Volatile organic compounds (VOCs) from green waste anaerobic digestion: degradation proceeding and sources assessment

Municipal waste (MW) degradation produces a wide range of volatile organic compounds (VOCs) having a potentially detrimental impact on environment and human health. Chemical composition of biogas in MW landfills is the result of contributions from many different waste categories. In this study, a laboratory experiment on anaerobic digestion (AD) of green waste (GW) was performed to investigate the composition of VOCs released in comparison to that of the overall MW biogas. Up to 29 different VOCs were recognized in gas samples from degrading GW during AD time, including terpenes, followed by alkanes, alkenes, aromatics, and halogenated compounds. The latter, as well as part of aromatics were considered xenobiotic compounds. O- and S-substituted compounds represented the minor fraction of VOCs. The presence of toxic VOC pollutants can make GW-biogas potentially harmful if released (without collection, control, and energy recovery) to the environment. Overall, speciation and total concentration of VOCs fluctuated over AD time. The variable compositional features and biogas production rate were likely caused by changes in microbial populations and metabolism during the AD evolution. As expected, VOCs speciation in GW biogas was lower than those identified in typical MW biogas, since the latter is produced by a large variety of organic and synthetic material following different subsequent oxidative degradation processes. These preliminary results suggest that further laboratory experiments carried out using different MW fraction types than GW, and under different chemical-physical and process conditions, could provide useful technical-scientific information to understand and predict the composition and the quantity of biogas produced and eventually released to the atmosphere from individual waste fractions at MW landfill environments.