Wankmuller, David

David Wankmuller is a Process Engineer at Hazen and Sawyer with 15 years of experience in wastewater treatment facility design and process optimization.

Titles from this speaker

Description: Application of Real-time Process Control and Post-anoxic Denitrification via...

Application of Real-time Process Control and Post-anoxic Denitrification via Internally Stored Carbon to Achieve Low TN Discharge Limits

Abstract

The North Durham Water Reclamation Facility (NDWRF) is a 20 mgd advanced biological nutrient removal (BNR) treatment facility, owned and operated by the City of Durham, NC that discharges to Falls Lake. In 2016 the facility began implementing state of the art control strategies in concert with excellent operational practices to reduce their effluent TN from approximately 6.0 mg/L in 2016 to 2.0 mg/L in 2019. From 2019 through 2022 the facility has averaged an effluent TN of 1.9 mg/L. Staff has been continuously optimizing the facility, pushing the limits of conventional treatment technologies. The NDWRF currently implements the following state of the art control strategies to reduce effluent TN and TP below typical conventional treatment limits: • Feed forward and feedback ammonia based aeration control (ABAC) • Nitrate based carbon addition • Diurnal ammonia based primary effluent equalization • Automatic SRT control utilizing solids analyzers • Sidestream Equalization This paper will i) detail the advanced control strategies the facility currently implements and the necessary operator maintenance/intervention necessary to ensure the programs run smoothly, and ii) demonstrate how process control has allowed operators to reduce supplemental carbon demand by leveraging internal carbon driven denitrification

The North Durham Water Reclamation Facility (NDWRF) is a 20 mgd advanced biological nutrient removal (BNR) treatment facility, owned and operated by the City of Durham, NC. In 2016 the facility began implementing state of the art control strategies in concert with excellent operational practices to reduce their effluent TN from approximately 6.0 mg/L in 2016 to 2.0 mg/L in 2019. From 2019 through 2022 the facility has averaged an effluent TN of 1.9 mg/L.

SpeakerWankmuller, David

Presentation time

09:10:00

09:30:00

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)

Wankmuller, David

Author(s)D. Wankmuller 1; J. Dodson 2 ; D. Wankmuller 1;

Author affiliation(s)Hazen and Sawyer 1; City of Durham 2 ; Hazen and Sawyer 1;

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2023

DOI10.2175/193864718825159063

Volume / Issue

Content sourceWEFTEC

Word count19

Description: In-Tank Carbon Generation as a Primary Benefit of RAS and MLSS Fermentation for...

In-Tank Carbon Generation as a Primary Benefit of RAS and MLSS Fermentation for Stabilizing Biological Phosphorus Removal Performance

Abstract

As effluent nutrient limits get more stringent, water resource recovery facilities (WRRFs) will continue to need to implement biological nitrogen (N) and phosphorus (P) removal. BPR systems in particular are susceptible to upset and many plants struggle to maintain BPR year-round because of a multitude of factors including, insufficient bioavailable carbon for PAOs, suboptimal aeration control, solids recycle impacts, excessive solids retention times, suboptimal pH operation, and secondary phosphorus release. Recently, it has been suggested that current BPR designs are inadequate for stable performance and that sidestream RAS fermentation is needed for stable BPR (Barnard et al., 2017). The authors propose that RAS fermentation selects for unique BPR organisms (e.g., Tetrasphaera) that can perform anoxic P uptake and also ferment in the presence of deeply anaerobic conditions redox conditions. Barnard et al., 2017 have also suggested that the influent wastewater characteristics can become irrelevant if RAS fermentation is implemented. Interestingly, recent work has shown that the microbial community in conventional BPR systems has less diversity than sidestream RAS or MLSS fermentation systems; however, the functional community (primary organisms responsible for BPR) were similar regardless of configuration (Onniss-Hayden et al., 2019). These authors were also careful to suggest that the "superior" performance of RAS/MLSS fermentation systems may be due to positive bias in their comparison of facilities. Given these findings, it is still not clear to practitioners what may be driving RAS/MLSS fermentation systems to have stable BPR. This work investigates the underlying reasons for scenarios where implementing RAS or MLSS fermentation as part of a flexible BNR design can help to stabilize BPR. Specifically, this work documents how RAS/MLSS fermentation can be used to ensure that sufficient carbon is provided to a BPR system that is subjected to shifts in wastewater characteristics in response to peak flow conditions and cold temperatures.

