Ahmad, Sakib

Sakib Ahmad is a speaker at WEFTEC 2026.

Titles from this speaker

Clarifier Capacity quantification and prediction: needs and limitation in a new era of process intensification

Abstract

[b]Introduction[/b] Clarifiers are often the bottleneck to process intensification (Sturm, 2020). As utilities move toward lower solids retention times (SRTs), higher peak flows, and new technologies, reliable clarifier capacity estimates are critical for design and operation. Pilot studies reduce uncertainty but cannot fully replicate full scale hydraulics or sludge behavior, leaving uncertainty in true settleability changes (Griborio, 2004). State Point Analysis (SPA), based on solids flux theory, remains the standard method for predicting secondary clarifier capacity. Under stable operation prior to blanket instability, clarifier performance is governed primarily by flocculent settling (Kinnear, 2002). However, flocculent settling is not directly quantified by conventional settleability measurements. Instead, SPA relies on Vesilind parameters derived from batch settling tests or empirical SVI30 relationships (Metcalf and Eddy, 2014), which assume fixed settling characteristics and do not resolve floc scale kinetics observed at full scale. A key limitation is that SPA focuses on blanket transport failure, while full scale clarifiers often experience effluent suspended solids (ESS) breakthrough at much lower loading rates (Cole, 2021; Kelly et al., 2009). The objective of this study is to assess whether SPA can reliably predict full scale clarifier capacity when performance is governed by flocculent settling and ESS breakthrough. [b]Materials and Methods[/b] Blue Plains AWWTP transitioned its secondary system from step feed (SF) to contact stabilization (CS) in 2020 (Ngo et al., 2021). SPA was performed using flux curves derived from Vesilind parameters obtained through batch settling tests (Ahmad et al., 2026). Full scale performance was evaluated using East train data from SF (2018 to 2020) and CS (2020 to 2024). Pilot scale flux curves from the secondary clarifier using inDENSE were used to quantify variability in settling behavior under similar SRT but varying F/M conditions (Diop et al., 2025). Clarifier stress testing was performed on five sludge types which has been described in Ahmad et al. (2026). Floc morphology and settling velocity data were collected using video analysis (Li et al., 2025). [b]Results and Discussion[/b] [b]Variability of Flux Curves increases clarifier capacity uncertainty[/b] Flux curves from East Secondary contact stabilization clarifiers showed substantial variability that could not be explained by solids retention time or seasonality (Fig. 1A & B). Pilot scale testing indicated that differences in feast and famine conditions within the same SRT range affected settleability and flux curve shape (Fig. 1C), demonstrating that a single flux curve cannot represent full scale performance. SPA of the full-scale contact stabilization dataset showed 25% variability in SOR and 43% variability in SLR around median values (Fig. 2). Lower variability under step feed reflects limited data rather than greater stability, highlighting the uncertainty of flux curve-based clarifier capacity estimates. [b]ESS failure determines clarifier failure more than blanket failure[/b] Clarifier stress testing revealed a gap between SPA predictions and observed performance limits. For the five sludge types evaluated (Table 2), blanket failure occurred at SORs consistent with SPA because representative flux curves were captured under controlled testing. In contrast, ESS deterioration occurred at approximately half of the SPA predicted SORs and ultimately defined usable clarifier capacity due to its direct impact on downstream performance and compliance. SPA based on measured flux curves showed no capacity gain under contact stabilization and in some cases suggested reduced capacity (Fig. 2), consistent with slightly higher SVI30 values (Table 1). However, full scale process data showed clear performance gains when effluent quality was considered (Fig. 3). Under contact stabilization, clarifiers maintained ESS ≤ 30 mg/L at the 75th percentile across higher SOR and SLR conditions, whereas step feed did not meet this limit under any observed condition. These improvements were only evident after implementation and were not captured by SPA, confirming that ESS failure precedes blanket failure and governs true clarifier capacity. [b]Is understanding flocculant settling key to better estimating clarifier capacity?[/b] The five sludge types evaluated showed similar weighted average floc settling velocities and velocity class distributions, resulting in closely aligned ESS failure points. Nevertheless, sludges with weighted average settling velocities above 14 m/h and >35 percent of flocs settling faster than 14 m/h achieved higher terminal SORs and greater clarifier capacity (Fig. 4). In contrast, SPA across 34 sludge types showed wide variability in predicted capacity under similar conditions. Future work should expand floc scale measurements to better link settling velocity spectra to ESS failure. [b]Conclusion[/b] SPA can predict blanket failure when representative flux curves are available, but full-scale flux curves vary widely across operating conditions, creating substantial uncertainty in clarifier capacity estimates. In practice, ESS failure rather than blanket failure governs clarifier capacity, highlighting the need for improved understanding of flocculent settling and new capacity assessment frameworks beyond traditional SPA.

