Chemical and odor characterization of gas emissions released during composting of solid wastes and digestates.

Hazardous and odorous gas emissions from composting and methanization plants are an issue of public concern. Odor and chemical monitoring are thus critical steps in providing suitable strategies for air pollution control at waste treatment units. In this study, 141 gas samples were extensively analyzed to characterize the odor and chemical emissions released upon the aerobic treatment of 10 raw substrates and five digestates. For this purpose, agricultural wastes, biowastes, green wastes, sewage sludge, and municipal solid waste (MSW) were composted in 300 L pilots under forced aeration. Gas exhausts were evaluated through dynamic olfactometry and analytical methods (i.e., GC/MS) to determine their odor concentration (OC in OUE m-3) and chemical composition. A total of 60 chemical compounds belonging to 9 chemical families were identified and quantified. Terpenes, oxygenated compounds, and ammonia exhibited the largest cumulative mass emission. Odor emission rates (OUE h-1) were computed based on OC measurements and related to the initial amount of organic matter composted and the process time to provide odor emission factors (OEFs in OUE g-1OM0). The composting process of solid wastes accounted for OEFs ranging from 65 to 3089 OUE g-1OM0, whereas digestates composting showed a lower odor emission potential with OEF fluctuating from 8.6 to 30.5 OUE g-1OM0. Moreover, chemical concentrations of single compounds were weighted with their corresponding odor detection thresholds (ODTs) to yield odor activities values (OAVs) and odor contribution (POi, %). Volatile sulfur compounds were the main odorants (POi = 54-99%) regardless of the operational composting conditions or substrate treated. Notably, methanethiol was the leading odorant for 73% of the composting experiments.

Characterizing the transformation and transfer of nitrogen during the aerobic treatment of organic wastes and digestates.

The transformation and transfer of nitrogen during the aerobic treatment of seven wastes were studied in ventilated air-tight 10-L reactors at 35 °C. Studied wastes included distinct types of organic wastes and their digestates. Ammonia emissions varied depending on the kind of waste and treatment conditions. These emissions accounted for 2-43% of the initial nitrogen. Total nitrogen losses, which resulted mainly from ammonia emissions and nitrification-denitrification, accounted for 1-76% of the initial nitrogen. Ammonification was the main process responsible for nitrogen losses. An equation which allows estimating the ammonification flow of each type of waste according to its biodegradable carbon and carbon/nitrogen ratio was proposed. As a consequence of the lower contribution of storage and leachate rates, stripping and nitrification rates of ammonia nitrogen were negatively correlated. This observation suggests the possibility of promotingnitrification in order to reduce ammonia emissions.