This work investigates the underlying reasons for scenarios where implementing RAS or MLSS fermentation as part of a flexible BNR design can help to stabilize BPR. Specifically, this work documents how RAS/MLSS fermentation can be used to ensure that sufficient carbon is provided to a BPR system that is subjected to shifts in wastewater characteristics in response to peak flow conditions and cold temperatures. Historical data, profile sampling, phosphorus release batch tests, and 48-hour fermentation batch tests were performed at two facilities to determine the carbon generation potential of the MLSS and RAS.

SpeakerWankmuller, David

Presentation time

14:00:00

14:15:00

Session time

13:30:00

15:00:00

SessionBringing It All Together: Guide to Designing and Implementing S2EBPR Processes

Session number502

TopicFacility Operations and Maintenance, Municipal Wastewater Treatment Design, Nutrients

Author(s)

David Wankmuller

Author(s)D. Wankmuller1; W. Khunjar2; D. Forney3; J. Pridgen4;

Author affiliation(s)Hazen and Sawyer, Raleigh, NC 1Hazen and Sawyer, Fairfax, NC 2Town of Cary, Cary, NC 3City of Wilson, Wilson, NC 4

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2021

DOI10.2175/193864718825158139

Volume / Issue

Content sourceWEFTEC

Word count19

Lessons Learned from Deammonification Startup treating THP Filtrate at two WRFs

Abstract

[b]Objectives[/b] This paper summarizes key lessons learned from startup of the first two U.S. AnitaMox IFAS deammonification systems implemented downstream of thermal hydrolysis (THP) and anaerobic digestion at the WSSC Water Piscataway Water Resource Recovery Facility (WRRF) (Figure 1) and the City of Raleigh's Neuse River RRF (NRRRF) (Figure 2). Both facilities transitioned to Class A biosolids production, increasing sidestream nitrogen loads from dewatering filtrate. Deammonification was incorporated to reliably maintain compliance with effluent discharge limits. Startup at Piscataway WRRF began in June 2024 and startup at NRRRF begin in May 2025.

[b]Status[/b] Piscataway WRRF The deammonification facility includes a common equalization tank and two AnitaMox IFAS reactors, each with a dedicated clarifier and RAS pump. Design loading rates are 0.52 kg NH3 N/m³ d (VLR) and 1.4 g NH3 N/m² d (SALR).
• Reactor 2 (R2) was seeded July 15, 2024 in MBBR mode. In early September, R2 experienced an acute toxicity event characterized by rapid nitrite accumulation and loss of attached biomass. Loading was reduced, and the reactor transitioned to IFAS mode in October. Over subsequent months, R2 fully recovered and now operates consistently at capacity.
• Reactor 1 (R1) was seeded October 15, 2024 and ramped up without performance issues. Both reactors have treated 500–750 kg NH3 N/day with 75–85% ammonia removal, occasionally operating above design SALR (Figure 3). NRRRF The NRRRF sidestream system consists of one EQ tank and one AnitaMox IFAS reactor with design loading rates of 0.66 kg NH3 N/m³ d VLR and 1.7 g NH3 N/m² d SALR. High filtrate TSS during early startup required holding flows constant until BFP performance improved, which is why SALR was constant for approximately 100 days (Figure 4). Normalized SALR trends show steady progress, and the reactor is currently treating ~315 kg NH3 N/day with 67% removal.

[b]Methodology[/b] Piscataway WRRF Historical influent data (TSS, COD, sCOD) and in-reactor nitrogen species were evaluated to understand R2's inhibition event. Batch activity tests performed by Hazen and Veolia assessed anammox and AOB activity and evaluated whether THP filtrate characteristics were inhibitory. Literature was reviewed to compare observed events with documented inhibition mechanisms. NRRRF Startup challenges included maintaining design temperature during early-season seeding and addressing high filtrate TSS. A rheology study performed by Bucknell University evaluated viscosity, shear-thinning behavior, and mixing needs for polymer conditioning to improve solids capture across the belt filter presses.

