South San Francisco-San Bruno Water Quality Control Plant Employs High-Solids Digestion to Enable Food Waste Co-Digestion & Advance Energy Neutrality with Lowest Lifecycle Cost

Executive Summary: South San Francisco-San Bruno Water Quality Control Plant (WQCP) incorporated high-solids digestion to cost-effectively provide necessary capacity and avoid the need to build additional digester tanks. Following third-party validation of healthy digester operations in high-solids conditions, the WQCP advanced a food waste co-digestion pilot in partnership with a nearby waste management authority. The pilot demonstrated significant increase in biogas production, without negative impact to digester health. The initiative not only sets the stage for the WQCP to advance energy-neutrality but also supports achievement of state-level organic waste diversion mandates in California to support greenhouse gas reductions. Background: The WQCP identified a need to rebuild the plant's three existing digesters due to seismic concerns and built additional AD to meet future load growth. While capital planning initially called for new construction of five (5) digesters total, Carollo Engineers conducted an alternatives analysis considering high-solids digestion to more cost-effectively achieve the required digester capacity. Carollo evaluated a scenario pairing two (2) conventional AD with one (1) high solids digester employing Anaergia's Omnivore high-solids digestion platform. 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. To ensure proper mixing in higher solids conditions, Omnivore employs Anaergia's high-torque submersible propeller mixers (Fig. 3). These high-efficiency mixers are designed to effectively mix high viscosities that result from thickening digestate and resist ragging. By applying mixing energy directly to digestate, 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 Carollo evaluation indicated that the proposed configuration leveraging high-solids digestion would deliver equivalent digester capacity to the originally planned five (5) tanks. Further, lifecycle cost analysis confirmed that the high-solids alternative would not only achieve CAPEX savings but OPEX savings, resulting in approximately 10% overall savings over the project life. OPEX savings to the WQCP primarily result from improved mixer energy efficiency and reduced polymer consumption associated with dewatering. Application: 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 Anaergia propeller mixers. Mixer selection was validated and informed by computational fluid dynamic (CFD) modelling, the results of which are summarized in Tables 1-4. The model demonstrated low energy demand, improved tank mixing, and energy-efficient mixing performance, particularly in high-solids conditions (6% TS), as compared to linear motion and pump mix alternatives. 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. 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 5): -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. 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 CFD modeling of mixer performance, which indicated superior performance compared to alternate mixing technologies. Co-Digestion Pilot: In July 2022, following successful completion of performance testing, the WQCP began a pilot program to receive 1-2 deliveries per day of slurried food waste. The organic slurry is produced at the South Bayside Waste Management Authority's Shoreway Environmental Center. The materials recovery facility (MRF) employs an Anaergia Organics Extrusion (OREX) press and Organics Polishing System (OPS) to remove contamination from commercial source separated organics and create a clean, homogenized, pumpable organic slurry. The slurry is pumped into 5,000-gallon liquid tanker trucks and hauled approximately 15 miles from the MRF to the WQCP. The slurry is fed to Dig. 1 for co-digestion with indigenous biosolids. The high-solids digestion upgrades have facilitated the pilot by provided excess capacity for external feedstock, as well as high-solids mixing capability within the digester to maintain digester health and mixing performance. WQCP monitoring indicates significant increases in biogas production since the initiation of high-solids digestion and co-digestion, respectively. Ongoing data collection during the pilot will inform planning and design of food waste receiving and biogas utilization upgrades to facilitate long-term co-digestion and advance energy-neutrality at the WQCP. Note that the high-solids digestion 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 expanded co-digestion. Conclusion: The Omnivore system effectively addressed the biosolids and residuals management needs of the WQCP leveraging existing infrastructure; provided third-party validated performance improvements; and facilitated a successful food waste co-digestion pilot. 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 co-digestion of organic waste -Increased biogas production and energy neutrality 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 co-digestion, and improve plant resiliency. A similar solution may be replicated at WRRF throughout North America to cost-effectively improve biosolids management while significantly enhancing the ability of municipalities to leverage existing infrastructure for meaningful resource recovery and energy neutrality.