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

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.