[b]Findings[/b] Piscataway WRRF Historical Data: R2 destabilized 53 days after seeding, with nitrite accumulation suggesting either temporary overloading or reduced anammox activity. Operators increased dilution, extended anoxic periods, and transitioned to IFAS mode on Day 108. Persistent nitrite variability led to a 'hard reset' on Day 113 by sharply reducing loading. R1, seeded later, exhibited stable startup, indicating the THP filtrate was not inherently inhibitory. Since recovery, both reactors have handled design and above-design loadings without further events. Activity Testing: Visual inspection and biomass density (Figure 5, R1: 30.9 g/m² vs. R2: 11.4 g/m²) indicate R2 lost anammox bacteria, reducing capacity. A variety of anammox activity tests were performed and the main conclusions from the activity testing include:
• Anammox activity increased in both reactors from November to December, indicating recovery in R2 and continued growth in R1 (Figure 6)
• R1 carriers + bulk liquid showed strong heterotrophic activity (simultaneous nitrite/nitrate removal), suggesting the need for COD fractionation analysis to determine impacts on TIN removal.
• AOB activity was present in both reactors, with limited NOB activity maintained by appropriate ammonia residuals. R2 AOB activity remained lower, consistent with prior biomass loss. Corrective Actions that were taken include:
• Reduced mechanical mixing and adjusted aeration intervals to stabilize biofilm.
• Improved BFP solids capture through optimized polymer and metal salt dosing.
• Adjusted THP operation (temperature, HRT) to reduce sCOD to deammonification. Literature indicates sCOD and colloidal organics can inhibit anammox and AOB, and THP parameter adjustments can reduce sCOD release. NRRRF THP digestate exhibited high viscosity and solids concentration, requiring increased mixing energy to achieve proper polymer conditioning. The rheology study recommended:
• High energy, adjustable mixing to provide sufficient velocity gradient.
• Dilution of digested sludge to lower viscosity.
• Multi point polymer addition to enhance floc formation and reduce demand. Raleigh installed an additional dynamic mixer ahead of the static mixer with dual polymer injection. Filtrate TSS has since remained below 750 mg/L, supporting stable IFAS loading.

[b]Significance[/b] Sidestream deammonification is essential for managing the increased nitrogen recycle associated with THP MAD systems at both WSSC Water and the City of Raleigh. The startup experience-including data evaluation, activity testing, filtrate quality improvement, and rheology-informed mixing strategies-provides practical guidance for future facilities implementing AnitaMox IFAS for THP filtrate treatment.

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

Presentation time

16:00:00

16:30:00

Session time

15:30:00

17:00:00

SessionRobust and Reliable Sidestream Nitrogen Removal

Session locationErnest N. Morial Convention Center

TopicFacility Operations and Maintenance, Municipal Wastewater Treatment Design, Nutrients, Research and Innovation

Author(s)

Wankmuller, David, Bailey, Erika, Fuentes, Silvia, Higgins, Matthew

Author(s)D. Wankmuller1, E. Bailey2, S. Fuentes3, M. Higgins4

Author affiliation(s)Hazen and Sawyer, 1City of Raleigh, 2Washington Sanitary Subruban Commission, 3Bucknell University, 4

