Preliminary Greenhouse Gas (GHG) Emissions Analysis of Four Gates Sanitation Systems: Emissions During Steady-State Operation

Abstract

This analysis estimates greenhouse gas (GHG) emissions associated with steady-state operation of four technologies in the Bill & Melinda Gates Foundation’s sanitation portfolio: the HTClean system (Helbling), the Empower Sanitation Platform (Duke Centre for WaSH-AID), the Electrochemical Reinvented Toilet (Eco-San), and the Janicki Omni-Processor (Sedron Technologies). Specifically, we estimated emissions from (i) the degradation of bodily waste during containment, treatment, and recovery; (ii) electricity and materials consumed during operation; and (iii) transportation of waste to its treatment and/or disposal site. We also estimated GHG offsets from recovery of fertilizer nutrients (nitrogen, phosphorus, potassium). This steadystate analysis does not include the emissions associated with the construction, maintenance, and end-of-life of these technologies. All estimates are expressed as equivalent kilograms of carbon dioxide per year, normalized to the estimated population served (i.e., kg CO2 eq·cap-1·year-1). With the exception of the HTClean system, direct emissions from the current portfolio technologies have the potential to compare favorably against pit latrines. Our results suggest that electricity demand tends to drive emissions trends across three of the four systems (excluding the Omni-Processor). HTClean is associated with the highest total GHG emissions per person (780-1,800 kg CO2 eq·cap-1·yr-1), with 90% of emissions coming from its large electricity demand. As the Omni-Processor is reported to require no outside electricity, it is associated with the lowest emissions (33-64 kg CO2 eq·cap-1·yr-1 for the full system including latrine containment and passive pretreatment). Latrine containment and pretreatment contribute most of the emissions related to the Omni-Processor system. The Empower (100-180 kg CO2 eq·cap-1·yr-1) and Eco-San (250- 470 kg CO2 eq·cap-1·yr-1) systems produce intermediate levels of emissions (compared with the HTClean and Omni-Processor systems), with the Eco-San total being larger due to its higher electricity demand. The Empower system offers two alternatives for final processing of dried solids (combustion or land application), but total emissions are similar for both options (as land-applied solids may continue to degrade). The Empower estimates are similar to preliminary values associated with degradation of bodily waste in pit latrines (as calculated by the University of Leeds CACTUS team in ongoing work; final values may differ slightly). In all full systems, recovered nutrients contribute offsets that are relatively small compared with total system emissions. Broadly, these preliminary results are associated with large uncertainties as the analysis required numerous assumptions. Results tend to be most sensitive to the emissions factor for electricity (i.e., kg CO2 eq·kWh-1) and multiple parameters related to direct emissions from bodily waste (e.g., carbon and nitrogen excretion, emissions during combustion). Electricity emissions will vary depending on the local source of electricity (e.g., coal, hydroelectric, solar), resulting in significant variability across implementation locations. This variability can be reduced if a power source is built into the system itself, and we show that integrating renewable energy sources (e.g., photovoltaics) may provide the greatest opportunity for reductions in total GHG emissions. Direct emissions vary based on storage and treatment conditions of bodily waste and will also depend upon implementation location, as local diets will affect excretion of carbon and nitrogen into sanitation systems. Moving forward, we hope to expand these analyses in collaboration with the design and modeling teams and develop a full life cycle assessment (LCA) of each system.