Case Study Highlighting the South San Francisco-San Bruno Water Quality Control Plant's Initiatives to Increase Digestion Capacity within Existing Plant Infrastructure via High-Performance Mixing, Recuperative Thickening, and High Solids Digestion.

l to both the solids retention time (SRT) and hydraulic retention time (HRT). Anaergia's two-part Omnivore AD platform triples capacity of existing digesters to maximize existing WRRF infrastructure and provide redundancy. Anaergia's sludge screw thickener (SST) (Fig. 1) recuperatively thickens digestate to decouple SRT from HRT, allowing digesters to operate at significantly higher solids content. By separating water from digestate, excess water is removed and returned to plant headworks for additional treatment and/or nutrient recovery, while thickened solids (typically 12% TS) are returned to the digester (Fig. 2). Digestate is therefore thickened up to three-fold (6-8% TS) thus tripling F:M ratio and enabling organic loading rates up to 0.33 lb-VS/ft3/day. By decoupling SRT from HRT, Omnivore ensures SRT well over 15 days required for regulatory compliance and stable digester operation. Mixing is an essential and often overlooked component of the AD process and has historically been limited to technologies that operate effectively at lower solids content. To ensure proper mixing in higher solids conditions, Omnivore employs Anaergia's robust low-speed and high-torque permanent synchronous magnet (PSM) mixers (Fig. 3). These high-efficiency propeller mixers are designed to effectively mix high viscosities that result from thickening digestate and resist ragging. Omnivore mixing achieves a high standard of mixing performance, over 90% of the digester volume continuously mixed above critical velocity (0.3 ft/s) ensuring maximum volatile solids destruction, biogas production, avoiding settling of grit, and homogeneity de-risking upset events such as rapid rise. The cost-effective retrofit has the ability to unlock additional digester capacity and increase redundancy while simultaneously improving digester performance. With the additional AD capacity created, WRRFs can rapidly accommodate increased internal sludge loads, high strength waste (HSW) for codigestion, and improve plant resiliency. Application: South San Francisco-San Bruno Water Quality Control Plant (WQCP) capital improvement planning identified a need for additional AD capacity and initially prescribed construction of a new AD facility. Design by Carollo Engineers incorporated Omnivore retrofits to the existing AD system to cost-effectively provide necessary capacity and avoid the need to build an additional digester. The retrofit also provides flexibility for future codigestion of HSW and increased biogas production. In late 2020, construction was completed to retrofit Digester 1 (0.83 MG) with the Anaergia Omnivore high-solids digestion (HSD) system. The technology package included one skid-mounted sludge screw thickener (SST) for recuperative thickening, three high-solids submersible PSM digester mixers, three mixer service boxes, and all ancillary equipment for a complete Omnivore system. Existing mixers were removed from Dig. 1 and replaced with new PSM mixers. Mixers were selected for low energy demand, low operation and maintenance cost, and efficient performance. Platforms installed provide access to Anaergia service boxes (Fig. 3), which enable in-situ mixer access without interrupting AD operations. Each mixer is mounted to a vertical post inside the digester allowing for height and attack angle adjustments to achieve optimal mixing, resuspend settled grit, and break scum accumulation in upper layers. The SST and accessory equipment were installed adjacent to Dig. 1, reducing pump energy, footprint, and impact on WQCP operations. Validating Omnivore HSD: Dig. 1 was brought online on January 5, 2021. Recuperative thickening was initiated in February and achieved steady performance in April (Fig. 4). Third-party performance testing was conducted in conjunction with Anaergia from April to July 2021. The below performance criteria were evaluated and achieved (Table 1): - Min. volatile solids reduction (VSR) above 57% for 80% of the time over 90-day test period, over a 30-day moving average (Fig. 5) - Demonstrate stable digester operation with volatile fatty acid (VFA)/alkalinity ratio <0.2 (Fig. 6) Testing further confirmed successful HSD in Dig. 1 as demonstrated by increased digester feed volumes, TS loading, and VSR, as well as steady OLR and VFA/alkalinity ratio. Note that design was developed to meet performance requirements and operational objectives requested by WQCP representatives and does not maximize use of Dig. 1 volume. Operation at higher solids would further increase digester capacity for future growth needs or codigestion. Mixer Performance Test: A third-party Lithium-ion Tracer test was conducted to confirm achievement of mixer performance requirements. The independent study (1) validated achievement of Anaergia's high-performance mixing standard (>90% of digester volume >0.3 ft/s) and (2) aligned with computational fluid dynamics (CFD) modeling of mixer performance, which indicated superior performance compared to alternate mixing technologies. Adequate mixing efficiencies were observed from the Tracer test washout curve (Fig. 7), which demonstrates low data scatter (high R2 value). Temperature profile indicated uniform temperature distribution within Dig. 1, with a 1-degree C maximum variance between maximum and minimum temperatures (Fig. 8). Total solids concentration was +/-10% of the mean value, indicating homogenous digester operation (Fig. 9). Tracer test findings are compatible with CFD analysis by Anaergia for the WQCP. The analysis modeled Dig. 1 and various mixer technologies present at the WQCP (Anaergia PSM, linear motion, and pump mix). The model was calibrated with published submittal sheet parameters for each mixer type and actual dimensions of Dig. 1. The model compared energy usage and mixing performance (as measured by percent of tank volume above critical velocity) for each technology at 2%TS and 6%TS. The model demonstrated that PSM mixers are more effective and efficient in delivering mixing performance (Tables 2-5), with equivalent or reduced energy usage compared to linear motion mixing (up to 54% reduction in energy consumption). Moreover, analysis demonstrated a greater portion of tank volume at or above critical velocity when using PSM mixers vs. the alternatives. Results of the CFD analysis demonstrate further advantages (i.e., greater delta in volume above critical velocity) in high solids conditions, such as those resulting from Omnivore thickening or codigestion with high-solids food waste and evaluated as part of the lithium tracer testing. Recuperative Thickening Performance Test: An SST-focused 5-day performance test demonstrated acceptable operation of the recuperative thickening system (Table 6). In addition, an extended 30-day test completed at the request of the WQCP demonstrated continuous and overnight operation. Summary: The Omnivore system effectively addressed the biosolids and residuals management needs of the WQCP leveraging existing infrastructure and provided third-party validated performance improvements. As a result of the project's success, the below benefits were delivered to the WQCP: - Avoided major capital expenditure of an additional digester - Addressed capital improvement needs - Redundant digester capacity - Reliable production of stabilized biosolids - Improved digester performance - Improved mixing performance - High-efficiency mixers with lowest overall lifecycle cost - Future-proofing for load growth and changes in feedstock composition - Enable codigestion of organic waste - Support for increased biogas production and energy neutrality A similar solution may be replicated at other WRRFs to cost-effectively improve biosolids management while significantly enhancing the ability of municipalities to leverage existing infrastructure for meaningful resource recovery. This session will discuss the considerations, advantages, and applications of implementing HSD at WRRF such as the WQCP. It will further detail third-party acceptance testing data demonstrating the successful operation of the Omnivore system and HSD processes, and achievement of a new standard in high-performance mixing. Discussion will include agreement between the in-field performance testing and modeled mixer perform