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

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.