, Belinda, Binty, Tasmin, Gagnon, Ali, Miller, Mark

Author(s)K. Bauhs1, J. Jimenez1, I. Huggins2, E. Coats2, B. Sturm3, T. Binty4, A. Gagnon5, M. Miller1

Author affiliation(s)Brown and Caldwell, 1Brown and Caldwell, 1University of Idaho, 2University of Idaho, 2Virginia Tech, 3University of Kansas, 4Hampton Roads Sanitation District (HRSD), 5Brown and Caldwell, 1

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Sep 2026

DOI10.2175/193864718825160280

Volume / Issue

Content sourceWEFTEC

Word count11

Investigating the Use of Internally Stored Carbon in Post-Anoxic Denitrification

Abstract

Three of Hampton Roads Sanitation District's (HRSD's) 5-stage conventional activated sludge Water Resource Recovery Facilities (WRRFs) add methanol as a carbon source for post-anoxic denitrification: the Virginia Initiative Plant (VIP), Nansemond Plant (NP), and Army Base (AB). From 2017-2020, the methanol dose at VIP averaged 0.50 lb COD/lb N removed across the treatment process, while much higher doses of 1.57 and 2.04 lb COD/lb N were required at NP and AB, respectively. Full-scale plant profile data at VIP indicates an estimated average of 33% of total NOx removal in the post-anoxic zone that cannot be attributed to methanol or endogenous decay (Figure 1). It was theorized that this remaining NOx removal is driven by anaerobically stored internal carbon, similar to or potentially the same as the storage of volatile fatty acids (VFA) as polyhydroxylalkanoate (PHA) by polyphosphate-accumulating organisms (PAOs) and glycogen-accumulating organisms (GAOs). The question explored here was the extent to which this internally stored carbon is carried through the pre-anoxic and aerobic zones unused and then contributes to denitrification in the post-anoxic zone. These results suggest that many factors affect this internally stored carbon-driven (internal C) denitrification, and it likely contributes to the significant methanol savings at VIP relative to NP and AB.
The capability for internal C denitrification was analyzed with mixed liquor collected from VIP, NP, and AB for lab-scale batch testing. These 'standard' batch tests included an anaerobic carbon storage phase spiked with acetate followed by aerobic and post-anoxic phases with and without external carbon addition (Figure 2). Internal C denitrification was observed in the batch reactors for mixed liquor from each WRRF, despite not being detected in all full-scale post-anoxic zones. The specific denitrification rate (SDNR) was measured after the mixed liquor was subjected to a 24-hour anoxic period to deplete any residual internal carbon, and these endogenous decay SDNRs were consistently lower than the standard batch test SDNRs when internal carbon is suspected to be present. This same temperature-controlled batch test was repeated over the course of several months for each WRRF, starting in late summer. The internal C SDNRs increased for the batch tests performed in winter, by as much as 1 mg N/g MLVSS/hr for VIP biomass (Figure 3). The ideal plug flow conditions in the batch test setting was potentially a reason for observing internal C denitrification with each[/i] mixed liquor; it occurs full-scale only at VIP, where the post-anoxic zone is much more plug flow-like. Specific factors affecting internal C denitrification were further evaluated through altering operation of the standard batch test, such as aerobic hydraulic residence time (HRT). Increasing the aerobic HRT by 2-4 hours lowered the SDNR by an average of 0.21-0.35 mg N/g MLVSS/hr. These results validate the importance of the aerobic phase operation to internal C denitrification, whether by controlling internal carbon availability or by some other mechanism. Biological phosphorus removal (bio-P) has been identified as a pre-requisite for internal C denitrification, likely because of the anaerobic storage phase (Vocks et al., 2005). Therefore, it was investigated whether or not mixed liquor from any[/i] bio-P plant was capable of internal C denitrification. Using mixed liquor from HRSD's bio-P plants that are non-nitrifying/denitrifying, no denitrification beyond that from endogenous decay was observed in the standard batch tests. Figure 4 shows an extended anoxic phase at the end of the test, and no change in the denitrification rate for the non-nitrifying WRRF biomass compared to a distinct change for VIP biomass when the internal carbon source presumably has been depleted. It was deduced that internal C denitrifiers need prior exposure to NOx, so bio-P biomass for this type of denitrification was not the only pre-requisite. Exposure to NOx is not a problem for WRRFs such as VIP, NP, or AB where nitrification/denitrification is obviously achieved, but it could factor into considerations as for aeration control that would affect NOx concentrations. These results also suggest that internal C denitrifiers are a distinct subset of heterotrophs. The effect of the amount of anaerobic acetate added in the batch test on the internal C SDNR was also evaluated, as anaerobic VFA augmentation has previously been shown to increase post-anoxic SDNRs (Coats et al., 2011). The increase in internal C SDNRs when the acetate dose was increased from 20 mg COD/L to 100 mg COD/L ranged from 0.06 to 0.28 mg N/g MLVSS/hr. Therefore, the quantity of carbon is one aspect of anaerobic storage that can control internal C denitrification.
Lastly, the internal C SDNRs were compared to the phosphorus (P) release and uptake rates for the batch tests. For each WRRF, there was a positive correlation between aerobic P uptake and internal C SDNR (Figure 5). There were no significant changes in the post-anoxic P concentration, so it is unlikely that denitrifying PAOs (dPAOs) are responsible for the internal C denitrification. Regardless, the trend with aerobic P uptake still indicates that internal C denitrification is tied to PAO activity. This is reflected in full-scale observations of the post-anoxic zone at VIP, where methanol addition and effluent P concentration tend to increase simultaneously (Figure 6). Higher effluent P concentrations from less effective bio-P performance can be expected in summer due to more competition from GAOs at higher temperatures (Lopez-Vazquez et al., 2009). If internal C denitrification is indeed linked to bio-P, poor bio-P could reduce contributions from internal C, and thus explain the higher methanol addition rate observed at these times. This dependence on bio-P could explain the increase observed in standard batch test internal C SDNRs during winter, because this is when bio-P performance is better at these WRRFs.
Understanding the conditions that promote internal C denitrification are required to develop effective strategies for full-scale implementation. For instance, the advantage of shorter aerobic HRT could be realized through lowering the aerobic DO setpoint. In that case, benefits could be two-fold to both external carbon and aeration energy savings. Another key aspect was the relationship between internal C denitrification and bio-P, so encouraging bio-P performance, such as through GAO out-selection, could improve not only P removal but nitrogen removal as well. The value of attaining internal C denitrification is exemplified by comparing methanol costs between WRRFs with and without this type of denitrification: while VIP treats almost three times as much flow as AB, their 2020 methanol cost was less than half of that of AB. A combination of operational changes that are advantageous to internal C denitrification based on the results of this study is likely to be a starting point for achieving methanol savings that would rival those at VIP.

