The Potential of Thermomechanical and Thermochemical Processes to Enable Sustainable Household Sanitation

Biological processes underpin centralized wastewater treatment but are difficult to deploy at a 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 pasteurization mechanical dewatering (PMD) and supercritical water oxidation (SCWO) treatment processes, using integrated process simulation, techno-economic analysis, and life cycle assessment under uncertainty. The total annualized expenditures (including capital and operating) are 1.41-1.87 (5th to 95th percentiles) and 1.85-2.45 USD·cap^-1·day^-1 for PMD and SCWO, respectively, placing both at the high end of global centralized treatment prices. The life cycle greenhouse gas (GHG) emissions span 321-452 and 362-520 kg CO₂-eq·cap^-1·year^-1 for PMD and SCWO, respectively, with the grid electricity contributing 87-90% in both HRTs. Poor solid-liquid separation disproportionately increases costs and GHG emissions for SCWO relative to PMD. 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 wage, 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.