Alternate: Evaluating Feasibility of Implementing Co-digestion at San Francisco International Airport

Background: This study was performed to evaluate feasibility of implementing co-digestion at San Francisco airport facility (SFO) in California. The overall objective was to obtain preliminary feasibility information regarding diverting organic wastes from the terminal restaurants to its wastewater treatment plant anaerobic digester to enhance biogas production for beneficial use. The study included preliminary regulatory, technical, economic as well as triple bottom-line evaluation. The key objectives of the study included evaluating i) how much organic waste can be added to the airport's anaerobic digester without impacting the digester stability, ii) can all of this capacity be met with the organic wastes generated at the airport, iii) how much biogas will be generated during co-digestion, iv) what are the potential beneficial uses for the quantity and quality of biogas produced that are in alignment with the airport's sustainability goals as well as current and impending regulations, v) what are the potential impacts (e.g. increase in wet cake, polymer demand, metals in biosolids) of this process, and vi) what is the preliminary cost estimate for this program. Regulatory Review: Regulatory evaluation included review of SFO's existing wastewater treatment plant permit requirements (e.g. effluent discharge, emissions) as well as recent changes to co-digestion related regulations. A review of the permits indicated i) discharge limits for ammonia, organics and select metals, and emissions limit for ammonia, select organics and hydrogen sulfide (H2S). The bench scale studies were designed to partially address impacts of co-digestion on these limits. Among the new/emerging regulations, the 'Advanced Clean Fleets' (ACF; Release #23-13) regulation put forward by California Air Resources Control Board (CARB) and approved by the State in 2023 requires all the heavy and medium duty vehicles to be phased out and replaced with zero emission vehicles (ZEV) within the next 20 years. This will have a significant impact on the use of biogas for vehicle fuel. Further, a proposed CARB amendment to suspend Low Carbon Fuel Standard (LCFS) incentives, if implemented, will further discourage pipeline injection of treated biogas (Renewable Natural Gas (RNG)). The impact of these and other regulations were taken into consideration during evaluation of beneficial use alternatives for biogas. Bench Scale Testing: A bench scale test was performed using the TWAS from the airport full scale sequential batch reactors and the organic waste from the airport restaurants to address various objectives of this study. The organic wastes were collected as mixed waste which was then extracted using a Thor process for addition to bench scale reactors. The wastes and TWAS were characterized for organic and inorganic constituents, including metals concentration. The data indicated that the organic content (COD) and degradable solids (VS) were much higher in the food waste than those for TWAS. The COD of the TWAS and organic wastes were approximately 48,000 and 245,000 mg/L respectively. The total and volatile solids of the TWAS were 4.6 and 82%, respectively and the total and volatile solids of the organic waste were 21 and 88%, respectively. Four 10 Liter bench scale digesters filled with 8 L of TWAS and appropriate amount of food waste was used in the bench scale studies (Figure 1). One of the digesters (Control) received only the TWAS. The other three digesters (R1, R2 and R3) received food waste at 20, 40 and 60% of volatile solids as the TWAS volatile solids content. On a volume basis the food waste added was 4.3, 8.3 and 12.25% of the TWAS volume in the three digesters, respectively. The digesters were operated in the anaerobic digestion lab (ADL) at Bucknell University (Lewisburg, PA) at 37 oF. Various operating parameters and biogas quality and quantity were measured during the four-month study. The bench scale data indicated that the control digester (i.e. TWAS-only) produced approximately 6,000 mls/day of biogas whereas the digester with the highest amount of food waste produced approximately 14,000 mls/day of biogas Figure 2). In addition to total biogas production, the biogas yield, i.e. the cubic feet of biogas produced per pound of volatile solids reduced also were higher for the food waste added digester. The biogas yield of the control and the highest food waste added digesters were 35.5 and 47 cu.ft./lb VS reduced, respectively. These data indicated that, the food waste was digested more readily than the TWAS and produced significantly higher amount of biogas at a much smaller volume added. Addition of food waste increases the solids added to the digester. This, in turn, may increase the amount of dewatered cake generated after digestion. Hence, dewatering studies were conducted using the digested sludge from the bench scale study. The data indicated that the net wet cake mass may decrease by about 7% at the loading used in R1 digester and increase by approximately 8 and 12% at the loading used in the R2 and R3 digester, respectively (Figure 3). Projections for Full Scale Operation: The bench scale study data was then used to project various parameters for the airport's full scale co-digestion operation. The evaluations indicated that nearly 1,900 tons of organic waste is required annually for the co-digestion process at the desired (food waste at 60% VS of TWAS VS) loading rates. The airport generates approximately 4,500 tons of organic waste each year. Hence, enough organic waste is available for co-digestion. Further, sufficient volume is available in the digester to receive the food waste is diluted up to 7% solids content for ease of pumping. This operation is estimated to produce approximately 38 to 45 standard cubic feet per minute (scfm) biogas. Based on bench scale data, addition of this amount of food waste may increase the dewatered cake produced by 1.3 wet tons per day and the polymer demand for dewatering by approximately 19.5 lb/day. A preliminary evaluation of regulatory compliance, limited to the data from the bench scale study, indicated compliance requirements for the tested parameters may be met after food waste addition to the digester. However, more detailed evaluation through a larger scale study and detailed discussions with regulatory agencies may be required during the next phases of the study to ensure compliance with regulatory requirements. Evaluation of End Use Alternatives: The above information/data were used to evaluate beneficial use alternatives for the biogas produced through co-digestion. The alternatives evaluated include i) combustion-based combined heat and power (CHP) generation technologies such as internal combustion engines (IC Engine) and microturbines, ii) non-combustion CHP technologies such as fuel cell systems and linear generation technology, iii) use of biogas for vehicle fuel, and iv) pipeline injection of treated biogas. The airport does not prefer combustion-based technologies due to environmental concerns. Further, the current and impending CARB regulations do not favor the use of biogas for vehicle fuel or pipeline injection in the long term. The non-combustion CHP technologies, while relatively new to the market, have made significant advancements in the recent years. Further, they produce much lower greenhouse gas emissions. Hence, these non-combustion technologies (fuel cell and Linear Generation) are recommended for further evaluation in the next phase for potential installation at the airport. Preliminary Cost Evaluation: A preliminary cost evaluation was performed for the implementation of the proposed program. The airport is currently evaluating its options for food waste collection, sorting and processing. Hence, the evaluation did not include food waste collection, extraction, storage and pumping to the digester. The preliminary cost estimate for the rest of the constituents (CHP, biogas treatment and installation, etc.) is approximately $2.95 M. The increased annual O&M cost is approximately $180,000/year. The estimated revenue from the savings in electricity is approximately $ 270,000/year. Triple Bottom-Line Evaluation: Further, a triple bottom-line cost-benefit analysis (financial, social and environmental) was performed for the proposed technology. The evaluation indicated that in the next twenty years the proposed technology will avoid a total of approximately 19,500 tons of CO2e emissions, mainly from diverted waste and fertilizers offset. It is also estimated that the proposed technology will mitigate emissions of approximately 5,200 pounds of contaminants through avoided natural gas purchases. No avoided emissions due to electricity generation is anticipated since SFO already purchases clean energy from San Francisco Public Utility Commission.