Significant methanol savings are hypothesized to result from anaerobic storage of internal carbon that is used for post-anoxic denitrification; up to a ten-fold reduction in methanol has been observed full-scale. An investigation into this internal carbon-driven denitrification was performed via a series of batch tests using biomass from Hampton Roads Sanitation District's water resource recovery facilities.

SpeakerBauhs, Kayla

Presentation time

13:55:00

14:10:00

Session time

13:30:00

15:00:00

TopicIntermediate Level, Municipal Wastewater Treatment Design, Nutrients, Research and Innovation

Author(s)

Bauhs, Kayla

Author(s)Kayla Bauhs1; Alexandria Gagnon2,3; Charles Bott3

Author affiliation(s)Brown and Caldwell, Nashville, TN1; Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA2; Hampton Roads Sanitation District, Virginia Beach, VA3

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2022

DOI10.2175/193864718825158617

Volume / Issue

Content sourceWEFTEC

Word count11

Description: Model-based Investigation of the Fate and Transport of Internal and External Carbon...

Model-based Investigation of the Fate and Transport of Internal and External Carbon Sources to Drive Partial deNitrification (PdN)-based Nitrogen Removal Processes

Abstract

This paper was presented at WEFTEC 2023 in Chicago, IL.

SpeakerAl-Omari, Ahmed

Presentation time

11:30:00

11:45:00

Session time

10:30:00

12:00:00

SessionPushing the Boundaries of Our Biological Models

Session locationRoom S404a - Level 4

TopicFacility Operations and Maintenance, Intermediate Level, Municipal Wastewater Treatment Design, Research and Innovation

Author(s)

Al-Omari, Ahmed

Author(s)A. Al-Omari 1; P. Regmi 2 ; T. Wadhawan 3; M.L. Bachmann 4; K. Bauhs 5; J.A. Jimenez 7; I. Takacs 8; CB Bott 4; A. Al-Omari 1;

Author affiliation(s)Brown and Caldwell 1; HRSD 2 ; Dynamita 3; HRSD 4; Dynamita 5; Dynamita 7; Dynamita 8; Dynamita 4; Brown and Caldwell 1;

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2023

DOI10.2175/193864718825159193

Volume / Issue

Content sourceWEFTEC

Word count22

Tale of Two WRRFs: Optimizing Internal Carbon for Meeting Low Nutrient Limits

Abstract

Introduction This investigation draws on comparative analyses from two neighboring WRRFs employing the 5-stage Bardenpho process. WSSC Water operates three other BNR facilities, encouraging sharing of valuable insights and strategies for broader use in the water resource recovery community. Accordingly, this paper delineates: 1) Operational approaches that leverage high Carbon-to-Nitrogen (C/N) ratio (~12) to enhance nutrient removal efficiency. 2) A comparative analysis of Nitrogen (N) and Phosphorus (P) removal mechanisms in two WRRFs sharing similar configurations but different operational approaches. 3) A conceptual framework to evaluate and optimize simultaneous nitrification-denitrification (SND), pre-anoxic, and post-anoxic denitrification through endogenous respiration and internally stored carbon. Methods Both WRRFs are in Montgomery County, MD, and eventually discharge into the Chesapeake Bay. The stringent nutrient limits are TN < 4 mg/L and TP < 0.27 mg/L annually. Fig. 1 shows the side-by-side comparison of BNR trains from the two facilities. Table 1 shows the key operational differences. The experimental plan included: - Reactor profiling for ammonia, orthophosphate, nitrate, and nitrite with morning and afternoon grabs; - Influent and effluent monitoring of carbon, N, and P species; - Batch testing to determine specific denitrification rates without supplemental carbon, sampling just before the post-anoxic zone; - Tracking airflow; - Collecting DNA samples