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Sep 2026

DOI10.2175/193864718825160540

Volume / Issue

Content sourceWEFTEC

Word count12

Lessons Learned from Deammonification Startup treating THP Filtrate at two WRFs

Abstract

Objectives This paper summarizes key lessons learned from startup of the first two U.S. AnitaMox IFAS deammonification systems implemented downstream of thermal hydrolysis (THP) and anaerobic digestion at the WSSC Water Piscataway Water Resource Recovery Facility (WRRF) (Figure 1) and the City of Raleigh's Neuse River RRF (NRRRF) (Figure 2). Both facilities transitioned to Class A biosolids production, increasing sidestream nitrogen loads from dewatering filtrate. Deammonification was incorporated to reliably maintain compliance with effluent discharge limits. Startup at Piscataway WRRF began in June 2024 and startup at NRRRF begin in May 2025. Status WSSC Piscataway WRRF The deammonification facility includes a common equalization tank and two AnitaMox IFAS reactors, each with a dedicated clarifier and RAS pump. Design loading rates are 0.52 kg NH3 N/m³ d (VLR) and 1.4 g NH3 N/m² d (SALR). - Reactor 2 (R2) was seeded July 15, 2024 in MBBR mode. In early September, R2 experienced an acute toxicity event characterized by rapid nitrite accumulation and loss of attached biomass. Loading was reduced, and the reactor transitioned to IFAS mode in October. Over subsequent months, R2 fully recovered and now operates consistently at capacity. - Reactor 1 (R1) was seeded October 15, 2024 and ramped up without performance issues. Both reactors have treated 500-750 kg NH3 N/day with 75-85% ammonia removal, occasionally operating above design SALR (Figure 3). City of Raleigh NRRRF The NRRRF sidestream system consists of one EQ tank and one AnitaMox IFAS reactor with design loading rates of 0.66 kg NH3 N/m³ d VLR and 1.7 g NH3 N/m² d SALR. High filtrate TSS during early startup required holding flows constant until BFP performance improved, which is why SALR was constant for approximately 100 days (Figure 4). Normalized SALR trends show steady progress, and the reactor is currently treating ~315 kg NH3 N/day with 67% removal. Methodology WSSC Piscataway WRRF Historical influent data (TSS, COD, sCOD) and in-reactor nitrogen species were evaluated to understand R2's inhibition event. Batch activity tests performed by Hazen and Veolia assessed anammox and AOB activity and evaluated whether THP filtrate characteristics were inhibitory. Literature was reviewed to compare observed events with documented inhibition mechanisms. City of Raleigh NRRRF Startup challenges included maintaining design temperature during early-season seeding and addressing high filtrate TSS. A rheology study performed by Bucknell University evaluated viscosity, shear-thinning behavior, and mixing needs for polymer conditioning to improve solids capture across the belt filter presses. Findings WSSC Piscataway WRRF Historical Data: R2 destabilized 53 days after seeding, with nitrite accumulation suggesting either temporary overloading or reduced anammox activity. Operators increased dilution, extended anoxic periods, and transitioned to IFAS mode on Day 108. Persistent nitrite variability led to a 'hard reset' on Day 113 by sharply reducing loading. R1, seeded later, exhibited stable startup, indicating the THP filtrate was not inherently inhibitory. Since recovery, both reactors have handled design and above-design loadings without further events. Activity Testing: Visual inspection and biomass density (Figure 5, R1: 30.9 g/m² vs. R2: 11.4 g/m²) indicate R2 lost anammox bacteria, reducing capacity. A variety of anammox activity tests were performed and the main conclusions from the activity testing include: - Anammox activity increased in both reactors from November to December, indicating recovery in R2 and continued growth in R1 (Figure 6) - R1 carriers + bulk liquid showed strong heterotrophic activity (simultaneous nitrite/nitrate removal), suggesting the need for COD fractionation analysis to determine impacts on TIN removal. - AOB activity was present in both reactors, with limited NOB activity maintained by appropriate ammonia residuals. R2 AOB activity remained lower, consistent with prior biomass loss. Corrective Actions that were taken include: - Reduced mechanical mixing and adjusted aeration intervals to stabilize biofilm. - Improved BFP solids capture through optimized polymer and metal salt dosing. - Adjusted THP operation (temperature, HRT) to reduce sCOD to deammonification. Literature indicates sCOD and colloidal organics can inhibit anammox and AOB, and THP parameter adjustments can reduce sCOD release. City of Raleigh NRRRF THP digestate exhibited high viscosity and solids concentration, requiring increased mixing energy to achieve proper polymer conditioning. The rheology study recommended: 1.High energy, adjustable mixing to provide sufficient velocity gradient. 2.Dilution of digested sludge to lower viscosity. 3.Multi point polymer addition to enhance floc formation and reduce demand. Raleigh installed an additional dynamic mixer ahead of the static mixer with dual polymer injection. Filtrate TSS has since remained below 750 mg/L, supporting stable IFAS loading. Significance Sidestream deammonification is essential for managing the increased nitrogen recycle associated with THP MAD systems at both WSSC Water and the City of Raleigh. The startup experience-including data evaluation, activity testing, filtrate quality improvement, and rheology-informed mixing strategies-provides practical guidance for future facilities implementing AnitaMox IFAS for THP filtrate treatment.

