Financial Viability and Environmental Sustainability of Fecal Sludge Treatment with Pyrolysis 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 (e.g., pyrolysis), 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 pyrolysis-based OP technology treating mixed and source-separated human excreta and to elucidate the key drivers of system sustainability. Overall, the daily per capita 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 per capita cost of 0.09 [0.08–0.14] USD·cap–1·d–1. Operation and maintenance of the OP is a critical driver of total per capita 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 per capita 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 pyrolysis-based OP technology can provide low-cost, low-GHG fecal sludge treatment while reducing global sanitation gaps.

Section S1. Description of the Biogenic Refinery (defining the system) Section S2. General approach to quantitative scenario modeling Section S3. Addition of agricultural residues to treatment of mixed excreta Section S4. Details on emptying and conveyance from pit latrines Supporting References  Table S1. Emissions estimates, based on relevant literature [1][2][3][4][5][6][7][8][9][10][11][12][13][14] and measured emissions for the Omni-Processor system. b Pyrolysis is assumed to produce emissions that are similar to combustion, with any methane from the reactor's pyrolysis zone being oxidized during catalysis. Table S2. Inputs for units used in the analysis. This information is from the code is that is openly available on Github. 15 (Table S2 is included as an Excel spreadsheet: Rowles_et_al_unit_inputs_Table_S2.xlsx) Figure S1. Two baseline scenarios for the Biogenic Refinery evaluated, including pit latrines and containerbased sanitation. Various subsets of these scenarios were evaluated across the simulation space of decision variables, contextual parameters, and technological parameters. The PCD is the middle section of the Biogenic Refinery and its primary responsibilities include pollution control and production of thermal energy for drying of the feedstock. Due to the inefficiencies of the pyrolysis process, there are typically pollutants in the exhaust. In order to treat these pollutants, the Biogenic Refinery has a catalyst, similar to a catalytic converter in a car, to ensure destruction of the pollutants before they can be released into the surrounding environment.

Collected fecal sludge (pit latrines)
The process of destroying the pollutants requires the catalyst to maintain temperatures above 315 ºC, and additional energy is released during this process. The temperature of the catalyst is closely monitored because the catalyst wash coat will start to degrade above 800 ºC.