for microbial analysis; and - Collecting Polyhydroxyalkanoates (PHA) and glycogen samples to understand carbon transformations in post-anoxic denitrification. Results and Discussion The Seneca WRRF has successfully implemented ammonia-based aeration control (ABAC) for about one year. Both facilities achieve excellent nutrient removal. The 90th percentile values are 2.5 mgN/L for TN and 0.18 mgP/L for TP at Seneca, while Damascus reported 2.1 mgN/L for TN and 0.13 mgP/L for TP. Notably, these outcomes are achieved without supplemental carbon. Furthermore, Seneca's ABAC system reduces aeration demand by 20% due to low dissolved oxygen (DO) operation compared to Damascus, which operates at high DO (Fig. 2). N Removal: SND represents the primary N removal mechanism, accounting for 69% of TN elimination at Seneca and 55% at Damascus (Fig. 3). Given the operational conditions-ABAC at Seneca and high DO at Damascus-the superior SND performance at Seneca aligns with expectations. Surprisingly, Damascus also exhibits substantial SND activity despite the higher DO. Both facilities demonstrate comparable pre-anoxic zone contributions to N removal. Nonetheless, Damascus depends more on post-anoxic denitrification, achieving 7.0 mgN/L vs. 3.5 mgN/L at Seneca, contributing to exceptional effluent TIN levels (Fig. 4). The post-anoxic zone makes up 20% of Damascus's total volume, double Seneca's 9% (Table 1). The maximum specific denitrification rates were also slightly higher at Damascus than at Seneca (Fig. 5). Additionally, denitrification rates in the absence of external carbon sources, measured at the end of the main aerobic zone-indicative of internal carbon storage-were higher at Damascus by ~0.3 mgN/gVSS/hr compared to Seneca. These measurements exceed the baseline endogenous denitrification rate of 0.35 mgN/gVSS/hr, suggesting a not-so-trivial role for internally stored carbon in the denitrification process. P Removal: Seneca and Damascus use both biological and chemical P removal to meet strict TP limits. Aluminum sulfate (alum) is added before the secondary clarifiers to precipitate residual P. Seneca relies more on bioP and uses half as much alum per million gallons per day (MGD) compared to Damascus (Fig. 6). Notably, recent operational shifts at both aim to reduce the alum dose (Seneca: 15 gallons of alum/MGD; Damascus: 35 gallons of alum/MGD). Enhanced P release in Seneca's pre-anoxic zone suggests more effective bioP activity than Damascus (Fig. 6). Additionally, a notable re-release of P is observed in Seneca's post-anoxic zone, likely due to a long SRT of ~25 days, which may lead to the decay of phosphorus-accumulating organisms. Despite a similar SRT, such a re-release is not evident at Damascus, potentially due to the inhibitory effects of high alum dosing on bioP processes (Fig. 7). With a strategic reduction in SRT and alum use, Seneca and Damascus are expected to rely on bioP to achieve the low TP limits. Particle Size Distribution and Settling: The extended SRT and high organic content of the influent contribute to a low anaerobic Food-to-Microorganism (F/M) ratio at both Seneca (1.8 gBOD/MLSS/d) and Damascus (1.4 gBOD/MLSS/d), which falls below the optimal ratio of 2.4 gBOD/MLSS/d recommended for favorable settling. Despite this, settling performance at both facilities remains commendably effective. Image analyses reveal a higher incidence of filamentous bacteria at Seneca than at Damascus; however, Seneca's biomass particles are larger (Fig. 8). Given that the lower SVI at Damascus than at Seneca, it can be inferred that filament abundance may significantly impact sludge settleability more than particle size.

