South San Francisco-San Bruno Water Quality Control Plant Digestion Capacity Expansion Within Existing Plant Infrastructure Via High Solids Digestion

Technology Overview:
Anaerobic digestion (AD) biologically degrades municipal wastewater solids and other organic waste streams. Typical AD at municipal water resource recovery facilities (WRRFs) receive primary sludge (PS) and thickened waste activated sludge (TWAS) at 4-5% total solids (TS) and operate at 2-3% TS. Volatile solids (VS) in TS are converted over ~15-day solids retention time (SRT) to produce biogas and stabilize sludge. In conventional digestion, SRT and hydraulic retention time (HRT) are equal. 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 at similar SRT and greatly reduced HRT. (Pre-thickening is available if preferable given existing site configuration.) Excess water is separated from digestate and returned to headworks for treatment and/or nutrient recovery, while thickened solids (~12%TS) are returned to the digester (Fig. 2). Digestate is thickened and returned to the digester, enabling the digester to operate at up to three-times the solids concentration (6-8% TS), with organic loading rates up to 0.33 lb-VS/ft3/day. By decoupling SRT from HRT, Omnivore provides SRT well over 15-day requirements for regulatory compliance and stable digester operation. Mixing is an essential component of digestion and has historically been limited to technologies that operate at lower solids content. To ensure proper mixing at higher solids, Omnivore employs robust low-speed, high-torque permanent synchronous magnet (PSM) mixers (Fig. 3). These high-efficiency submersible propeller mixers effectively mix high viscosities resulting from digestate thickening. Omnivore achieves a high standard of mixing performance, over 90% of digester volume continuously above critical velocity (0.3 ft/s) ensuring maximum volatile solids destruction, biogas production, and limited grit settling or upset events such as rapid rise. The cost-effective retrofit unlocks digester capacity and increases redundancy while improving digester performance. With additional AD capacity, WRRFs can rapidly accommodate increased internal sludge loads, external high strength waste (HSW) for co-digestion, and improve 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 co-digestion of HSW and increased biogas production. In late 2020, construction was completed to retrofit Digester 1 (0.83 MG) with Anaergia's Omnivore high-solids digestion (HSD) system. The Omnivore package included one skid-mounted SST for recuperative thickening, three high-solids submersible PSM digester mixers, three mixer service boxes, and all ancillary equipment. Existing mixers were removed from Digester 1 and replaced with Anaergia mixers, selected for low energy demand, low operation and maintenance cost, and efficient performance. Platforms installed provide access to Anaergia service boxes (Fig. 3), enabling in-situ mixer access without interrupting 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. SST and accessory equipment were installed adjacent to Digester 1, reducing pump energy, footprint, and impact on WQCP operations.
Validating Omnivore HSD: Digester 1 was brought online 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. Performance criteria were evaluated and achieved (Table 1): - Minimum volatile solids reduction (VSR) above 57%, for 80% of the 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 Digester 1 as demonstrated by increased digester feed volumes, TS loading, and VSR, and 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 Digester 1 volume. Operation at higher solids would further increase digester capacity for future growth needs or co-digestion.
Mixer Performance Testing: A third-party Lithium-ion Tracer test was conducted to confirm achievement of mixer performance requirements. 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 versus alternate mixing technologies. From Tracer testing, mixing efficiencies were confirmed from the washout curve (Fig. 7). Temperature profile indicated uniform temperature distribution within Digester 1, with a 1-degree C maximum variance between maximum and minimum temperatures (Fig. 8). Total solids concentration fell within +/-10% of the mean, indicating homogenous digester operation (Fig. 9). Tracer test findings are compatible with Anaergia's CFD analysis for the WQCP. Digester 1 was modeled with 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 Digester 1 dimensions. The model compared energy usage and mixing performance (measured by percent of tank volume above critical velocity) for each technology at 2%TS and 6%TS. Anaergia's model demonstrated 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). Moreover, analysis demonstrated a greater portion of tank volume at or above critical velocity when using PSM mixers versus the alternatives. Results of 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 HSW co-digestion.
Summary: Omnivore leveraged existing infrastructure to effectively address biosolids and residuals management needs of the WQCP and provided third-party validated performance improvements.
The WQCP realized these benefits: - Avoided major capital expenditure of additional digester - Addressed capital improvement needs - Redundant digester capacity - Reliable stabilized biosolids production - Improved digester and mixing performance - High-efficiency mixers with lowest lifecycle cost - Future-proofing for load growth and feedstock composition changes - Enabled co-digestion of HSW - Support for increased biogas production and energy neutrality
Similar solutions may be replicated at other WRRFs to cost-effectively improve biosolids management while enabling municipalities to leverage existing infrastructure for meaningful resource recovery.
Session will discuss considerations, advantages, and applications of HSD at WRRFs. It will further detail third-party acceptance testing data demonstrating successful operation of Omnivore and HSD processes, and achievement of a new standard in high-performance mixing. Discussion will include agreement between in-field performance testing and modeled mixer performance for the WQCP, and implications.