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

Presentation time

14:00:00

14:30:00

Session time

13:30:00

15:00:00

SessionDAS Design New Frontiers

Session locationErnest N. Morial Convention Center

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

Author(s)

Ahmad, Sakib, Li, Yuang, Ngo, Khoa Nam, Mendoza, Maria, Okechukwu, Godswill, Azam, Hossain, Gu, April, Riffat, Rumana, De Clippeleir, Haydee

Author(s)S. Ahmad1, Y. Li2, K. Ngo3, M. Mendoza4, G. Okechukwu4, H. Azam4, A. Gu3, R. Riffat1, H. De Clippeleir3

Author affiliation(s)George Washington University, 1Cornell University, 2DC Water, 3University of the District of Columbia, 4University of the District of Columbia, 4University of the District of Columbia, 4George Washington University, 1DC Water & Sewer Authority, 3

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Sep 2026

DOI10.2175/193864718825160306

Volume / Issue

Content sourceWEFTEC

Word count16

Determining Activated Sludge Characteristics Impacting Clarifier Capacity and Performance through Two Simple Measurements

Abstract

Introduction Improving settleability and clarifier capacity is an important driver in current technology innovations (Sturm & De Clippeleir, 2020). To date, sludge volume index (SVI) is still a standard settling measurement method to determine settleability. However, SVI measurements are not sensitive enough to detect small differences in settling behavior. SVI just describes non-stokesian settling (hindered and compression settling zone), while there has been strong evidence that flocculent settling (stokesian settling) is the main settling regime in clarifiers and hindered settling only occurs during clarifier failure (Kinnear, 2002). Therefore, an alternative measurement, the 'threshold of flocculation (TOF)' method (Mancell-Egala et al. 2017a) to quantify the stokesian settling (flocculant settling). TOF describes collision efficiency particles and is thus dependent on the non-settling fraction, particle charge, particle density, particle size, extracellular polymeric substances (EPS) characteristics etc. Initial work has shown a direct correlation between TOF and effluent TSS (Ngo et al., 2021a) and showed high sensitivity detecting small changes in behavior. On a more fundamental level, increased collision efficiency (lower TOF) was directly linked with increased EPS-TKN/COD ratios and introduced as a parameter to improve clarifier models (Ngo et al., 2021b). In addition, based on the wide range of systems studied so far (Table 1), more insights have been gathered on how a combination of TOF and SVI data can provide a better interpretation of sludge settling, sludge characteristics and system behavior. This paper compiles the current lessons learned and demonstrates the power of knowing both settling parameters which can potentially avoid the need for more complicated measurements such as particle size, EPS etc. Methodological Approach A throughout literature review of reported combinations of SVI30 (7 to 1051 mg L/g TSS) and TOF (11 to 1500 mg TSS/L) under various configurations, operational conditions, and wastewater characteristics were summarized in Figure 1 and Table 1. This captured a wide range of different sludge morphologies, settling characteristics, wastewater types and operational conditions. TOF and SVI method are described in Mancell-Egala et al. 2017a. Results & Discussion Flaws of only focusing on SVI (non-stokesian settling) or TOF (stokesian settling) Most systems presented in Figure 1 showed good hindered settling behavior with only a few examples with SVI> 100 mL/g TSS (Fig. 1). When looking into a narrow range of SVI of 50-100 mL/g TSS, a wide range of systems and system behaviors can be found (Fig. 1, Table 1). One can understand that these sludge types have vastly different morphologies resulting in difference in clarifier performance. Effluent quality in this narrow SVI range was at a minimum 2 mg TSS/L and at maximum 87 mg TSS/L (Table 1). When looking into a good flocculation behavior range described by TOF < 300 mg TSS/L (Ngo et al., 2021a), a vastly different behavior in SVI within the same TOF range can also be observed (Figure 1). Also, here a wide range of systems and sludge morphologies fell within this TOF ranges (Fig. 1, Table 1). This implies that relying solely on TOF may lead to encountering clarifier capacity limitations. Power of combining TOF and SVI for interpretation of sludge characteristics Figure 2 illustrates floc morphology, drawing insights from the patterns observed in Figure 1 across various system types. TOF is more directly linked to particle size with decreasing TOF with increasing floc or particle size. SVI is more directly linked to density with higher density and more compacted flocs leading to lower SVI. Combining the 2 characteristics leads to an overall morphology assessment (Fig. 2). Case study 1: Optimization of high-rate activated sludge systems Overall, one can see the importance of substrate and feast-famine management in high-rate systems and how these impacts settling behavior and settling limitations. EX: Increase TOF within the same SVI range typically occurs when switching to an A-stage system versus a contact stabilization (CS) system [Fig.3, arrow A, (Rahman et al., 2019)]. More details are described in figure 3. Case study 2: Cyclone selection for densification (inDENSE and DEMON example): With the hydrocyclone one selected for denser and larger particles and thus improved SVI and TOF, while one wasted out the smaller and more fluffy flocs with increased TOF and increased SVI (Figure 4A). For activated sludge systems applying cyclone wasting the differences are smaller and the wasted fraction is mostly defined by increased TOF (lower collision efficiency) rather than a true SVI and potentially density difference (Figure 4B). This example demonstrates the benefit of being able to quantify the selection over a cyclone in more detail and describe what might be limited and reported in Avila et al. (2021). Conclusion Overall, this paper provides a framework for interpretation and quantification of settling within activated sludge systems. It shows the power of using two practical measurements, SVI and TOF, to describe sludge characteristics and to help optimizing systems as shown in the case study examples. With further adoption of the TOF combined with SVI measurements, the proposed framework will be further enhanced.

