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

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.