Composting is generally accepted as the sustainable recycling of biowaste into a useful and beneficial product for soil. However, composting processes can produce gases that are considered air pollutants. In this dataset, we summarized the spatial and temporal distribution of process gases (including rarely reported carbon monoxide, CO) generated inside full-scale composting piles. In total 1375 cross-sections were made and presented in 230 figures. The research aimed to investigate the phenomenon of gas evolution during the composting of biowaste depending on the pile turning regime (no turning, turning once a week, and turning twice a week) and pile location (outdoors, and indoors in a composting hall). The analyzed biowaste (a mixture of tree leaves and branches, grass clippings, and sewage sludge) were composted in six piles with passive aeration including additional turning at a municipal composting plant. The chemical composition and temperature of process gases within each pile were analyzed weekly for ~49–56 days. The variations in the degree of pile aeration (O2 content), temperature, and the spatial distribution of CO, CO2 and NO concentration during the subsequent measurement cycles were summarized and visualized. The lowest O2 concentrations were associated with the central (core) part of the pile. Similarly, an increase in CO content in the pile core sections was found, which may indicate that CO is oxidized in the upper layer of composting piles. Higher CO and CO2 concentrations and temperature were also observed in the summer season, especially on the south side of piles located outdoors. The most varied results were for the NO concentrations that occurred in all conditions. The dataset was used by the composting plant operator for more sustainable management. Specifically, the dataset allowed us to make recommendations to minimize the environmental impact of composting operations and to lower the risk of worker exposure to CO. The new procedure is as follows: turning of biowaste twice a week for the first two weeks, followed by turning once a week for the next two weeks. Turning is not necessary after four weeks of the process. The recommended surface-to-volume ratio of a compost pile should not exceed 2.5. Compost piles should be constructed with a surface-to-volume ratio of less than 2 in autumn and early spring when low ambient temperatures are common.

Carbon monoxide (CO), an air pollutant and a toxic gas to humans, can be generated during aerobic digestion of organic waste. CO is produced due to thermochemical processes, and also produced or consumed by cohorts of methanogenic, acetogenic, or sulfate-reducing bacteria. The exact mechanisms of biotic and abiotic formation of CO in aerobic digestion (particularly the effects of process temperature) are still not known. This study aimed to determine the temporal variation in CO concentrations during the aerobic digestion as a function of process temperature and activity of microorganisms. All experiments were conducted in controlled temperature reactors using homogeneous materials. The lab-scale tests with sterilized and non-sterilized mix of green waste, dairy cattle manure, sawdust (1:1:1 mass ratio) were carried out for 1 week at 10, 25, 30, 37, 40, 50, 60, 70°C to elucidate the biotic vs. abiotic effect. Gas concentrations of CO, O2, and CO2 inside the reactor were measured every 12 h. The CO concentrations observed for up to 30°C did not exceed 100 ppm v/v. For 50 and 60°C, significantly (p < 0.05) higher CO concentrations, reaching almost 600 ppm v/v, were observed. The regression analyses showed in both cases (sterile and non-sterile) a statistically significant effect (p < 0.05) of temperature on CO concentration, confirming that the increase in temperature causes an increase in CO concentration. The remaining factors (time, O2, and CO2 content) were not statistically significant (p > 0.05). A new polynomial model describing the effect of temperature, O2, and CO2 concentration on CO production during aerobic digestion of organic waste was formulated. It has been found that the proposed model for sterile variant had a better fit (R2 = 0.86) compared with non-sterile (R2 = 0.71). The model predicts CO emissions and could be considered for composting process optimization. The developed model could be further developed and useful for ambient air quality and occupational exposure to CO.

Composting processes reduce the weight and volume of biowaste and produce products that can be used in agriculture (e.g., as fertilizer). Despite the benefits of composting, there are also problems such as odors and the emission of pollutants into the atmosphere. This research aimed to investigate the phenomenon of process gas (CO, CO2, NO, O2) evolution within a large-scale municipal composter. The effects of turning frequency and pile location (outdoor vs. indoors) on process gas and temperature spatial and temporal evolution were studied in six piles (37‒81 tons of initial weight) over a six-month period. The biowaste consisted of green waste and municipal sewage sludge. The chemical composition and temperature of process gases within four cross sections with seven sampling locations were analyzed weekly for ~7–8 weeks (a total of 1375 cross sections). The aeration degree, temperature, CO, CO2, and NO concentration and their spatial and temporal distribution were analyzed. Final weight varied from 66% reduction to 7% weight gain. Only 8.2% of locations developed the desired chimney effect (utilizing natural buoyancy to facilitate passive aeration). Only 31.1% of locations reached thermophilic conditions (necessary to inactivate pathogens). Lower O2 levels corresponded with elevated CO2 concentrations. CO production increased in the initial composting phase. Winter piles were characterized by the lowest CO content. The most varied was the NO distribution in all conditions. The O2 concentration was lowest in the central part of the pile, and aeration conditions were good regardless of the technological regime used. Turning once a week was sufficient overall. Based on the results, the most favorable recommended procedure is turning twice a week for the first two weeks, followed by weekly turning for the next two weeks. After that, turning can be stopped unless additional removal of moisture is needed. In this case, weekly turning should continue until the process is completed. The size of the pile should follow the surface-to-volume ratio: <2.5 and <2 for cooler ambient conditions.

The paper presents, for the first time, the results of fuel characteristics of biochars from torrefaction (a.k.a., roasting or low-temperature pyrolysis) of elephant dung (manure). Elephant dung could be processed and valorized by torrefaction to produce fuel with improved qualities for cooking. The work aimed to examine the possibility of using torrefaction to (1) valorize elephant waste and to (2) determine the impact of technological parameters (temperature and duration of the torrefaction process) on the waste conversion rate and fuel properties of resulting biochar (biocoal). In addition, the influence of temperature on the kinetics of the torrefaction and its energy consumption was examined. The lab-scale experiment was based on the production of biocoals at six temperatures (200–300 °C; 20 °C interval) and three process durations of the torrefaction (20, 40, 60 min). The generated biocoals were characterized in terms of moisture content, organic matter, ash, and higher heating values. In addition, thermogravimetric and differential scanning calorimetry analyses were also used for process kinetics assessment. The results show that torrefaction is a feasible method for elephant dung valorization and it could be used as fuel. The process temperature ranging from 200 to 260 °C did not affect the key fuel properties (high heating value, HHV, HHVdaf, regardless of the process duration), i.e., important practical information for proposed low-tech applications. However, the higher heating values of the biocoal decreased above 260 °C. Further research is needed regarding the torrefaction of elephant dung focused on scaling up, techno-economic analyses, and the possibility of improving access to reliable energy sources in rural areas.

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