The paper addresses the limitations of the traditional Sludge Volume Index (SVI) in measuring wastewater settleability and introduces the 'threshold of flocculation' (TOF) as a sensitive alternative, correlating with effluent TSS. Combining SVI and TOF offers a comprehensive understanding of sludge characteristics, optimizing system performance. Case studies on high-rate activated sludge systems and cyclone densification demonstrate the practical benefits of this approach.

SpeakerAhmad, Sakib

Presentation time

13:30:00

14:00:00

Session time

13:30:00

15:00:00

SessionInnovative Approaches to Address Traditional Operational Challenges

Session number211

Session locationRoom 344

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

Author(s)

Ahmad, Sakib, De Clippeleir, Haydee, Ngo, Nam, Riffat, Rumana, Gu, April, Li, Yuang

Author(s)S. Ahmad1, H. De Clippeleir2, N. Ngo3, R. Riffat4, A.A. Gu5, Y. Li6

Author affiliation(s)1George Washington University, DC, 2DC Water & Sewer Authority, VA, 3DC Water, VA, 4George Washington University, VA, 5, NY, 6Cornell University

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2024

DOI10.2175/193864718825159614

Volume / Issue

Content sourceWEFTEC

Word count14

Description: Improving Settleability and Clarifier Capacity for High-rate Contact-stabilization...

Improving Settleability and Clarifier Capacity for High-rate Contact-stabilization Systems Through inDENSE Application

Abstract

Introduction
High-rate activated sludge systems (HRAS) have the potential to become a central part of an energy-efficient water resource recovery facility (WRRF); they are engineered to treat higher loads under short solid retention time (SRT), minimizing carbon oxidation and redirecting the maximum of organic carbon into a concentrated sludge fraction that can be recovered as a source of organic energy or bioproducts. (Rahman 2017, Rahman et al., 2020). The low SRT makes this technology less stable in operation due to the poorer physical behaviour of the sludge, therefore resulting in highly variable effluent quality (Rahman et al., 2020; Sancho et al., 2019). Typically, sludge from HRAS systems is described as weak and fluffy (Ekama et al., 2018). Improvements of feast famine through implementation of high rate contact stabilization (CS) rather than traditional plug flow improved flocculation significantly (Ngo, 2022, Van Winckel et al., 2022). However, it also showed slightly increased sludge volume index (SVI) as flocs became sticky and started to repel each other. This could potentially limit the capacity of the high-rate CS systems. Improved settleability (flocculant and hindered/compression) is needed to enhance capacity of high-rate CS systems while maintaining good effluent quality.

Capacity enhancement through densification of activated sludge systems using hydrocyclones (i.e. inDENSE technology) for selective wasting has been applied for long SRT systems such as biological nutrient removal systems (Gemza and Ku›nierz, 2022, Regmi et al., 2022). The intensification of these processes is achieved by selective wasting of fluffy material and selective retention of dense material. It has allowed the transition to densified biomass achieving SVI 76 mL/g (Avila et al., 2021), and seeing increased fractions of granules, defined by a size > 250 µm, thus increasing hydraulic capacity. However, it was also reported that effluent quality becomes challenging when pushed too far (Avila et al., 2021).

This paper evaluates the application of inDENSE on high-rate CS systems and looks into the impact of settleability, process stability and carbon capture.

