Evans, Eric

Evans, Eric

Eric Evans is a speaker at WEFTEC 2026.

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About Evans, Eric

Eric Evans is a speaker at WEFTEC 2026.

Evans, Eric

Eric Evans is a speaker at WEFTEC 2026.

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Densified Sludge Model Sensitivity Analysis and Pilot Data Calibration

Abstract

Background
The densified sludge process is a popular method by which to intensify secondary wastewater treatment (Daigger et al., 2023; Winkler et al., 2018). While batch processes are more widely utilized to date, continuous flow processes using hydraulic cyclones have generated significant interest (Bauhs et al., 2024). Unfortunately, there is limited understanding of good modeling practices for hydrocyclone-based densified systems. Process models take different approaches to model densified sludge behavior and selective retention by cyclones. One method is to define the fractions of state variables retained by cyclones, but these fractions are poorly understood and difficult to measure (Uri et al., 2017). Alternatively, a zero-dimensional densified sludge (DS) model has been proposed to streamline cyclone modeling by differentiating granule biomass from floc biomass, allowing for selective retention of granule mass fraction by the cyclone (Baeton et al., 2018). Granular biomass half-saturation coefficients are multiplied by a diffusion resistance (DR) term, simulating restriction of access to substrate within granules and subsequent reduction in granule organism growth kinetics (Baeton et al., 2018). However, there remains limited documentation regarding parameter sensitivity and calibration of this approach.

To help provide an improved practical understanding of the application of both the state variable and DS model approaches to hydrocyclone simulation, this paper undertakes (1) a sensitivity analysis of key input parameters for both approaches and (2) a calibration of both approaches to long-term pilot data.

Methods
Sensitivity analyses were conducted using the A2O plant + Cyclone simulation available in the SUMO library following the installation of the DS model addon (Dynamita, SUMO22) (Figure 1). A key benefit of densification is the elevation of nitrification capacity without capital expansion (Bauhs et al., 2024). For this reason, key performance indicators related to nitrogen removal were used as dependent variables during the sensitivity analysis. These include densified fraction of total biomass (FGranule), fraction of nitrifying biomass, and effluent quality.

Several parameters were evaluated in the DS model sensitivity analysis (Table 1). The variation range for each parameter was typically calculated using a change of +/-25% & 50% from the default value, though alterations were made as appropriate (Baeten et al., 2018; Cosenza et al., 2013). Further details regarding model setup and simulation will be included in the final paper, as will the parameters used for sensitivity analysis of the state-variable cyclone approach and their respective ranges.

Results
DR and mass split to cyclone underflow emerged as particularly impactful calibration parameters. The mass split to underflow is an operational parameter which sets the fraction of flocculant suspended solids retained by the cyclone. This abstract focuses on the impact of these parameters, though the final paper will include a deeper analysis.

Diffusion Resistance. FGranule is inversely correlated with DR, whilst maintaining the same fraction of granule mass retention by the cyclone (Figure 2a). As DR for all substrates increases, FGranule decreases due to the simulated inhibition of granule biomass. High DR conditions resulted in marginally reduced nitrification but an overall lower effluent total nitrogen (Figure 2b), potentially due to increased anoxic biomass within granules. At a DR of 1.0, ordinary heterotrophic organisms (OHO) were the most abundant granular organism group, as one would expect when the 'diffusion handicap' is removed (Figure 2c). Phosphorus accumulating organisms (PAO) preferentially granulate and were insensitive to DR, though granule ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) were nearly eliminated under high DR conditions (Figure 2d). It is crucial to note that individual substrate DR can be calibrated in the model using field data or literature values (Baeten et al., 2018); the sensitivity of individual substrate DR will be included in the final paper.

Mass Split. FGranule is positively correlated with the mass split to underflow since more solids, and thus more granules, are retained within the system (Figure 3a). As the mass fraction retained by the cyclone increased, MLSS and SRT also increased, with total SRT increasing from 14 days (-50%) to 39 days (+50%). Nitrification was maximized at high granule retention (Figure 3b) however, wasting would need to be appropriately adjusted to yield an operable MLSS. Interestingly, AOB and NOB composition in the granules remained relatively unchanged. Granular biomass was maximized with increased mass retention, for OHO and PAO in particular. While PAO growth was insensitive to DR, their growth is directly correlated to mass split to underflow (Figure 3c and d).

Next Steps. Results from the sensitivity analysis of the state variable model will be compared to the results of the DS model analysis. Finally, a DS model will be calibrated to hydrocyclone field data, and calibrated parameters will be compared to hydrocyclone vendor-supplied recommended values.

