A recent analysis of pit latrines concluded that globally, pit latrines accounted for 1% of anthropogenic CH4 emissions. The relatively large contribution of pit latrines to global CH4 sources can be attributed to the global extent of pit latrine use e approximately one-quarter of the global population e as well as the wet and unventilated conditions that drive anaerobic CH4 production. EcoSan relies on aerobic conditions to treat waste and has the potential to considerably reduce the GHG footprint of waste management. In aerobic thermophilic composting, CH4 emissions are typically low because of the presence of oxygen. However, anaerobic microsites created by uneven distribution of water in pores and hot spots of labile carbon can create conditions leading to CH4 emissions. The use of bulking agents and pile turning can be used to reduce the occurrence of anaerobic CH4-producing conditions and, when effective, carbon emissions from composting are in the form of CO2, which is considered to be climate-neutral because of its biogenic origin. Composting can, however, produce biogeochemical conditions prime for N2O emissions through nitrification or denitrification, including large sources of reactive nitrogen, dynamic and spatially varying levels of oxygen, and labile carbon sources. Quantifying the magnitude and balance of CH4 and N2O emissions in a given sanitation system is critical as the two gases have 100-year global warming potential values of 34 and 298, respectively. EcoSan systems utilizing aerobic, thermophilic composting are promising because they may mitigate GHG emissions from the waste and agricultural sectors, however these emissions reductions have not yet been quantified. Further, measurements of GHG emissions from management of solid organic wastes are especially limited from tropical climates , vertical farm system where implementation of EcoSan solutions are likely to be greatest.
To our knowledge, no direct measurements of GHG emissions exist from EcoSan systems that deploy container-based toilets and thermophilic composting of human excrement. Our primary objective was to characterize and quantify the GHG emissions resulting from the aerobic composting of human waste in EcoSan settings. We considered two operations that employed similar compost practices, but differed in the physical infrastructure that could alter biogeochemical conditions mediating GHG dynamics. We also compared the GHG footprint of EcoSan with alternative waste management pathways present in the region, including waste stabilization ponds and unmanaged disposal on grass fields. Finally, we undertook an investigation of the effects of compost management options that help reduce EcoSan GHG emissions.Greenhouse gas fluxes were measured from three sanitation pathways in Haiti: two waste stabilization ponds, two EcoSan operations, and a grass field where the illegal disposal of sewage was observed. The waste stabilization ponds were located in Croix ed Bouquets near Port-au-Prince, Haiti and operated by the Haitian government agency, Direction Nationale de l’Eau Potable et de l’Assannissement. Ponds consisted of uncovered concrete basins with effluent pipes connected to secondary overflow ponds. Two ponds were included in the sampling: a pond that received mostly septic tank waste , and another that received mostly pit latrine waste. Solid sludge was scraped out occasionally and stockpiled on-site. Solid and liquid waste from septic tanks and pit latrines were transported to the site and emptied into the waste stabilization ponds. The waste stabilization ponds represent the primary pathway of centralized waste treatment as advanced municipal wastewater treatment technologies are not present in the country. The EcoSan compost facilities were located in Port-au-Prince, Haiti and Cap Haiti€ en, Haiti. The sites received approximately 65 MT yr 1 and 440 MT yr 1 of human waste, respectively. Eighteen and 54 L container-based toilets were collected from households and communities, respectively, in each of the regions. Container-based toilets separated urine and feces into different compartments.Urine was disposed of on-site, and only solid material was transported to the compost facilities. Both facilities used a similar aerobic composting process consisting of a static thermophilic stage, followed by pile turning and maturation in windrows.
At the Cap-Haiti€ en operation, hereafter referred to as “Compost CH,” the ground was lined with cement to prevent leaching and an aluminum roof covered the area. Roofs and cement-lined floors were absent at the Port-au-Prince EcoSan operation, hereafter referred to as “Compost PAP”. During the initial thermophilic stage, approximately 2700 kg of fresh material from container-based toilets was added to a ~27 m3 bin consisting of air-permeable walls and an open top. Coarsely ground sugarcane bagasse was used as a bulking agent to create interstitial air spaces within the pile and as a 15 cm deep covering material. The material remained in the bin for about two months or until confirmation of Eschericia coli elimination, during which time it underwent static, thermophilic composting, reaching a peak minimum temperature of 50 C for at least 7 days. Following the thermophilic stage, the material was removed from the bin onto a flat surface, formed into windrows and aerated by weekly manual turning for two to three months. Finally, matured compost was then sieved and bagged for use as a composted soil amendment. The third waste disposal pathway was an unmanaged grass field near Quartier Moren, Haiti. Unregulated emptying of septic and pit latrine waste is not uncommon and has been observed here for at least five years and within a few months of sampling. While the frequency and magnitude of waste disposal in the grass field was unknown, there was apparent build up of organic material that resembled a moist, viscous sludge several inches to feet deep.Greenhouse gases were measured once at each site within seven days in July 2014. Fluxes of CO2, CH4, and N2O were measured using vented static flux chambers constructed of 25.4 cm diameter Schedule 80 PVC collars and chamber tops. 1.5 m tall collars were placed carefully in the semi-solid sludge in the waste stabilization ponds,vertical indoor farming approximately 1 m from the edge.Four areas were sampled in Pond 1, and six areas were sampled in Pond 2. Gas samples collected within ponds were treated as replicates and used to determine mean fluxes from each pond. Six 0.3 m tall collars were placed randomly in the Grass Field and allowed to settle for 1 h before the chamber tops were connected. Gas measurements were also made within and across compost piles of different stages. While sampling was conducted at each site at one time point, compost piles exist along a gradient of ages from freshly collected waste to mature compost. This design allows us to effectively substituting space for time to determine mean flux from each EcoSan system over the entire composting process.
