Techno-Economic Analysis of Advanced Solids Treatment by Anaerobic Co-Digestion and Hydrothermal Liquefaction

Summary

The solids treatment options in wastewater treatment facilities are at a crossroad. While recent technological advances provide opportunities to increase resource recovery concerns about emerging contaminants pose challenges for beneficial use. For example, addition of non-sludge organic wastes to the anaerobic digesters has the potential to increase biogas production while diverting organic wastes from disposal in landfills where they produce greenhouse gases. However, there are emerging concerns about the presence of per- and polyfluoroalkyl substances (PFAS) that can render land application of biosolids unviable. The biosolids would then have to be disposed of in landfills which is associated with a large hauling costs with no beneficial use. To address these concerns, existing WWTP can be retrofitted with new technology that can minimize solids production and produce alternate beneficial products.

Co-digestion is an anaerobic digestion option where food waste is added into the sludge feed to the digester in order to increase biogas production and generate revenue through a tipping fee. This requires additional infrastructure to accommodate increased loading such as, food waste storage, additional CHP engines, and biogas storage. Hydrothermal liquefaction (HTL) is an emerging technology where the solid waste produced from anaerobic digestion is processed in a high heat and pressure environment in order to produce biofuel. This generates revenue through selling biofuel and eliminates hauling cost of solid waste. More importantly, HTL destroys most of the PFAS compounds in the solids produced. Equipment to facilitate the biocrude reaction and biocrude storage would have to be purchased and installed.

Technical Approach/Methodology

Co-digestion or HTL can be implemented to generate more revenue or lower costs. Co-digestion creates more biogas in turn lowers the electrical cost of operating the plant. HTL removes the hauling cost and creates biofuel which can be sold, generating revenue and lowering operation costs.

In order to analyze the viability of incorporating co-digestion or HTL to an existing anaerobic digestion facility the following variables were considered. First, the capital costs of each option were complied. This includes the cost of equipment and their installation, as well as intangible costs associated with construction. Additionally, a life cycle analysis on the co-digestion and HTL was completed in order to determine if and when the technology would become profitable.

For the HTL alternative, Pacific Northwest National Laboratory (PNNL) reports were used to support the process design basis, biocrude production assumptions, and economic modeling approach. These reports provided guidance for using dewatered sludge as the feedstock, estimating biocrude production, and evaluating HTL through capital cost, O&M cost, revenue, fuel credits, and avoided sludge disposal. Based on these considerations, a life cycle analysis can be done and a recommendation on which technology to implement can be done

Outcomes

The life cycle cost analysis shows that co-digestion produces the strongest overall economic performance, with the highest NPV, highest IRR, and shortest breakeven period among the evaluated alternatives. Sensitivity analysis further confirms that energy recovery and electricity value are the primary economic drivers of the co-digestion process. HTL shows potential advantages related to sludge disposal reduction, renewable biocrude production, and future PFAS management concerns. In summary, the results of this project support continued development and implementation of advanced sludge treatment technologies to improve energy recovery, reduce disposal challenges, and increase the sustainability of wastewater treatment operations.