Meeting stringent nutrient limits increases operational costs for WRRFs due to high energy demands for aeration and the need for supplemental chemicals. Traditional BNR processes can exacerbate costs through inefficient carbon management. This study compares two WRRFs using the 5-stage Bardenpho process, revealing strategies to optimize BNR and reduce external chemical reliance via SND and post-denitrification driven by internally stored carbon.

SpeakerRegmi, Pusker

Presentation time

09:30:00

10:00:00

Session time

08:30:00

10:00:00

SessionInternal Stored Carbon for Nutrient Removal

Session number309

Session locationRoom 243

TopicAdvanced Level, Nutrients, Research and Innovation

Author(s)

Regmi, Pusker, Nguyen, Caroline, Bauhs, Kayla, Yeakle, Brad, Marinelli, Michael, Rowe, Allen

Author(s)P. Regmi1, C.K. Nguyen2, K. Bauhs3, B.C. Yeakle4, M. Marinelli5, A. Rowe6

Author affiliation(s)1Brown and Caldwell, MD, 2WSSC Water, MD, 3Brown and Caldwell, TN, 4WSSC, MD, 5Washington Suburban Sanitary Commision, MD, 6Washington suburban sanitary commission, MD

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2024

DOI10.2175/193864718825159579

Volume / Issue

Content sourceWEFTEC

Word count13

Towards Sustainable WRRFs: Exploring the Potential of Internally Stored Carbon for Efficient BNR

