Financial viability and environmental sustainability of fecal sludge treatment with Omni Processors

Omni Processors (OPs) are community-scale systems for non-sewered fecal sludge treatment. These systems have demonstrated their capacity to treat excreta from tens of thousands of people using thermal treatment processes, but their relative sustainability is unclear. In this study, QSDsan (an open-source Python package) was used to characterize the financial viability and environmental implications of fecal sludge treatment via OP technology treating mixed and source-separated human excreta, and to elucidate the key drivers of system sustainability. Overall, the daily user cost for the treatment of mixed excreta (pit latrines) via the OP was estimated to be 0.05 [0.03 – 0.08] USD·cap-1·d-1, while the treatment of source-separated excreta (from urine-diverting dry toilets) was estimated to have a user cost of 0.09 [0.08 – 0.14] USD·cap-1·d-1. Operation and maintenance of the OP is a critical driver to total user cost; whereas the contribution from capital cost of the OP is much lower because it is distributed over a relatively large number of users (i.e., 12,000 people) for the system lifetime (i.e., 20 yr). The total emissions from the source-separated scenario were estimated to be 11 [8.3 – 23] kg CO2 eq·cap-1·yr-1, compared to 49 [28 – 77] kg CO2 eq·cap-1·yr-1 for mixed excreta. Both scenarios fall below the estimates of greenhouse gas (GHG) emissions for anaerobic treatment of fecal sludge collected from pit latrines. Source-separation also creates opportunities for resource recovery to offset costs through nutrient recovery and carbon sequestration with biochar production. For example, when carbon is valued at 150 USD·Mg-1 of CO2, the user cost of sanitation can be further reduced by 44% and 40% for the source-separated and mixed excreta scenarios, respectively. Overall, our results demonstrate that OP technology can provide low-cost, low-GHG fecal sludge treatment while reducing global sanitation gaps.