Lessons Learned from a Pilot-Scale inDENSE Application: Improving High-Rate Contact-Stabilization 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 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.