IntensiCarb for Anaerobic Digestion Intensification: A Techno-economic Analysis

Intensicarb® is a vacuum-based sludge concentration technology, that when installed to intensify anaerobic digestion at a Water Resource Recovery Facility (WRRF) could decrease hydraulic retention times (HRT) while maintaining solids retention times (SRT), providing various benefits towards process intensification. Previous work has demonstrated Intensicarb® feasibility for fermentation intensification (Okoye et al. 2022) and digestion intensification (Khadir et al. 2024) at lab scale as well as its economic feasibility for use as fermentation intensification for onsite WRRF supplemental carbon production (Armenta et al. 2023). This study presents a techno-economic analysis (TEA) of the Intensicarb® process when used to intensify mesophilic anaerobic digestion (MAD) at WRRFs of multiple sizes (10 million gallons per day (MGD), 50 MGD, and 250 MGD of influent flow). Significant reductions of overall lifecycle costs, including reductions in process volume and facility footprint were seen for all 3 facility sizes analyzed. Methodology Three facility sizes were evaluated, based on 10, 50, and 250 MGD influent flows, representing small, medium, and large facilities. This was correlated to digester solids loadings of 11,000, 54,000, and 270,000 lb/day respectively. Four scenarios of increasing intensification were developed and evaluated for each facility size. These scenarios were based on the decoupling of HRT and SRT provided by the IntensiCarb® technology. Average operating SRT was assumed to be 30 days for all scenarios, and the HRT was changed from 30 days (conventional MAD), to 15, 7.5, and 5 days with IntensiCarb®. These were designated as intensification factors (IFs) 1, 2, 4, and 6. Simplified process diagrams showing the conventional MAD process and MAD with IntensiCarb® are shown in Figures 1 and 2, respectively. Flows and loads used in each scenario were developed in Excel (Microsoft), with the assumption that volatile solids destruction was constant over the range of intensification studied (Khadir et al. 2024). Further assumptions are shown in Tables 1 and 2. These parameters were then used to develop the TEA, using a Net Present Cost approach and assuming a 30-year project lifespan. A nominal discount rate of 6% and an escalation rate of 3% were assumed (Construction Cost Index 2019-2022 2022; Guidelines and Discount Rates for Benefit-Cost Analysis of Federal Programs 2023). Costs associated with this TEA were developed based on vendor budgetary estimates for vacuum evaporators from PRAB and IWE, and AD tank mixers from Anaergia. Other equipment estimates, including conventional concrete AD tanks and associated pumps were developed based on previous Brown and Caldwell designs. Operational costs outside of labor, including electrical energy costs for pumping and mixing, sludge heating energy costs, and rehabilitation and replacement costs were developed based on the process modeling outputs and the budgetary estimates outlined above. A sensitivity analysis based on several key unit costs was developed to better estimate lifecycle costs for sites in North America with different operating costs. To simplify the analysis and provide a more conservative estimate of IntensiCarb®'s potential cost savings, two key potential benefits of the IntensiCarb® process were not included in this analysis. These were biogas recovery enhancement, and reduction of nitrogen levels in the sludge (Muller et al. 2024). Additionally, vendor estimates were used to develop physical layouts for all scenarios, which were compared on a volume and footprint basis to determine any physical space savings provided by the Intensicarb® technology. Results and Discussion The results of the TEA indicated significant reductions in process volume and footprint, particularly in larger facilities, leading to potential cost savings. These results are shown in Figure 3. The economic analysis showed that higher intensification factors generally result in lower life cycle costs, with the most substantial savings observed at intensification factors of 4 and 6. However, the benefits diminish at higher intensification levels for midsize plants due to the discrete sizes of evaporator equipment available. Life cycle costs for the 50 MGD facility are shown in Figure 4. The study concludes that IntensiCarb® technology is viable at small, medium, and large scales, with the largest savings seen at IF 6 for all sizes (27%, 26%, 16% NPC savings over conventional MAD seen at IF 6 for the small, medium, and large facilities respectively). Through a sensitivity analysis, key factors driving life cycle costs were identified, with natural gas, electricity prices, and IntensiCarb® facility construction costs causing the most impact on NPC. The baseline values for these costs, as well as their identified upper and lower bounds, are shown in Table 3. Future work should focus on empirical testing and theoretical analysis to refine cost assumptions and explore additional benefits like biogas quality improvement and nutrient recovery.