This paper was presented at the WEF Residuals, Biosolids, and Treatment Technology Conference in Kansas City, MO, May 11-14, 2026.

Presentation time

15:30:00

15:45:00

Session time

15:30:00

17:00:00

SessionLessons Learned in Deammonification/ANAMOX Sidestream Treatment

Session locationKansas City Convention Center

TopicSidestream Treatment/Solid Recycle Streams

Author(s)

Wankmuller, David, Bailey, Erika, Fuentes, Silvia, Ortenzio, Louis, Higgins, Matthew

Author(s)D. Wankmuller1, E. Bailey2, S. Fuentes3, L. Ortenzio, M. Higgins4

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date May 2026

DOI10.2175/193864718825160229

Volume / Issue

Content sourceResiduals, Biosolids and Treatment Technology Conference

Word count12

Description: Thermal Hydrolysis of Biosolids: Reducing Microbial Inhibition in Sidestream...

Thermal Hydrolysis of Biosolids: Reducing Microbial Inhibition in Sidestream Treatment

Abstract

Introduction
As part of its 'Bio-energy Recovery Project,' the Neuse River Resource Recovery Facility (NRRRF) in Raleigh, NC will implement new processes to be more energy-neutral. In the new system, biosolids will go through a thermal hydrolysis process (THP), anaerobic digestion (AD), and dewatering. While THP will increase biogas production and reduce the overall biosolids production, the liquid from dewatering THP-AD sludge (THP-filtrate) becomes a sidestream that must be further treated due to its high levels of ammonia. The facility will implement ANITA™Mox IFAS sidestream treatment with aerobic and anaerobic ammonia oxidizing bacteria (AOB and AMX, respectively) to perform deammonification, which can theoretically remove greater than 90% of ammonia-N and 75-85% of total N.

However, THP releases recalcitrant organics that are inhibitory to AOB and AMX, posing a major challenge in plants that need to remove nitrogen. We hypothesized that pre-treatment of the sidestream will reduce the inhibition of AOB and AMX compared to no pre-treatment. The selected methods included dilution, coagulation with alum sulfate, adsorption to powder activated carbon, and oxidation with ozone. The impact of these treatment methods on the AOB and AMX maximum activity rates in unacclimated biomass (from an ANITA™Mox MBBR) were quantified through batch activity tests. Reducing the inhibition to AOB and AMX is a major challenge in facilities employing THP, and the results of this study will provide needed guidance on addressing this crucial issue.

Methods
Thermal hydrolysis
Dewatered cake was collected from the NRRRF and processed through a CAMBI THP pilot (Figure 1) three times between June 2022 and May 2023. The dewatered cake (~19% total solids (TS)) consisted of a blend of primary sludge and waste activated sludge, which was diluted to 12% TS before adding it to the pressure vessel. The sludge was thermally hydrolyzed at 165°C for 30 minutes.

Anaerobic digestion and dewatering
A 6-L mesophilic anaerobic digester (Figure 2) was operated as a sequencing batch reactor for 1.5 years at an SRT of 15 days. The digester was fed thermally hydrolyzed sludge following the projected VS loading rate at the NRRF. TS, VS, pH, and methane content were routinely measured. The digestate was dewatered with the same cationic polymer used at the NRRRF at a dose of 30 lb/dt (Figure 3). The polymer dose was selected based on the final specifications of the belt filter presses at the plant. After flocculation, the filtrate was collected via gravity filtration with a belt filter sample, then stored at 4°C.