Methodology
A pilot system (8 m3) was designed to replicate the operational conditions of the high-rate secondary system at Blue Plains and is fed with primary effluent, as shown in Figure 1. The operational conditions are detailed in Table 1.

Results and Discussions
High-rate CS operation without selective wasting (day 0-64)
The pilot aerobic SRT was gradually decreased during baseline testing from 4.8 to 3.8, 2.5, and 1.8 days (Figure 2A). The decreased SRT resulted in limitations in SVI when reaching an SRT of 3.8 and limitations in both SVI and TOF below an SRT of 2.8 (Figure 3A, 3B & 3D). This correlated with increased effluent suspended solids > 64.2 mg TSS/L at the latter SRT. At an SRT of 2.5, sludge blankets started to reach higher levels, and the distribution of solids over zones A, B, and C (Figure 1) shifted more toward zone C, indicating a higher fraction of slow-settling solids (Figure 4D). In terms of COD and nutrient mass balances, decreased aerobic SRT led to 40.6% carbon capture, 47.4% N capture, and 62.7% P capture at an SRT of 2.5 days. When decreased further, reaching settling limitations, a decrease in carbon capture efficiency to 26.2% was observed (Figure 5).

High-rate CS operation with selective wasting (day 64 onwards)
On day 64, wasting was directed through a hydrocyclone, creating a mass split of 72/28 between overflow and underflow and a hydraulic split of 82.3/17.7 (Figure 4A, 4C). After 26 days of operation (approximately 2.2 sludge ages), we began to observe improved flocculation behavior and effluent quality (Figures 2B, 3A & 4B). From day 90 onwards, SVI showed a decreasing trend, reaching levels of 107 mL/g under current conditions (Figure 3B). With the decreased SVI, an increase in underflow mass was also observed (Figure 4A, 4C). This aligns with a reduction in mass settling in zone C (Figure 4D). Stabilization has not yet been achieved, as SVI continues to improve. SVI test conducted on day 120 (Figure 6), demonstrates better compression in the underflow compared to MLSS, while the overflow exhibits a higher value of 89.3 mL/g. This value remains within acceptable settleability limits, supporting ongoing improvements in settling behavior.

With hydrocyclone separation, COD capture increased to 55.5% (Figure 5A), a 15% improvement over the baseline at similar SRTs. P capture reached 67.4% and N capture 42.2% (Figure 5B & 5C), both showing trends of improvement as settleability enhanced. Improved settling, driven by isolating denser particles, reduced effluent losses, and increased retention and capture of carbon, phosphorus, and nitrogen.

Conclusion
This study shows promising results for the application of inDENSE on high-rate systems, improving both flocculant and hindered/compression settling and expands the application range for this technology.

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

Presentation time

11:00:00

11:30:00

Session time

10:30:00

12:00:00

SessionBeyond BNR: Pushing the Envelope of Intensification Technologies

Session locationMcCormick Place, Chicago, Illinois, USA

TopicLiquid Stream Treatment Technology - Secondary & Tertiary Treatment

Author(s)

Diop, Arame, Mendoza Ramirez, Maria, Ahmad, Sakib, Li, Yuang, Ngo, Khoa Nam, Massoudieh, Arash, Azam, Hossain, Gu, April, Riffat, Rumana, De Clippeleir, Haydee

Author(s)A. Diop1, M. Mendoza Ramirez1, S. Ahmad1, Y. Li1, K. Ngo1, A. Massoudieh2, H. Azam3, A. Gu5, R. Riffat4, H. De Clippeleir1

Author affiliation(s)DC Water/ Catholic University1, The Catholic University of America2, University of the District of Columbia3, George Washington Univeristy4, Cornell University5

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2025

DOI10.2175/193864718825160088

Volume / Issue

Content sourceWEFTEC

Word count13

Lessons Learned from a Pilot-Scale High-Rate Contact Stabilization System with inDENSE Application: Coupling Biological and Physical Selection to Improve Performance and Capacity

