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

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.