Enhancing Secondary Clarifier Capacity through Hydrocyclone Implementation at Full-Scale Wastewater Treatment Plants

BACKGROUND Hydrocyclone technology was tested at the Los Angeles County Sanitation Districts' (Districts) Whittier Narrows Water Reclamation Plant (WNWRP). The WNWRP is a Modified Ludzack Ettinger (MLE) facility with a permitted design capacity of 15 mgd. However, the facility currently operates below its rated flow capacity due to secondary clarifier loading limitations. Additionally, the plant faces seasonal challenges with high sludge volume index (SVI), particularly during winter, which impacts system performance. The plant's location within a floodplain further complicates matters, as construction beyond the existing facility footprint is prohibited. To address the secondary clarifier capacity constraints and winter SVI challenges, the World Water Works inDENSE® hydrocyclone system, equipped with five cyclones, was evaluated at the WNWRP (Figure 1). The hydrocyclone system, applied to the return activated sludge (RAS) stream, separates lighter biomass (overflow) from denser biomass (underflow). The denser fraction is recycled back into the RAS, while the lighter fraction is discarded, enhancing sludge settleability. RESULTS AND DISCUSSION The hydrocyclone's performance was assessed by measuring the solids concentration and SVI in the hydrocyclone inflow, underflow, and overflow streams (Figure 2). Overall, the results revealed a consistent pattern across samples: SVI values followed the order of overflow > inflow > underflow, while TSS concentrations were ranked as underflow > inflow > overflow. Long-term impacts on the full-scale activated sludge process were evaluated by monitoring SVI over time. Figure 3 illustrates SVI trends for the three MLE aeration units (Units 1, 2, and 3) throughout 2024. Hydrocyclone implementation improved sludge settleability, reducing SVI from approximately 175 to 125 mL/g within a few weeks of operation. Continued use further decreased SVI to about 50 mL/g; however, this unexpectedly resulted in higher TSS and turbidity levels in the secondary effluent (Figure 4). In response, the hydrocyclone system was temporarily shut down until effluent quality improved, after which four out of the five cyclones were reactivated. The deterioration in effluent quality was attributed to a loss of extracellular polymeric substances (EPS) within the sludge flocs. EPS forms a matrix that binds microbial cells and traps pinpoint flocs, promoting the formation of cohesive flocs. The high-speed centrifugal forces within the hydrocyclone disrupt the EPS matrix (Xu and Wang, 2019; Xu et al., 2019), weakening cell adhesion. This disruption results in the formation of smaller, fragmented flocs and untrapped fine particles, both of which settle poorly and are more likely to be carried over into the secondary effluent. Scatter plots in Figure 5 illustrates the relationship between average SVI for the three aeration units and TSS and turbidity in the secondary effluent. Figure 5a reveals a positive correlation: higher SVI, indicative of poorer sludge settling, results in increased TSS and turbidity in the effluent. This trend remained consistent after hydrocyclone implementation (Figure 5b). Conversely, excessively low SVI also correlated with elevated effluent solids, likely due to smaller, fragmented flocs and untrapped pinpoint flocs caused by EPS disruption. This U-shaped relationship suggests that maintaining SVI within an optimal range is critical for minimizing effluent solids. Microscopic analysis of biological flocs in the inflow, underflow, and overflow samples (Figure 6) further elucidated these observations. Initial hydrocyclone operation improved settling (~125 mL/g SVI) by regulating filamentous bacteria populations (Figures 6a, 6c, and 6e) within three weeks. However, extended hydrocyclone operation reduced or eliminated filament bridging within flocs (Figures 6b, 6d, and 6f), which contrasts with typical biological flocs where filamentous bridging provides structural integrity. Table 1 summarizes the size distribution of biological flocs across inflow, underflow, and overflow samples collected on four dates. Mid-sized flocs (100—500 µm) were dominant. Over time, smaller flocs (<100 µm) increased, while larger flocs (>500 µm) decreased, indicating that hydrocyclone shear forces break larger, fragile flocs into smaller, denser ones. SUMMARY AND RECOMMENDATIONS Hydrocyclone technology has demonstrated its value as a cost-effective solution for enhancing secondary clarifier capacity by improving sludge settleability. This improvement has allowed the WNWRP to sustain efficient operations at higher flow rates. However, optimizing hydrocyclone operating conditions is essential to balance improved settling performance with maintaining effluent quality. In 2024, the hydrocyclone system was intermittently shut down to address effluent quality concerns, with only four of the five cyclones in operation during the latter half of the year. Beginning in 2025, the system will transition to continuous operation using three cyclones, and findings from this updated configuration will be presented. Building on this success, the Districts are advancing the installation of two additional hydrocyclone systems at the Pomona and San Jose Creek WRPs. Performance data from these installations is expected by mid-2025, and the presentation will include key insights and results from these new applications.