The learnings presented in this paper will provide practitioners insight into the use and sensitivity of two hydrocyclone process models, as well as practical good modeling practices for calibration.

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

Presentation time

08:30:00

09:00:00

Session time

08:30:00

10:00:00

SessionDensified Activated Sludge: Practical Design and Modeling Experiences Converge

Session locationMcCormick Place, Chicago, Illinois, USA

TopicProcess Control and Modeling

Author(s)

Croll, Henry, Schlepp, Geneva, Mehrdad, Mahsa, Thompson, Luke, Takacs, Imre, Figdore, Bryce, Benisch, Mario, Evans, Eric

Author(s)H. Croll1, G. Schlepp1, M. Mehrdad1, L. Thompson1, I. Takacs2, B. Figdore1, M. Benisch1, E. Evans1

Author affiliation(s)HDR Inc1, Dynamita2

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2025

DOI10.2175/193864718825159970

Volume / Issue

Content sourceWEFTEC

Word count10

Getting a Grip on AGS Waste Solids: Settleability and Phosphorus Release

Abstract

INTRODUCTION The Aerobic Granular Sludge (AGS) and other densified processes offer the promise of biomass intensification with the added benefit of nutrient removal. A key element needed to support AGS is the selective wasting of flocculant biomass and particles from the process. Wasting occurs from the top of the settled solids or by other physical methods (e.g. hydrocyclones). While this wasting approach is essential to the AGS process, it results in waste solids (WAS) streams that differ from conventional activated sludge (CAS) with higher flow rates, lower total suspended solids (TSS) concentrations, and biological nutrient removal (BNR) activity. Therefore, the AGS WAS must be managed in a completely different way from CAS WAS. Two key WAS management challenges need to be considered. The first challenge is managing intermittent AGS WAS flows, which are on the order of 5 to 15 times higher than CAS WAS flow rates on average and approaching 100 times higher for instantaneous flow rates. The second challenge for AGS WAS management is understanding the impact of the phosphorus accumulating organisms (PAOs) on the solids handling system and managing the potential for struvite formation. The objectives of the project work are the following: *Define AGS WAS settleability characteristics *Expand the understanding of phosphorus release potential (total OP release) and rates METHODS & APPROACH Solids Settling Characterization All WAS samples were taken directly from the pipe that transfers WAS from the AGS reactors to the solids buffer tank (Figure 1). Settlability was evaluated using a 5.5-inch diameter column settler (Figure 2). The settling column was filled with 10 L of WAS sample for each test. Sample ports, located every 2 L and at the bottom of the column, were used to collect supernatant and settled solids samples. Settled WAS and supernatant samples were taken and TSS was measured in both samples. Sludge volume index (SVI) values were also estimated. Phosphorus Release Rates and Potential Phosphorus release testing has been conducted using gang stirrer apparatuses with 1.5 L samples (Figure 3) and low-rate mixing (30-50 s-1). A carbon source was added to 10 of the samples giving F:M ratios ranging from 0.0 to 0.5 g-COD/g-VSS. Additionally, the initial TSS concentration was varied by providing some initial thickening in some samples resulting in a range from 1,500 to 5,000 mg/L. The initial total phosphorus and phosphate concentrations were measured in the raw WAS sample. Samples were subsequently taken from the release tests through a 34 h time period. Samples were filtered and phosphate measured. Results & Discussion AGS WAS Settling Rates The WAS volume was recorded as a function of time by measuring the depth of the sludge layer in two separate test batches. Initial WAS concentrations were 2,500-5,000 mg/L and thickened to 8,500-10,000 mg/L by the end of the test period. The final settled volume was reached within 4-12 minutes. The maximum settling velocity was 3.9-11.9 m/hr at time 2-4 minutes and decreasing over time (Figure 4). SVI5 values were estimated in the range 110-200 mL/g, and SVI30 values were calculated in the range 100-120 mL/g. The TSS concentration in the supernatant was 8-40 mg/L. Evaluation of modeled settling in a clarifier was conducted using a BioWin simulator based on the calculated Vesilend model clarifier coefficients. Model sludge blanket height and effluent suspended solids remained within the desired range, even with the additional solids. A state point analysis from BioWinTM indicated that the AGS WAS operating condition was within the solids-flux limitation of the clarifier (Figure 5). Phosphorus Release Rates and Potential Phosphorus release testing showed phosphorus release rates varied initially in response to carbon dosing with higher short-term release rates followed by reduced long-term release rates (Figure 7). Short-term (or initial) release rates varied from 0.4 mg-P/g-TSS/h for no carbon addition to 4.0 mg-P/g-TSS/h for carbon addition (acetic acid with F:M of ~ 0.4 g/g). The long-term release rates varied from 0.1 to 0.7 mg-P/g-TSS/h for initial TSS concentrations in the range 1,500-5,000 mg/L (Table 1 and Figure 8). The effect of starting TSS concentration was more important than carbon addition for this test batch with rates normalized to volume increasing exponentially as shown in Figure 8. The overall phosphorus released varied in the range 7-22% of the initial TP measured. A follow-up test was conducted showing a release of 25-30% of the initial TP over a 72-h release period. A phosphorus mass balance on the plant aligns with maximum releasable TP observed based on the phosphorus removed from the liquid stream. An evaluation of the settling characteristics of AGS WAS showed high settling velocities resulting in WAS TSS concentrations similar to CAS WAS TSS concentrations with low DSS concentrations. Based on the results of phosphorus release testing, struvite mitigation is recommended for anaerobic digestion of AGS WAS.

