The potential of thermomechanical and thermochemical processes to enable sustainable household sanitation

Biological processes underpin centralized wastewater treatment but are difficult to deploy at small scale. Thermomechanical and thermochemical approaches could enable household-level sanitation, yet their economic and environmental potential remains unclear. We assessed two prototype household reinvented toilets (HRTs), with either mechanical compression (MC) and supercritical water oxidation (SCWO) treatment processes, using integrated process simulation, techno-economic analysis, and life cycle assessment under uncertainty. Household-level sanitation costs are 1.41 to 1.87 (5th to 95th percentiles) for MC and 1.85 to 2.45 USD·cap-1·day-1 for SCWO, placing both at the high end of global centralized treatment prices. The life cycle GHG emissions span 321 to 452 and 362 to 520 kg CO2-eq·cap-1·year-1 for MC and SCWO, respectively, with the grid electricity contributing 87 to 90% in both HRTs. Poor solid–liquid separation penalizes SCWO more than MC, elevating per-capita costs and GHG emissions. In the short term, optimizing a few levers—number of users, flush water volume, and the detailed design of the SCWO unit—can significantly reduce cost and emissions. In the long term, operating at maximum efficiency reduces both cost and emissions by approximately 70%. Deployment in locations with low wages, low-carbon electricity, low price levels, and large household sizes offers the greatest potential, positioning HRTs as viable advanced decentralized sanitation options in specialized settings.