Abstract

Introduction High-rate activated sludge (HRAS) systems operate with short SRTs and HRTs, which often experience deteriorated sludge settleability (Rahman et al., 2020). To mitigate this challenge, selective wasting strategies have been investigated, including surface wasting (Haaksman et al., 2024), hydrocyclone wasting (Avila et al., 2021), screen wasting (Boltz and Daigger, 2022), and velocity-based wasting (Sun et al., 2019). Among them, inDENSE selection has shown strong potential for improving and stabilizing settleability, particularly in BNR systems under seasonal temperature fluctuations (Shao et al., 2025; Regmi et al., 2022; Hunter et al., 2024; Roche et al., 2022). This study evaluated the effects of inDENSE selection, alone and combined with biological control strategies, on sludge settleability and overall performance in a pilot-scale HRAS system, with the goal of identifying approaches to optimize both. Method and Materials A pilot-scale contact stabilization system (Figure 1), replicating secondary treatment at the Blue Plains AWTP, was used to evaluate the combined effects of inDENSE selection and biological control strategies. The system included four reactor zones and three clarifiers. Zone 1 operated as a high-DO stabilizer receiving only RAS to induce famine conditions, while Zones 2 and 3 functioned as contactors with primary effluent feed. Hydrocyclone wasting (inDENSE) was applied on the wasting line. The study comprised four runs: a baseline without inDENSE, followed by Runs 1-3 incorporating physical and biological controls, yielding six stable phases (Baseline, 1A, 2A, 2B, 3A, and 3B). A summary of system information is provided in Table 1. Sludge settleability was evaluated using TOF, SVI, and solids flux analyses, while system performance was assessed through daily solids and nutrient mass balances. Famine conditions were quantified by sCOD in the stabilizer, with lower values indicating stronger famine. Results and Discussion inDENSE Selection improves first flocculation, then compression InDENSE selection significantly improved sludge settleability and overall performance in the high-rate secondary system. Following the baseline run, inDENSE was implemented during Run 1. The introduction of inDENSE selection resulted in significant improvements in SVI, TOF and ESS (Figure 2). ESS improved first after 0.4 sludge ages and stavlized within one sludge age. TOF improved subsequently after 2.3 sludge ages and stablized after 3.3 sludge ages. SVI was last to respond after two sludge ages and stabilized after 4.1 sludge ages. This progression indicates that inDENSE initially enhances flocculation, as reflected by reductions in ESS and TOF, and subsequently improves sludge compression, as reflected by reductions in SVI. Combining F/M Control and Famine Control with inDENSE Selection The findings indicate that optimal settleability and system performance were achieved when inDENSE selection was combined with high F/M ratios and strong famine conditions (Figure 3). Under comparable SRTs (Figure 3C) and famine conditions (Figure 3B), comparison of Runs 2A, 2B, and 3A showed that higher F/M ratios with inDENSE selection reduced SVI, TOF, and ESS, reflecting improved sludge compression and flocculation. By contrast, the baseline run, which lacked inDENSE selection, exhibited poor settleability and elevated ESS despite operating at a high F/M ratio and similar famine conditions, underscoring that increased F/M enhances settleability and performance only when paired with inDENSE selection. Comparison of Runs 3A, 1A, and 3B, with similar F/M ratios but varying famine conditions, showed that stronger famine improved sludge settleability with inDENSE selection. Notably, in Runs 1A and 3B, which experienced poor famine conditions, TOF was lower and resulted in improved ESS. However, the substantial increase in SVI made these conditions undesirable. These results emphasize the need to balance ESS, TOF, and SVI when regulating system famine. The improved settleability and performance are likely linked to increased EPS production, as high F/M and famine conditions enhance EPS formation in high-rate CS systems (Rahman et al., 2020). Elevated EPS promotes floc aggregation, producing larger and denser particles that improve sludge compression and reduce SVI. Increased EPS also enhances flocculation in the clarifiers, thereby lowering TOF and ESS. Additionally, the inDENSE process amplifies these benefits by selectively retaining denser particles, which are likely composed of faster-growing microorganisms with high EPS production rates. InDENSE Selection Enhances Clarifier Capacity With inDENSE selection, both high F/M ratios and strong famine conditions improved clarifier capacity. Comparing Runs 3A, 2B, and 2A (Figure 4), which had similar famine conditions (Figure 3B) and SRTs (Figure 3C) but different F/M ratios (Figure 3A), showed capacity increased with F/M. At F/M < 1 kg sCOD/kg VSS·day (Run 2A), the flux curve area was smallest, indicating low capacity. Moderate F/M (1–2 kg sCOD/kg VSS·day, Run 2B) slightly improved capacity, while substantial gains occurred at F/M > 2 kg sCOD/kg VSS·day (Run 3A). Comparison of Runs 3A, 1A, and 3B at similar F/M (Figure 4) highlighted the role of famine conditions in clarifier capacity. Runs 1A and 3B, with Zone 1 sCOD > 50 mg/L (weaker famine), had smaller flux areas, whereas Run 3A, with Zone 1 sCOD < 50 mg/L, showed markedly higher flux, reflecting improved capacity. Overall, Run 3A achieved the highest clarifier capacity, demonstrating that optimal performance requires both F/M > 2 kg sCOD/kg VSS·day and strong famine conditions. Conclusion: This study demonstrates that inDENSE selection significantly enhances sludge settleability and overall performance in high-rate secondary systems, consistent with previous findings (Shao et al., 2025; Regmi et al., 2022; Roche et al., 2022). The results further show that optimal settleability and system performance were achieved when inDENSE was combined with high F/M ratios and strong famine conditions, underscoring the importance of integrating physical selection with biological strategies to maximize system performance.