The final section of the Biogenic Refinery is the Heat Exchanger (HX). Biomass Controls offers three types of heat exchanger methods, a Forced Air Heat Exchanger (FAHX), a Hydronic
Heat Exchanger (HHX), and a Combined Heat and Power (CHP) system. The purpose of the heat exchanger is to utilize the thermal energy that is created in the pyrolysis process while ensuring that the stack maintains temperatures above 110 deg C to prevent condensation from lining the stack wall. Creosote is a carbonaceous material that is formed during pyrolysis.
In the FAHX system, there is an exchange between the exhaust gas and ambient air to use the thermal energy to heat a nearby space. These types of systems are typically used in cold climates as it does not require any water that would otherwise freeze, and the heat can be used to heat a cabin for the operator.
The HHX system is used for applications that require drying of the feedstock before the refinery is capable of processing the material. The heat is exchanged between the exhaust gas and water, which is then pumped into radiators connected to a dryer. The refinery monitors the S10 temperature of the water to ensure that the feedstock is being sufficiently dried before entering the refinery.
The CHP system is used to generate additional electricity that the refinery and/or facility can use to decrease the unit's electrical demand on the electrical grid. This type of system is required for ISO 31800 certification as the treatment unit needs to be energy independent when processing fecal sludge. S11 Section S2. General approach to quantitative scenario modeling Section S2.1. Economic analysis. For the Biogenic Refinery, we calculated the total system material cost using the bill of materials. The design team provided a bill of materials that included several of the main components, their weight, and material composition. A more detailed bill of materials was developed using the system's user manual and related patents. Each part was included in its respective unit process for the estimates of capital costs. Specifically, initial capital costs are distributed over the lifetime of the system (20 years), with a discount rate (2-8%) adjusting for the diminishing value of money over time. To provide a conservative estimate of per capita costs associated with serial production of each system at scale, we used a generalized learning curve function 16 to estimate the capital cost of the 10,000 th unit. Operation and maintenance (O&M) costs included ongoing costs for materials and labor. Assumptions for O&M were developed from a detailed maintenance activity schedule to estimate the need for replacement parts, along with costs of labor wages associated with the different levels of skills required for maintenance (e.g., service team, electrician, Biomass Controls, etc.). Finally, costs from electricity requirements were estimated based on the energy needs of specific components and parts, along with typical electricity costs per kilowatt-hour. For the energy requirements of specific components and parts, data from a published study on the system. 17 Section S2.1. Environmental analysis. Capital impacts were calculated using the bill of materials and vendor websites to identify materials and processes associated with each. We then compiled an inventory of emissions associated with each material and process or used similar items as surrogates when necessary. We used global emissions values for each item. Emissions were then converted to unit global warming potential (e.g., kg of CO2 equivalent per kilogram of material). Impacts associated with electricity were calculated from the OP's energy requirements and the unit environmental impacts associated with grid electricity.
For environmental impacts originating directly from the excreta, we estimated direct GHG emissions, which include methane and N2O released from the degradation of bodily waste. By mass, methane from biogenic sources is estimated to contribute 28 times the climate change impact of non-biogenic carbon dioxide, while N2O has an impact 265 times that of carbon dioxide. 18 Total CH4 and N2O emissions were multiplied by these factors to represent all direct emissions as an equivalent mass of carbon dioxide. Methane and N2O released from bodily waste in sanitation systems depend on the environmental conditions present and the treatment processes being employed, and they are directly related to the quantities of carbon (assumed to be proportional to COD 19 ) and nitrogen excreted in bodily waste. We used general ranges of COD S12 and nitrogen excretion found in the literature (Table S1). 20 In our process models, the specific configurations determine what fraction of excreted carbon and nitrogen enter certain treatment processes. The analysis focused on emissions that may occur during active treatment processes (e.g., thermal drying, pyrolysis) with expected emissions based on data from relevant literature (Table S1). While carbon dioxide emissions contribute no global warming impacts (as they are biogenic), they are incorporated to track carbon losses throughout each system. Thermal drying processes are expected to release minimal quantities of methane and ammonia (a fraction of which may transform to N2O in the atmosphere). 1,2 Pyrolysis of bodily waste is assumed to release N2O of up to approximately 3% of total nitrogen. 2 Other emissions that were documented in a field study for OP were also included in our analysis. These included SO2, CO, Hg, Cd, As, and dioxin and furans, all of which were estimated to be emitted at the exhaust flow rate of the system.
Using the maintenance activity schedule and consumables from system processes, the impacts due to O&M of the OP were estimated. The CO2 emissions associated with transportation were estimated based on transport distances and the volume transported. 21 S13 Section S3. Addition of agricultural residues to treatment of mixed excreta.
The addition of an agricultural residue is considered for the treatment of mixed excreta with 10,000 users. Details on this analysis are shown in the _models.py file as (Scenario C or sysC). The mass of residue added is equivalent to achieving the same mass loading rate to the Carbonizer Base as the scenario with 12,000 users (i.e., the mass loading rate into pyrolysis is the same for scenario with 12,000 users as the scenario with 10,000 users plus agricultural residue). For this system, rice husks were supplemented as the agricultural residue with a typical caloric value of 14.693 MJ·kg -1 , moisture content of 8.47 %, C content of 38.5%, and N content of 0.45% from the literature. 22

Section S4 Details on emptying and conveyance from pit latrines
Baseline assumptions related to emptying and conveyance were adopted from Trimmer et al.
2020. 23 Details on these assumptions and probability density distributions are shown in the _trucking section of Table S2 and the Python files (_trucking.py, _systems.py, and _models.py).
The conveyance of the sludge first requires pumping the sludge from pit latrines to a tanker truck.
This truck then transports the collected sludge to a central facility where it is subsequently treated by the pyrolysis Omni Processor. Our analysis included interdependences between emptying period, trucking interval, waste density and volume, and emptying fee. First, trucking interval was defined as the emptying period (i.e., time interval between trips). Next, the truck load (mass per load) was based on the sludge mass from the toilets, trucking interval, and number of toilets. The transportation fee (transportation fee per trip) was then calculated considering truck load and sludge density (to convert to volume of sludge) using a power law regression (as shown in the code on Github 15 ).