Assessing hydrothermal carbonization as sustainable home sewage management for rural counties: A case study from...

Improper management of home sewage treatment systems (HSTS) presents major challenges in the developing world, and even in many parts of developed countries, which contribute to health, environmental, economic, and social problems. Hydrothermal treatment, a thermochemical conversion process that is particularly useful for wet wastes, can produce fertilizers from septic tank wastes while eliminating human pathogens. However, hydrothermal treatment requires high temperature and high pressure, which might need additional economical justification when targeted for rural communities. The aim of this study was to investigate the economic feasibility of a hydrothermal treatment facility that can treat septic waste generated in Athens County, Ohio, a rural Appalachian county in the U.S., where failing HSTS have been observed quite frequently. Two different cases were considered for economic analysis, where Case I assumed a decentralized facility and Case II assumed a centralized facility. Results showed that both cases are economically feasible, where Case I and II reached breakeven in years 4 and 6, respectively. Additionally, despite the greater capital investment requirement for Case I, there was also a greater return on investment (ROI) of 2.85 compared to 1.52 for Case II. A sensitivity analysis was examined to determine the effect of solid content in either the septic tank or the reactor's feed, selling price of the fertilizer, and septage pumping costs on the project feasibility. The sensitivity analysis showed that pumping cost is the most significant factor affecting the project feasibility, while the ROI varied about ±80% and ± 200% for Case I and II, respectively, due to only about ±18% change in pumping cost.