Activity tests
Activity tests (Figure 4) were conducted in triplicate. Synthetic wastewater was used as positive control. Fresh media was collected the morning of each batch test; 25 media pieces that had a uniform biofilm thickness were selected for each test. A negative control was conducted with only THP-filtrate and the same number of media pieces. The impact of THP-filtrate on AOB activity was first tested by exposing the biomass to increasing levels of filtrate (10%, 20%, 30%, and 50% THP-filtrate on a volume basis). Next, two pre-treatment methods, alum coagulation and PAC adsorption, were tested. Alum was added at two doses (2000 and 4000 mg/L); each bottle was agitated at 300 rpm for 90 seconds, then 60 rpm for 10 minutes. The solids were removed via centrifugation. PAC was also tested at two doses (500 and 1000 mg/L); each bottle was agitated at 80 rpm for 24 hours, and the PAC was removed via centrifugation.

The initial ammonia was between 300 and 350 mg NH3-N/L. Samples were taken at 20-minute intervals and the test lasted 80 minutes, and ammonia, nitrate, and nitrite were measured on 0.45 um filtered samples using Hach kits. The maximum AOB activity was taken as the linear portion of the nitrite generation curve, which was observed in the first 40 minutes.

Results and Discussion
As expected, addition of THP filtrate to synthetic wastewater resulted in a decrease in nitrite generation rates of AOB. This is consistent with the production of recalcitrant nutrient products during TH. These include melanoidins produced during Maillard reactions occurring in thermal hydrolysis. Increasing THP filtrate addition to synthetic wastewater resulted in similar decreases in AOB activity in terms of nitrite generation (Figure 5). Compared to the synthetic control, a 10%, 20%, and 30% THP-filtrate addition decreased the activity by 31%, 30%, and 35%, respectively. For alum and PAC treatment, the results were compared to 50% THP filtrate addition with no treatment (chosen to simulate a filtrate with significant inhibition). Treating with 2000 mg/L alum and 500 mg/L PAC increased the activity by 13% and 25%, respectively, with higher alum and PAC doses not resulting in significantly higher activity increases (Figure 6).

Additional data from AOB and anammox tests will be included in the presentation. The sCOD of different size fractions and 3D-EEM will be shown. The results from this study will provide Raleigh Water with specific insights and guidance on how to manage inhibition in the sidestream treatment unit. The results are also of interest to other enhanced nutrient removal facilities planning on incorporating THP to increase anaerobic biogas production. To our knowledge, this is the first time that alum coagulation and PAC addition have been tested as sidestream pretreatment methods to reduce inhibition.

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

SessionA Story of Struvite Survival and a Sidestream Saga

Session locationMcCormick Place, Chicago, Illinois, USA

TopicBiosolids & Residuals

Author(s)

Morales, Michaela, De Los Reyes, Francis, Bailey, Erika, Khunjar, Wendell, Bilyk, Katya, Wankmuller, David

Author(s)M. Morales1, F. De Los Reyes2, E. Bailey3, W. Khunjar1, K. Bilyk1, D. Wankmuller1

Author affiliation(s)Hazen and Sawyer1, North Carolina State University2, City of Raleigh3

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Sep 2025

DOI10.2175/193864718825160115

Volume / Issue

Content sourceWEFTEC

Word count11

Description: Towards Unifying Densification and Low TN / TP Operation at the South Durham Water...

Towards Unifying Densification and Low TN / TP Operation at the South Durham Water Reclamation Facility

Abstract

Introduction
Densified activated sludge (DAS) is a process intensification technology that can allow water resource recovery facilities (WRRFs) to utilize a combination of biological (metabolic and kinetic) and physical selection to improve the settleability of the sludge. Intensification via DAS can allow WRRFs to operate at elevated MLSS concentrations (>5,000 mg/L) and solids loading rates (>40 lb/ft2-day) to clarifiers. Beyond capacity benefits, there is great interest in leveraging DAS to reliably achieve low TN/TP standards (e.g., TN < 5 mg/L and TP < 0.2 mg/L) while minimizing aeration and supplemental carbon demand.

This paper will provide insights from a successful multi-year effort to combine DAS with low DO operation at the 20 mgd South Durham Water Reclamation Facility (SDWRF). The focus of the optimization has been to enhance post-anoxic denitrification WITHOUT supplemental carbon addition to achieve TN <5 mg/L, while maintaining DAS properties.

Facility Description
The SDWRF is an advanced biological nutrient removal (BNR) treatment facility, owned and operated by the City of Durham, NC. The SDWRF consists of influent screenings and grit removal, primary clarification, 5-stage BNR, secondary clarification, tertiary filtration, and UV disinfection (Figure 1).

