Flexibility During Design — How Goleta Sanitary District (GSD) adapted their long-term bioenergy recovery strategy to rapidly changing external factors

This presentation will demonstrate how Goleta Sanitary District (GSD) benefited from having a flexible bioenergy recovery plan and analytical tools to adapt to changing external factors such as funding availability, capital costs, biosolids hauling costs and evolving emission regulations. Attendees will learn the value of revisiting previous decisions when new data becomes available or conditions change and the advantage of having an analysis tool to visualize new outcomes. GSD developed a Biosolids and Energy Strategic Plan (BESP) in 2019 to better capitalize on the energy available from gas produced by their anaerobic digesters at the Goleta Water Resource Recovery Facility (WRRF). The project team used an interactive energy, mass, and financial balance tool (Energy Balance and Analysis Tool - EBAT) to evaluate the benefit from multiple bioenergy recovery alternatives including a combined heat and power (CHP) system, renewable natural gas and sludge drying. Digestion expansion, gas storage and CHP were recommended for the near team. Construction of a biosolids dryer was recommended for solids management about 10 years after digestion expansion to reduce hauling costs and produce a Class A product. GSD began preliminary design work in 2020 to implement the recommendations from the strategic plan. The design included a CHP system with a 450kW digester gas fueled engine generator, digester gas treatment, emissions controls to meet the regional emission limit rules, and a gas storage sphere to provide a consistent gas supply and operational flexibility for the CHP engine. The CHP system would initially operate at 75 percent of its rated output, increasing to full output upon implementation of the planned fats, oils, and greases (FOG) co-digestion program. During the preliminary design phases several external factors changed that extended the payback period of the recommended bioenergy recovery strategy (CHP), including: 1.Capital costs of the CHP engine increased 5-10 percent due to the increasing costs of raw materials. 2. Dialog with the Santa Barbara County Air Pollution Control Department (SBCAPCD) indicated that the 450kW engine would fall under the 'Best Achievable Control Technology' (BACT), requiring continuous emissions monitoring system (CEMS) and a higher level of emission reduction equipment which would increase operating costs. 3. Biosolids disposal and hauling costs increased at a higher rate than expected and could justify expediting the sludge dryer installation. As these conditions evolved, the EBAT model was used to re-examine the long-term bioenergy recovery strategy to determine if an alternative strategy would provide a higher level of value. The team tested many different energy scenarios simultaneously. Scenarios were various combinations of one 450 kW engine generator, one or two 160 kW engine generators, SCR and CMS, and funding received from the Self Generation Incentive Program (SGIP). GSD and the design team discussed the risks and benefits of moving forward with a larger system compared to a smaller system such as availability of grants to fund the project, capital at risk, disruption to operations during construction and adaptability to capitalize on future plant modifications such as co-digestion. As demonstrated in Figure 1, the analysis found that the 160kW CHP system provided a shorter payback period than the 450kW system (13 years vs 22 years) primarily due to the lower capital costs per kW, which was achieved by removing the need for digester gas storage and the continuous emissions monitoring system. While the smaller CHP system would not use all the digester gas produced, the addition of a thermal dryer in the near term (at about 5 years) could provide a means to beneficially utilize the digester gas volume that exceeds the CHP capacity. To test this idea, the team again returned to the EBAT model, incorporating the dryer into the financial and energy balance calculations. The adjustable inputs within the model such as dryer uptime, dryer installation year, FOG co-digestion volume and revenue from FOG tipping fees were varied to understand the individual and cumulative impact on the financial outcome of installing a dryer in the near term. GSD used the interactive, Power-BI based dashboard to experiment independently with CHP sizes, dryer installation years and capital cost reductions from funding opportunities. Figure 2 is a screen shot of the EBAT dashboard. Ultimately, the EBAT model guided GSD to select the smaller CHP system as the bioenergy recovery strategy that provides them higher value with less capital risk. The installation year for a thermal dryer is still under evaluation. This case study illustrates the importance of continuously evaluating and comparing different bioenergy alternatives as conditions such as capital costs, emission regulations, hauling costs, market conditions and other variables evolve during the development of a bioenergy recovery project. The development of a flexible long term bioenergy recovery plan as well as an interactive bioenergy recovery analytical tool provided GSD a means to easily navigate changing conditions and ensure the optimal solution in implemented.