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

Presentation time

10:30:00

11:00:00

Session time

10:30:00

12:00:00

SessionKeeping the Good Bugs and Losing the Fluff: Advances in Selective Wasting

Session locationKansas City Convention Center

TopicInnovative and Advanced Treatment for Achieving Limit-of-Technology Performance

Author(s)

Li, Yuang, Ahmad, Sakib, Ngo, Khoa Nam, Diop, Arame, Mendoza, Maria, Okechukwu, Godswill, Massoudieh, Arash, Azam, Hossain, Riffat, Rumana, Gu, April, De Clippeleir, Haydee, Goldberg, Emily

Author(s)Y. Li1, S. Ahmad2, K. Ngo3, A. Diop, M. Mendoza4, G. Okechukwu3, A. Massoudieh5, H. Azam4, R. Riffat2, A. Gu, H. De Clippeleir3, E. Goldberg5

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date May 2026

DOI10.2175/193864718825160236

Volume / Issue

Content sourceResiduals, Biosolids and Treatment Technology Conference

Word count23

Lessons Learned from a Pilot-Scale inDENSE Application: Improving High-Rate Contact-Stabilization Performance and Capacity

Abstract

Introduction High-rate activated sludge (HRAS) systems operate with short SRTs and HRTs, which often experience deteriorated sludge settleability (Rahman et al., 2020). To mitigate this challenge, selective wasting strategies have been investigated, including surface wasting (Haaksman et al., 2024), hydrocyclone wasting (Avila et al., 2021), screen wasting (Boltz and Daigger, 2022), and velocity-based wasting (Sun et al., 2019). Among them, inDENSE selection has shown strong potential for improving and stabilizing settleability, particularly in BNR systems under seasonal temperature fluctuations (Shao et al., 2025; Regmi et al., 2022; Hunter et al., 2024; Roche et al., 2022). This study evaluated the effects of inDENSE selection, alone and combined with biological control strategies, on sludge settleability and overall performance in a pilot-scale HRAS system, with the goal of identifying approaches to optimize both. Method and Materials A pilot-scale contact stabilization system with four reactor zones and three clarifiers was used for testing (Figure 1). Zone 1 operated as a high-DO stabilizer receiving only RAS to induce famine, while Zones 2 and 3 acted as contactors with primary effluent feed. inDENSE was applied on the wasting line. Four runs were performed: a baseline without inDENSE (wasting from Zone 4) and Runs 1–3 with physical and biological controls, resulting in six phases (Baseline, 1A, 2A, 2B, 3A, 3B). Sludge settleability was assessed via Threshold of Flocculation (TOF), Sludge Volume Index (SVI), and solids flux curve, while system performance was evaluated through daily solids and nutrient mass balances. Famine intensity was quantified using sCOD in the stabilizer, with lower values indicating stronger famine. Results and Discussion inDENSE Selection improves first flocculation, then compression Implementing inDENSE enhanced sludge settleability and system performance. Following the baseline run, inDENSE was implemented during Run 1 and resulted in significant improvements in SVI, TOF and ESS (Figure 2). ESS improved first after 0.4 sludge ages and stabilized within one sludge age. TOF improved subsequently after 2.3 sludge ages and stabilized after 3.3 sludge ages. SVI was last to respond after two sludge ages and stabilized after 4.1 sludge ages. This progression indicates that inDENSE initially enhances solids flocculation and subsequently improves sludge compression. Combining F/M Control and Famine Control with inDENSE Selection Optimal settleability and system performance were achieved when inDENSE selection was combined with high F/M ratios and strong famine conditions (Figure 3). At comparable SRTs and famine conditions, comparison of Runs 2A, 2B, and 3A showed that higher F/M ratios with inDENSE selection reduced SVI, TOF, and ESS, indicating improved sludge compression and flocculation. In contrast, the baseline run without inDENSE selection exhibited poor settleability and high ESS, showing that high F/M alone is insufficient to achieve imprevements. Comparing Runs 3A, 1A, and 3B (similar F/M, varying famine) revealed that stronger famine improved settleability under inDENSE selection. Notably, runs 1A and 3B, with weaker famine, had lower TOF and improved ESS, but their substantially higher SVI made these conditions suboptimal, highlighting the need to balance ESS, TOF, and SVI when managing famine. The improved settleability and performance are likely linked to increased EPS production, as high F/M and famine conditions enhance EPS formation in high-rate CS systems (Rahman et al., 2020). Elevated EPS promotes floc aggregation, producing larger and denser particles that improve sludge compression and reduce SVI. Increased EPS also enhances flocculation in the clarifiers, thereby lowering TOF and ESS. Additionally, the inDENSE process amplifies these benefits by selectively retaining denser particles, which are likely composed of faster-growing microorganisms with high EPS production rates. InDENSE Selection Enhances Clarifier Capacity With inDENSE selection, both high F/M ratios and strong famine conditions improved clarifier capacity. Comparing Runs 3A, 2B, and 2A (Figure 4A), which had similar famine conditions (Figure 3B) and SRTs (Figure 3C) but different F/M ratios (Figure 3A), showed capacity increased with F/M. At F/M < 1 kg sCOD/kg VSS·day (Run 2A), the flux curve area was smallest, indicating low capacity. Moderate F/M (1–2 kg sCOD/kg VSS·day, Run 2B) slightly improved capacity, while substantial gains occurred at F/M > 2 kg sCOD/kg VSS·day (Run 3A). Comparison of Runs 3A, 1A, and 3B at similar F/M (Figure 4B) highlighted the role of famine conditions in clarifier capacity. Runs 1A and 3B, with Zone 1 sCOD > 50 mg/L (weaker famine), had smaller flux areas, whereas Run 3A, with Zone 1 sCOD < 50 mg/L, showed markedly higher flux, reflecting improved capacity. Conclusion: This study demonstrates that inDENSE selection significantly enhances sludge settleability and overall performance in high-rate secondary systems, consistent with previous findings (Shao et al., 2025; Regmi et al., 2022; Roche et al., 2022). The results further show that optimal settleability and system performance were achieved when inDENSE was combined with high F/M ratios and strong famine conditions, underscoring the importance of integrating physical selection with biological strategies to maximize system performance.