Kinetic and metabolic selection facilitated DAS at the SDWRF
Settling properties (SVIs, settling velocity and compaction coefficients) are summarized in Table 1. Median SVIs have remained below 90 L/mg and the facility has largely been able to minimize settling property variability, as illustrated with 95/50th SVI ratios consistently below 1.25. These results indicate that SDWRF has successfully achieved DAS with biological selection only; physical selection was not necessary. Achieving and maintaining DAS at this facility is critical as it allows the plant to minimize the number of aeration basins and clarifiers in service, reliably treat peak flows (PF of 5) while maintaining solids inventory, reducing operating costs and deferring the need for costly repairs of aging infrastructure.

50% reduction in effluent TN was facilitated by stimulating endogenous denitrification through low DO/post-anoxic zone operation
Effluent nutrient, operating SRT and primary effluent BOD/TKN ratios are summarized in Table 2. Effluent TN has decreased from 6 mg/L in 2019 to less than 4 mg/L in 2024. This decrease in effluent TN was accomplished WITHOUT supplemental carbon addition to the post-anoxic zones. To accomplish this, SDWRF staff reconfigured bioreactors to: i) reduce the pre-anoxic volume, ii) extend post-anoxic volume by operating lower DO through aerobic zones (< 0.5 mg/L), and iii) increase MLSS concentration/total SRT in the basins (Table 2).

Nutrient profiling data (Figure 2) confirms that basin and operational modifications reduced nitrate concentrations throughout the aerobic and post-anoxic zones. This reduction is likely due to increased endogenous denitrification that occurs because of the lengthened post-anoxic operation/low DO operation.

Ex-situ denitrification batch testing indicated that specific denitrification rates observed at SDWRF have changed since process configuration changes. Prior to 2022, batch testing indicated that SDWRF possessed a distinct fast and slow kinetic profile that that has been previously seen in systems hypothesized to have internal carbon driven denitrification. Testing in 2024 indicated that SDWRF continues to show a fast/slow kinetic profile; however, the 'fast' endogenous denitrification rate is significantly lower than that observed in 2021 (Table 3).

Given the magnitude of TN reduction observed since 2022, it is hypothesized that the process changes at SDWRF are allowing the biomass to effectively consume the fraction of biodegradable carbon that could previously stimulate 'fast' endogenous denitrification. This reduction of the 'fast' carbon bank results in lower nitrate in-situ but effectively reduces the specific denitrification rate observed during ex-situ testing.

Further investigation in the source of the electron donors for denitrification observed at SDWRF is underway to more fully understand endogenous denitrification trends.

<B>Optimization of TN reduction must be balanced with impacts to settling properties
While SDWRF has maintained settling qualities typical of DAS, the initial settling velocity (Vo) has decreased since 2022, when changes were made to improve TN performance. EPS characterization has suggested an increase in the loosely bound fraction since operational changes have been made. This increase in EPS fraction appears to correlate with the increase in SVI (Figure 3).

Cumulatively, these results suggest that it is possible to leverage low DO and extended post-anoxic zone operation to reduce TN while maintaining low variability in sludge settling characteristics. However, there may be some tradeoff on initial settling velocity properties. Further work is underway to determine how to best leverage kinetic selective pressure to increase settling velocity to prior levels while maintaining enhanced endogenous denitrification observed recently.

We envision that the lessons from this work will provide insights on how to design and operate WRRFs to consistently achieving low SVIs while also achieving low effluent TN and TP limits with no supplemental carbon addition.

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

Presentation time

15:50:00

16:00:00

Session time

15:30:00

17:00:00

SessionDensified Activated Sludge Full Scale Performance and Optimization

Session locationMcCormick Place, Chicago, Illinois, USA

TopicLiquid Stream Treatment - Nutrient Removal and Recovery

Author(s)

Wankmuller, David, Cartner, Jeremy, Cocker, Charles, Montgomery, Lori, Stiegel, Patricia, Bilyk, Katya, Pathak, Ankit, Khunjar, Wendell, Sun, Yewei