Abstract

Introduction Water resource recovery facilities (WRRFs) may rely on external carbon sources like methanol for denitrification. However, carbon may be stored within the biomass to be used later in the biological nutrient removal (BNR) process for denitrification either post-anoxically or via simultaneous nitrification-denitrification (SND) and may reduce these external carbon requirements. Evidence of internally stored carbon (ISC) denitrification has been observed at full-scale (Bauhs et al., 2022; Regmi et al., 2023), although to a limited extent. 5-stage BNR facilities that are the focus of this analysis include the Seneca WRRF and Damascus WRRF (Washington Suburban Sanitary Commission [WSSC]), and the Virginia Initiative Plant (VIP) and Nansemond Treatment Plant (NTP) (Hampton Roads Sanitation District [HRSD]). Seneca and Damascus show indications of ISC denitrification: low to no methanol addition and yet 4-7 mg N/L of post-anoxic nitrate reduction. Similarly, profiles at VIP attribute 33% of post-anoxic N removal to ISC. NTP serves as a reference facility with no ISC denitrification. This analysis presents a framework for further implementation of ISC denitrification, drawing on an extensive literature review and observations from full-scale WRRFs. Operational Factors Several factors have been theorized to contribute to ISC denitrification at the WSSC and HRSD facilities. High selector F/M and influent C/N: High food-to-microorganism (F/M) ratios promote substrate storage in selector zones, as is the case for densified systems (Sturm 2020). The F/M at VIP far exceeds the recommended value of 2.4 g BOD/g MLSS/d for anaerobic selectors (Jenkins et al., 2003). All three ISC denitrifying facilities have high influent carbon-to-nitrogen (C/N) ratios above 12. Low IMLR: Lower internal mixed liquor recycle (IMLR) rates promote deeper anaerobic conditions upfront in the process for better carbon storage. IMLR rates at Seneca were reduced from 400% to 200%, and Figure 1 shows the significantly lower batch test post-anoxic ISC denitrification rates for the 400% IMLR rate. Low DO: This results in less oxidation of ISC prior to denitrification. Implementation of ABAC at Seneca (averaging 0.3 mg/L DO) improved SND and reduced post-anoxic methanol addition (Regmi et al, 2023). Batch tests at HRSD showed longer aerobic HRT led to lower ISC denitrification rates (Bauhs et al., 2022). Bio-P performance: Sources have identified PHA (Carvalho et al., 2007; Qin et al., 2005) and/or glycogen (Coats et al., 2011; Vocks et al., 2005; Winkler et al., 2011) as the likely form of ISC for denitrification, both of which are forms of carbon in the metabolism of organisms involved in bio-P. Batch testing of HRSD biomass showed a positive correlation of ISC denitrification rates with P uptake rates (Bauhs et al., 2022). Plug flow reactor configuration: The high substrate gradient may enable access to this carbon source. The post-anoxic zone configuration at VIP is equivalent to over 30 continuously stirred tank reactors (CSTRs) in series, compared to facilities like NTP with 3 CSTRs. Figure 2 shows an aerial of the post-anoxic zones at all four facilities. Benefits There are many benefits to implementation of ISC, as follows:

*Chemical Savings: Elimination of external carbon reduces chemical costs. Furthermore, less external carbon addition lowers sludge production and associated solids handling costs.

*Energy Savings: Lower IMLR reduces associated pumping energy while low DO operation leads to significant aeration energy savings.

*Settleability Improvements: Use of a selector zone to promote carbon storage is similar to biological selection factors for densification, so promoting ISC could improve settleability.

