Abstract
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.
Highlights
CO is described as the “silent killer”—CO binds to the iron atoms in hemoglobin, with an affinity 200–250 times that of O2, and impairs O2-carrying capacity of the blood causing hypoxia and highly toxic to living organisms (Kaymak and Basar, 2010; Bürstel et al, 2016)
It was shown that CO production had both abiotic and biotic character and observed CO concentrations depended mainly on the temperature and changed within the time during the organic waste aerobic decomposition process
It was shown that under sterile conditions the influence of temperature on CO production was more significant than in samples not subjected to sterilization, in which the influence of process duration was visible, which indicates the role of microorganisms in net CO production
Summary
CO is described as the “silent killer”—CO binds to the iron atoms in hemoglobin, with an affinity 200–250 times that of O2, and impairs O2-carrying capacity of the blood causing hypoxia and highly toxic to living organisms (Kaymak and Basar, 2010; Bürstel et al, 2016). Neurological injuries can be fatal (Schallner and Otterbein, 2015). Emissions of CO have indirect effects on climate through enhanced levels of tropospheric O3 and CH4 as a result of its reaction with HO (Hellebrand et al, 2006; Talaiekhozani et al, 2018). It is well-known that CO emissions are associated with anthropogenic sources such as transportation. It is not well-known that biowaste and its treatment (e.g., via composting) are a source of CO
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