The aerobic granular sludge (AGS) process offers strong potential for both incorporating nutrient removal and process intensification for wastewater treatment. Some outstanding research needs have come to the forefront. This paper seeks to address questions that have arisen regarding waste solids. In particular, the work evaluated the settleability and phosphorus release potential of AGS waste solids.

SpeakerEvans, Eric

Presentation time

14:00:00

14:30:00

Session time

13:30:00

15:00:00

SessionEvaluating Plantwide Impacts of AGS and DAS

Session number606

Session locationRoom 343

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

Author(s)

Evans, Eric, Geesman, Ashley, Devitt, Dillon, Thompson, Matthew, Sova, Ronald, Kigin, Abby

Author(s)E.A. Evans1, A. Geesman2, D. Devitt3, M.J. Thompson4, R.J. Sova5, A. Kigin5

Author affiliation(s)1HDR, IA, 2, IA, 3HDR ENGINEERING, INC., NE, 4, NE, 5HDR Inc, NE

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2024

DOI10.2175/193864718825159522

Volume / Issue

Content sourceWEFTEC

Word count12

Optimization Strategies: BOD to P (Des Moines Case Study)

Abstract

The Des Moines WRA’s nitrification activated sludge process has been previously optimized for energy efficiency by reducing airflow with staggered dissolved oxygen (DO) concentrations in passes one through four of their six-basin process. A second optimization step to provide enhanced biological phosphorus removal (EBPR) has been tested by decreasing and cycling air flow in the first pass to achieve anoxic/anaerobic conditions with DO concentrations measuring less than 0.1 mg/L and average oxidation reduction potential (ORP) concentration of about -130 mV. EBPR activity was successfully established in the modified basins with average phosphorus mass content of the waste sludge 3.2 ± 0.2% (95% confidence) compared to 1.6 ± 0.1% in the control basin. A wet weather scenario was also tested by operating in contact stabilization mode in order to protect the first pass from high DO entrained influent waste streams. Results showed effluent phosphorus concentrations less than 1 mg-P/L could be achieved and operation in contact stabilization mode provided some protection to the first pass.

SpeakerEvans, Eric

Presentation time

10:35:00

10:50:00

Session time

08:30:00

12:00:00

SessionStrategies to Optimize WRRFs for Nutrient Removal

Session number510

TopicNutrients, Research and Innovation

Author(s)

E.A. EvansE.A. Evans

Author(s)E.A. Evans1; E.A. Evans1;

Author affiliation(s)HDR, IA1

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2020

DOI10.2175/193864718825157510

Volume / Issue

Content sourceWEFTEC

Word count10

Rebooting a WRRF Site for Resiliency, Capacity, and Nutrient Removal

Abstract

PROBLEM AND OBJECTIVES The Des Moines Wastewater Reclamation Authority (WRA) is at a crossroads at their Wastewater Reclamation Facility (WRF). It has performed well for decades providing a high-quality, nitrified effluent protective of receiving waters with a facility bult on a tightly constrained site, but it is due for renewal. A planning and modeling effort, oriented toward answering challenging questions about the facility, was completed. Drivers and questions for the future:

• Aging Infrastructure – Parts of the WRF have not been replaced or rehabilitated in over 40 years are approaching the end of useful life. What is the best path forward for this infrastructure?
• Resiliency of WRF – Wet Weather and flooding caused problems historically. Can future work increase flood resiliency with appropriate building elevations and setbacks while minimizing failure points?
• Nutrient Removal – Nutrient removal is a future requirement. Can the process be modified to provide for total nitrogen (TN) and total phosphorus (TP) of 10 mg-N/L and 1 mg-P/L; and can it tighter future limits?
• Capacity and Wet Weather Needs – The existing WRF hydraulic capacity has been 200 MGD since 1982. Can the existing, constrained site be adapted to treat 400 MG?
• Sustainability – Sustainability is a key value of the WRA. Are there opportunities that grow energy efficiency and resource recovery?
• High-risk infrastructure – What is the best way to address:
° Headworks with critical pumping and electrical constraints,
° Yard piping, stretching for miles, showing signs of stress failures at joints,
° Equipment (e.g. blowers) reaching 40+ years of age?

OVERVIEW OF WRF The WRA serves approximately 550,000 people in 18 member communities with an average flow of 70 MGD through 125 miles of conveyance piping. It treats industrial hauled waste from 90 customers and produces 500,000 MMBTU of RNG yearly. The configuration of the WRF from the 1980s has a capacity of roughly 200 MGD with primary clarifiers, secondary nitrification, and anaerobic digestion (Figure 1). The existing WRF site is constrained with the Des Moines River on the south, a railroad on the north, and wetlands on the east and west sides (Figure 2). Sewers stretch across the WRF routing flow to the headworks and influent pumping from all directions. Limitations and opportunities persist at the WRF. Processes no longer in use should be removed. During the 1980s, the WRA committed to a centralized treatment facility. More recently, the WRA has invested heavily into conveyance providing a route for flow to the treatment facility. This planning cycle focuses on treatment facilities, and reviews the challenges with a centralized facility on a constrained site. METHODOLOGY A unique approach to facility planning was conducted. For this plan, the evaluation was conducted as a fully coordinated effort. WRA staff worked hand in hand with the treatment and conveyance planning teams as a unified effort that included frequent check-ins. A key team decision was that everything is on the table; no process or infrastructure is sacred. FINDINGS Several alternatives were considered for WRA to meet their goals. One is building EQ basins in the conveyance system. Another alternative is in-kind expansion that builds on the baseline configurations. Satellite treatment was reviewed as well. These alternatives were found to be complex, cost prohibitive, and challenging. The preferred alternative is to [b]re-configure the WRRF[/b] at the existing site. While the WRF is site constrained and does not support in-kind expansion, an in-depth analysis with a novel reconfiguration of processes showed that the site can support WRA's goals and initiatives while solving treatment challenges, unlocking capacity and providing nutrient removal as shown in the modeled scheme (Figure 3). A site reset was proposed (Figure 4) that relocates and reconfigures infrastructure increasing capacity from 200 MGD to over 400 MGD by implementing novel intensification concepts. Examples of efficiencies are weaved throughout the plan. The hydraulic profile of the site is reset with new influent pumping to support future capacity while eliminating intermediate pumping. Primary clarifiers are relocated adjacent to secondary treatment [b]eliminating single points of failure[/b] associated with a pipeline in a levee. Circular clarifiers are replaced with rectangular tanks using baffling. This approach is primed for surface overflow rates exceeding 4,000 lb/ft[sup]2[/sup]/d (Gerges et al., 2024). The cost evaluation shows savings compared to satellite treatment, and a shift from conveyance to the treatment system (Figure 5). As with others, the WRA seeks the highest level of fiscal responsibility. Therefore, a sensible phasing plan was developed that supports the improvements based on appropriate trigger points (Figure 3). SUMMARY The Des Moines WRF underwent planning to support a number of drivers ranging from water quality to capacity while overcoming challenges ranging from site constraints to aged infrastructure with reliability concerns. Staff employed a collaborative planning effort oriented toward unlocking capacity, improving resilience, and identifying optimizations and synergies. The result is a reset of the WRRF with a new approach to treatment.

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

Presentation time

08:30:00

08:45:00

Session time

8:30:00

10:00:00

SessionFacility Planning is not Just Master Planning

Session locationErnest N. Morial Convention Center

TopicFacility Operations and Maintenance

Author(s)

Evans, Eric, Trpkosh, Bryce, Bakke, Brian, Brown, Patrick, Schlickbernd, Melissa

Author(s)E. Evans1, B. Trpkosh1, B. Bakke2, P. Brown2, M. Schlickbernd

Author affiliation(s)HDR, 1HDR Inc, 1Des Moines Wastewater Reclamation Authority, 2

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Sep 2026

DOI10.2175/193864718825160349

Volume / Issue

Content sourceWEFTEC

Word count11

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Evans, Eric

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