At Compost PAP, eight piles ranging from <1, 1, 2, 4, 6, 8, 10, and 13 months were sampled, with six static flux chambers randomly placed in each pile. At Compost CH, six piles ranging from <1, 3, 4, 5, and 12 months old were sampled, with three static flux chambers randomly placed in each pile. Linear interpolation between age classes was used to calculate the net GHG emissions from EcoSan compost operations. Mean GHG emissions were determined by weighting fluxes by age of the pile. Gas samples were collected from the static flux chamber head space at 0, 5, 10, 20, and 30- minute intervals, immediately transferred to evacuated 20-mL Wheat on glass vials outfitted with 1-inch butyl septa. Samples were transported to the Cary Institute of Ecosystem Science in Millbrook, New York for analysis on a gas chromatograph. Methane concentrations were analyzed using a flame ionization detector. An electron capture detector was used to analyze N2O concentration, and CO2 concentrations were analyzed using a thermal capture detector. Samples with concentrations exceeding the maximum detection limit on the gas chromatograph were diluted at 1:10 or 1:100 with N2. Fluxes were calculated using an iterative exponential curve- fitting approach.To explore how compost management impacts GHG emissions, we established an additional experiment at the Compost CH site in August 2016. First, to test the effects of pile lining permeability, we measured GHG emissions using the procedure described above from a bin with a soil floor and from a bin with a cement floor and a blocked PVC overflow pipe , which were filled at approximately the same time. Second, to test the effects of pile turning, we measured GHG emissions one and three days after unlined bin material was turned for the first time, and compared emissions to those before turning. Twelve fluxes spaced evenly on a four x three grid were measured for each bin in the first stage to explore spatial variability of GHG emissions within the pile.
Six fluxes were measured per pile in the second turned stage, spaced on a three x two grid. The grid design also allowed us to explore the effects of pile structure and geometry on GHG emissions in greater detail.One-way analyses of variance followed by TukeyKramer means comparison tests were used to identify statistically significant effects of waste treatment pathway on mean CO2, CH4, and N2O fluxes. Treatment fluxes sampled across waste pathways were calculated as the mean and standard error of replicate samples collected within each system. Standard errors were propagated when considering mean GHG fluxes throughout the entire compost process at the EcoSan sites. For the compost management experiments, fluxes are represented as the mean and standard error of sample replicates. Fluxes of each gas species were considered separately and in combination using units of CO2-equivalents, using the 100-year global warming potential of 298 for N2O and 34 for CH4. Gas flux data were log-transformed to meet assumptions for ANOVA. Data are reported either as mean values±one standard error. Statistical significance was determined as P < 0.05 unless otherwise noted.We found pile lining permeability and pile turning significantly altered GHG emissions during EcoSan composting. A permeable soil lining lowered GHG emissions with pile CH4 and N2O emissions four and three-fold lower, respectively, in the unlined pile compared to the cement lined pile. Emissions also followed spatial patterns within the piles with CH4 emissions generally increasing from corners and edges to the center of the pile, with the opposite trend observed for N2O. Finally, substantial changes were observed in all GHGs after pile turning: CO2 and N2O emissions approximately doubled while CH4 emissions dropped almost to zero.We found that composting human fecal matter resulted in significantly lower overall GHG emissions than those observed from waste stabilization ponds and the illegal disposal of human waste on unmanaged grass fields. In EcoSan systems, CO2 was a major constituent of the overall GHG footprint, making up 42e62% of net CO2-eq emissions. In contrast, CO2 made up only 12% from grass fields used for dumping of untreated waste and 4e9% of the net CO2-eq emissions from waste stabilization ponds, suggesting that the ponds are not completely anaerobic. The difference in CO2 loss is likely driven by differences in oxygen availability among the waste treatment pathways. Composting systems enhance aerobic conditions that stimulate the microbial oxidation of organic carbon to CO2 , whereas biological treatment systems with oxygen-depleted conditions stimulate methanogenesis and production of CH4. Carbon dioxide produced from the decomposition of human waste is of biogenic origin, and therefore not considered a net source of GHG emissions from a climate change perspective. Conditions that evenly aerated compost piles, such as forced aerated and pile turning, tend to maximize CO2 production relative to CH4 and N2O. However, compost piles tend to have high levels of bio-geochemical heterogeneity due to within pile spatial variations in organic compounds, physical size and structure of material, oxygen diffusivity, density, porosity, and moisture content. Therefore, compost pile management can play a strong role in altering biogeochemical conditions that effect the composition of GHG fluxes. In the two EcoSan systems that we studied, we saw significantly higher CH4 fluxes from Compost CH, where static piles sat atop a concrete floor during the thermophilic phase. The presence of the floor likely built up moisture in the pile and increased the probability of anaerobic microsites. Methane emissions from Compost CH rapidly declined after the first two to three months, when piles were moved into the actively turned stage. In contrast, compost piles at Compost PAP were constructed atop compacted soil and without a roof covering, allowing for infiltration of leachate and higher rates of evaporation.