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

TL;DR

This paper evaluates two advanced household sanitation systems and finds they are costly and emit significant greenhouse gases, but could be viable in specific settings with optimizations.

Contribution

The study provides a novel techno-economic and environmental assessment of two household-level sanitation technologies under uncertainty.

Findings

PMD and SCWO household toilets have high annualized costs compared to centralized systems.

SCWO has higher costs and emissions than PMD due to poor solid-liquid separation.

Optimizing design and operation can reduce costs and emissions by up to 70%.

Abstract

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.

Figures