*Simplification of Process: Pre-anoxic zones could ultimately be eliminated (use an anaerobic/oxic/anoxic process as in Coats et al. (2023)). Furthermore, pending SND performance, the post-anoxic zone could be as well. Discussion Several knowledge gaps still remain, which if addressed could significantly increase implementation of ISC denitrification. First, how resilient is ISC for N removal? Responses to variation in WRRF loading or other process upsets is not yet understood. For instance, Figure 3 shows the inconsistency in methanol dose at VIP over many seasons. The viability of operating without a 'backup' system (i.e. external carbon addition) should be explored. Next, while several factors may promote ISC denitrification, which factors are the highest priority and what level of control do they offer? Table 1 summarizes these factors for each of the four full-scale facilities, but notably for Damascus, most are absent. Clearly some aspects of operation/configuration may be more important than others, or haven't yet been identified. Some, like ABAC for low DO, may be relatively straightforward. Others, like boosting C/N, may require more guidance. Related to the somewhat surprising observations of low methanol use at Damascus, there arises another question-does reduction of methanol itself promote ISC denitrification? In other words, there could be some interference of methylotrophs with ISC denitrifying organisms (Bauhs 2021). Nitrate concentrations in these systems are not limiting; nevertheless, cease of external carbon feed could trigger a microbial population shift.

Evidence of internally stored carbon (ISC) denitrification has been observed at multiple of Washington Suburban Sanitary Commission's and Hampton Road Sanitation District's Water Resource Recovery Facilities (WRRFs), where minimal post-anoxic methanol addition is required to meet effluent total nitrogen limits. A framework for further implementation of ISC denitrification based on results from these WRRFs is presented, including key factors, benefits, and knowledge gaps.

SpeakerBauhs, Kayla

Presentation time

08:30:00

09:00:00

Session time

08:30:00

10:00:00

SessionInternal Stored Carbon for Nutrient Removal

Session number309

Session locationRoom 243

TopicAdvanced Level, Nutrients, Research and Innovation

Author(s)

Bauhs, Kayla, Regmi, Pusker

Author(s)K. Bauhs1, P. Regmi2

Author affiliation(s)1Brown and Caldwell, TN, 2Brown and Caldwell, MD

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2024

DOI10.2175/193864718825159723

Volume / Issue

Content sourceWEFTEC

Word count14

Description: Understanding the Kinetics of Densified Activated Sludge: Implications in Design and...

Understanding the Kinetics of Densified Activated Sludge: Implications in Design and Optimization

Abstract

INTRODUCTION
For densification at full-scale continuous flow water resource recovery facilities (WRRFs), biological selection principles derived from selector design and aerobic granular sludge (AGS) concepts have been coupled with physical selection to lower the sludge volume index (SVI) and achieve superior sludge settling. In densified activated sludge (DAS), flocs co-exist with granules, and significant efforts have been made to research their settling behavior. However, the respective roles of flocs and different sized granules in biological nutrient removal kinetics and their impact on process performance and design of densified WRRFs remain unclear. The relative abundance and microbial activity distribution of ammonia oxidizing bacteria (AOB), nitrite oxidizing bacteria (NOB), and ordinary heterotrophic organisms (OHO) among flocs and granules and its influence on the process performance was investigated. This study focused on the kinetic contribution of different particle size-classes in DAS.

MATERIAL AND METHODS
DAS samples from the full-scale WRRF at Pueblo CO's James R. Dilorio WRRF (referred herein as Pueblo) and from the DAS demonstration treatment train at Metro Water Recovery's Robert W. Hite Treatment Facility (referred herein as Metro) were collected and characterized.

Size-class distribution of DAS was determined by sieving: flocs were 200 µm or smaller particles, small granules were 200-600 µm and large granules were 600 µm and larger. Particle size distribution analysis was conducted for each size-class.

Samples for microbial community analysis were collected from each size-class and stored at -18°C. Microbial community analyses were conducted with Illumina-based 16S rRNA amplicon sequencing. Quantitative PCR (qPCR) was performed on the DNA extracts to quantify total bacteria (16S). Species-level microbial communities were determined with long-read amplicon sequencing.

The biological kinetic rates for nitrification, denitrification and SND for each size-class were assessed through lab-scale studies.