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

Presentation time

11:00:00

11:30:00

Session time

10:30:00

12:00:00

SessionDensified Activated Sludge: From Startup to Optimization

Session locationErnest N. Morial Convention Center

TopicFacility Operations and Maintenance, Municipal Wastewater Treatment Design

Author(s)

Li, Yuang, Ahmad, Sakib, Ngo, Khoa Nam, Diop, Arame, Mendoza, Maria, Okechukwu, Godswill, Goldberg, Emily, Massoudieh, Arash, Azam, Hossain, Riffat, Rumana, Gu, April, De Clippeleir, Haydee

Author(s)Y. Li1, S. Ahmad2, K. Ngo3, A. Diop4, M. Mendoza4, G. Okechukwu4, E. Goldberg5, A. Massoudieh5, H. Azam4, R. Riffat2, A. Gu, H. De Clippeleir3

Author affiliation(s)Cornell University, 1George Washington University, 2DC Water, 3University of the District of Columbia, 4University of the District of Columbia, 4Catholic University of America, 5Catholic University of America, 5University of the District of Columbia, 4George Washington University, 2DC Water & Sewer Authority, 3

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Sep 2026

DOI10.2175/193864718825160296

Volume / Issue

Content sourceWEFTEC

Word count14

Description: Using video analysis to better understand Settleability in High-Rate Activated...

Using video analysis to better understand Settleability in High-Rate Activated Sludge Systems

Abstract

Introduction:
The high-rate activated sludge (HRAS) system is characterized by low sludge retention time (SRT) and hydraulic retention time (HRT). However, shorter SRTs can potentially impair sludge settleability (Md. Arifur Rahman et al., 2020). To address this, selective wasting strategies include surface wasting (Haaksman et al., 2024), hydrocyclone wasting (Avila et al., 2021), screen wasting (Boltz and Daigger, 2022), and velocity-based wasting (Sun et al., 2019) have been explored. To evaluate the effectiveness of these strategies, a deeper understanding of the settling characteristics is necessary to identify desirable and undesirable properties, going beyond basic parameters like threshold of flocculation (TOF), sludge volume index (SVI), and Vesilind Parameters (VPs). We propose using a camera-based method, inspired by Mancell-Egala et al. (2017), to analyze settling flocs under realistic clarifier conditions. This paper presents findings and assesses the key properties for improving settleability in HRAS systems.

