Optimizing Suspended Solids Removal in Secondary Clarifiers from DAS Systems

Introduction:
Metro Water Recovery's Robert W. Hite Treatment Facility (Hite) in Denver, CO, is permitted for a maximum month flow (MMF) of 220 mgd and an average daily flow (AADF) of 130 mgd. Hite includes the North Secondary (NSEC) treatment train, rated at 106-mgd MMF and 94-mgd AADF, which utilizes an MLE process with a sidestream anaerobic reactor (SAR) for nutrient removal. In 2018, Metro's facility plan highlighted the need for increased aerobic solids retention time (aSRT) to comply with potential future total nitrogen and total phosphorus regulations. Metro commissioned a full-scale demonstration train (NAB2) to test densified activated sludge (DAS) to avoid costly expansion.

While DAS is effective at improving settleability and increasing treatment capacity, it can also lead to higher secondary clarifier effluent suspended solids (ESS) (Figure 1). This study investigates the causes for elevated ESS from DAS and identified strategies for improving ESS by modifying existing clarifiers utilizing comprehensive field sampling and an updated 2Dc computational fluid dynamics (CFD) model.

Secondary Clarifier Capacity Evaluation:
This study builds on previous 2Dc modeling efforts evaluating the capacity of NSEC's secondary clarifiers under projected future flows and loads with DAS implementation. Stress testing of the NAB2 DAS pilot indicates that NAB2 could be loaded at surface overflow rates (SORs) > 700 gpd/sf and solids loading rates (SLRs) > 60 pounds per day per square foot (ppd/sf) while still maintaining reliable blanket control and stable ESS (Figure 2). Whole plant process and 2Dc modeling indicate that DAS could provide Metro sufficient capacity to address design flows/loads at the NSEC while also meeting stringent nutrient limits without new basins or clarifiers (1).

Weir Sampling to Inform Modeling:
The NSEC clarifiers have inboard (two weirs) and outboard (one weir) launders, and different effluent quality associated with each weir. In both the conventional activated sludge (CAS) control and NAB2 DAS pilot systems, the narrow space between the inboard and outboard launder is believed to create a higher velocity current up the side wall of the clarifier where approximately 2/3 of the effluent flow leaves the clarifier. In the DAS system, this effect may be exacerbated by the unique properties of the densified particles, including higher settling velocities and reduced agglomeration as indicated by increased dispersed suspended solids (DSS) (Figure 3) (2).

To support this investigation, comprehensive field sampling was performed on the NAB2 DAS pilot to gather ESS data from each weir across four clarifier quads (Figure 4). The data confirm higher ESS associated with the outboard launder and support updated 2Dc model calibration to evaluate potential clarifier modifications to reduce ESS.

Modeling Approach and Key Findings:
The updated 2Dc modeling code was enhanced to depict a more accurate representation of NSEC's dual launder system and the ability to report ESS per weir (including blocking flow from individual weirs). This enhanced model was used to perform a sensitivity analysis including weir blocking, center well extensions, and addition of a Stamford baffle. The alternative improvements were evaluated under the following NSEC design conditions: MMF (106 mgd) and load, AADF (94 mgd) and load, and peak flow (150 mgd) with annual average load. Results indicate that multiple alternatives may significantly reduce ESS:

Alt 1: Block Weir 3: Blocking the outboard launder weir is a simple, low-cost modification, and is predicted to reduce ESS by 25-50% through addressing uneven solids distribution identified in field data.

Alt 2: Block Weir 3 + Center Well Extension: Extending the center well by 2' is predicted to reduce ESS by 30-70%; however, structural modifications to the mechanism arm and center well would be required.

Alt 3: Block Weir 3 + Stamford Baffle: Adding a Stamford baffle is predicted to reduce ESS by 40-70%, with improvements attributed to better sludge blanket stability and optimized flow patterns.

Alt 4: Combine All Modifications: Integrating all strategies is predicted to have the highest ESS reduction (45-75%), but careful sludge blanket monitoring and return activated sludge (RAS) flow control would be required to keep the blanket below the Stamford baffle (Figure 5).

Predicted ESS improvements were applied to historic NAB2 ESS data to determine the anticipated ESS ranges for each alternative (Table 1).

Conclusion:
This study showcases an enhanced 2Dc modeling approach for optimizing ESS in DAS systems. For DAS systems experiencing ESS challenges, utilities may consider clarifier modifications such as blocking specific weirs, extending the center well, and adding a Stamford baffle to help address uneven solids distribution, mitigate density currents, and optimize clarifier performance. These modifications show potential ESS reductions ranging from 25-75%, offering practical, site-specific solutions. 2Dc modeling is a valuable tool for diagnosing elevated ESS issues and identifying opportunities to reduce variability, helping utilities make informed decisions to optimize clarifier performance. By combining advanced modeling tools with targeted clarifier improvements, utilities can effectively reduce ESS variability and position themselves to meet future regulatory requirements.