RESULTS AND DISCUSSION
Nitrification Activity for Different Size-Class Distribution: Figure 1 presents the microscopy imaging for the three particle size classes investigated during this study for Metro. Specific nitrification rate (NH4RR) experiments were performed, since abundance and activity of nitrifiers among the different size-classes in DAS are not necessarily proportional. In general, the larger the size of the bio-aggregates, the slower their specific AOB and NOB activities (Figure 2). Small aggregates or flocs have higher specific NH4RR at DO concentrations ranging from 0.2 to 3.0 mg/L. Figure 3 depicts the maximum nitrification rates for the size-classes for Pueblo and Metro's DAS samples; the highest rates were observed for the small granules. In the case of Pueblo, the large granules offered the lowest nitrification rates. For Metro, the maximum rates for flocs and large granules were comparable. Therefore, the benefit of large granules is unclear when it comes to nitrification in DAS.

Figure 4 shows the observed half-saturation oxygen concentration for AOB (KDO, AOB) for the size-classes determined from lab tests. As the size of aggregate increases, there is greater diffusion resistance, so the higher their apparent KDO, AOB (similar results for NOB were observed). The KDO, AOB for Pueblo are generally lower than those estimated at Metro, as Pueblo operates at comparatively low DO concentrations.

Denitrification and SND Activity for Different Size-Class Distribution: The larger the size of the bio-aggregates, the smaller their specific denitrification rates (SDR). In the case of Metro, 4.58, 3.80, and 3.11 mg N/g MLVSS/hr were measured for flocs, small granules, and large granules, respectively. TIN removal efficiencies via SND for the bio-aggregate classes from Pueblo were measured and results are presented in Figure 5. Lower DO concentrations increased SND for all the size-classes. The highest SND efficiencies were observed at DO concentrations of 0.3 mg/L, 0.75 mg/L and 1.0 mg/L for flocs, small and large granules, respectively. For granules, SND reactions were nitrification-limited at low DO conditions, since the operating DO was much lower than the apparent KDO, AOB (reported in Figure 4). After maximum TIN removal efficiencies were achieved for each size-class, higher DO values decreased the TIN removal as the oxygen in the bulk liquid penetrated deeper inside the granules reducing the anoxic zone. A main challenge is finding a balance for aeration: to achieve full nitrification while also establishing anoxic zones in the core of the granules to preserve SND.

Microbial Ecology for Size-Class Distribution: Results will be included for the paper and presentation.

CONCLUSION
The study confirmed that diffusion and counter-diffusion resistance of substrates are inversely correlated with the size of the particles. Nitrification is maximized as particle size is decreased or DO is increased. SND is maximized in flocs when DO is decreased because nitrification is not limited, while it is maximized in small and large granules at higher DO. Controlling the size-class distribution in DAS must be considered for the design and operation of BNR systems, as optimized size will balance settleability and nutrient removal benefits.

This paper was presented at WEFTEC 2025, held September 27-October 1, 2025 in Chicago, Illinois.

Presentation time

14:00:00

14:30:00

Session time

13:30:00

15:00:00

SessionDensified Activated Sludge (DAS) Fundamentals

Session locationMcCormick Place, Chicago, Illinois, USA

TopicLiquid Stream Treatment - Nutrient Removal and Recovery

Author(s)

Bauhs, Kayla, Jimenez, Jose, Al-Omari, Ahmed, Miller, Mark, Garrido, Manel, Freedman, Daniel, Maltos, Rudy, McGowan, Patrick, Sturm, Belinda

Author(s)K. Bauhs1, J. Jimenez1, A. Al-Omari1, M. Miller1, M. Garrido1, D. Freedman2, R. Maltos2, P. McGowan3, B. Sturm4

Author affiliation(s)Brown and Caldwell1, Metro Water Recovery2, City of Pueblo3, University of Kansas4

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Sep 2025

DOI10.2175/193864718825160029

Volume / Issue

Content sourceWEFTEC

Word count13

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