Method and Materials:
Samples were collected from a 433 secondary pilot system operated through four phases, with SRT drops from 3.4 to 1.5 days. Settling parameters, including TOF, SVI, and VPs were measured. Videos of settling flocs were recorded from a pilot-scale clarifier (Figure 1) using a Canon REBEL T3i (Figure 2), and analyzed with the TrackMate plugin in ImageJ (Ershov et al., 2022) (Figure 3). Floc size, circularity, area, and settling velocity were directly measured from the video. Floc relative density calculation was adapted from Tambo et al. (1978). In addition, sludge samples were collected and tested from the over- and underflow of a hydrocyclone and from clarifier zones with different surface overflow rates (SOR): Zone A (1.8 m/h) and Zone C (0.6 m/h).

Results and Discussion:
Impact of SRT on Settling Characteristics:
Table 1 shows that as SRT decreased, both TOF and SVI increased, indicating deteriorating sludge settleability. The rise in SVI from the 3.4-day SRT phase to the 2.8-day SRT phase was more pronounced than the increase in TOF, while from 2.8 to 1.5 days, TOF increased much more than SVI. This suggested that when SRT dropped from 3.4 days to 2.8 days, SVI was predominantly impacted, which limited sludge compressibility. Once the SRT fell below 2.8 days, TOF was more adversely affected, indicating flocculation limitations.

Relationship between Floc Characteristics, Settling Characteristics, and SRT:
Figure 4A shows that slowly settling flocs were observed at lower SRTs. While the Feret diameter remained constant (Figure 4B), at 1.5 days SRT, there was a loss of flocs with larger surface areas (Figure 4C). Combining this information with the slight decrease in circularity observed at lower SRTs (Figure 4D), it can be concluded that reduced SRT led to the formation of more irregularly shaped particles with minimal change in size. This irregularity in floc shape might cause limitation in sludge compressibility (increased SVI) as more charge repulsion might occur. This might also lead to a decrease in collision efficiency, and therefore, an increased TOF (Table 1). The density change between SRTs of 3.4 and 2.8 days appeared more pronounced, which may correlate with changes in SVI (Figure 4E). In contrast, the density variation between SRTs of 2.8 and 1.5 days was primarily associated with an increase in the fraction of lower-density flocs, which may be linked to flocculation limitations reflected in the change in TOF (Figure 4E).

Proposed Physical Selection Strategies to Improve Sludge Settleability:
The image analysis suggests two potential physical selection strategies to address settling limitations caused by low SRT: (i) selecting based on floc settling velocity, and (ii) selecting based on floc density. To assess the first strategy, sludge samples were collected and tested from different clarifier zones with different SORs. The initial analysis (Figure 5) showed only minor improvements in both TOF and SVI. Knowing the shift in settling velocity as SRT decreases (Figure 4A), it appears that clarifier zones may not be effective enough to select flocs based on settling velocity. Further tests for wasting and recycling from the clarifier zones is planned, which will allow for a more thorough evaluation. We believe selection based on density is more effective, as initial tests with hydrocyclone over- and underflow show clearer improvements in SVI and TOF, particularly at lower SRTs (2.8 days vs. 3.4 days) (Figure 6). We are conducting image analysis on the over- and underflow fractions to validate the floc properties this strategy selects. The pilot system has also begun wasting via the hydrocyclone, and we will report on the changes in settling properties in the final paper.

Conclusion:
This paper demonstrated that high-rate systems are constrained by floc settling velocity as SRT decreases, which is caused by a reduction in density and the formation of irregularly shaped flocs. This leads to less efficient sludge compression and ultimately impacting collision efficiency. To overcome this limitation, selection based on density using a hydrocyclone appears to be the most promising approach. The results of this testing will be included in the final paper. Overall, this study provides new insights into floc properties, enhancing our understanding of settleability and the potential improvements or limitations of settleability.

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

Presentation time

16:30:00

17:00:00

Session time

15:30:00

17:00:00

SessionUnderstanding Clarifier Settleability from Fundamental to Intensification Process

Session locationMcCormick Place, Chicago, Illinois, USA

TopicResearch

Author(s)

Li, Yuang, Ahmad, Sakib, Ngo, Khoa Nam, Diop, Arame, Ramirez, Maria, Massoudieh, Arash, Azam, Hossain, Riffat, Rumana, Gu, April, De Clippeleir, Haydee

Author(s)Y. Li1, S. Ahmad2, K. Ngo3, A. Diop4, M. Ramirez5, A. Massoudieh4, H. Azam5, R. Riffat2, A. Gu1, H. De Clippeleir2

Author affiliation(s)Cornell University1, George Washington University2, DC Water and Sewer Authority3, The Catholic Univeristy of America4, University of the District of Columbia5

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Sep 2025

DOI10.2175/193864718825160026

Volume / Issue

Content sourceWEFTEC

Word count13

Base text size -

Ahmad, Sakib

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