Literature search shows that there are not updated information regarding the process of anaerobic digestion of vegetable waste from fennel (F) and onions (O). The only international scientific paper related to this topics concerns a work done on the Anaerobic Digestion (AD) of onion scraps from which it stems out that the methane yield is rather low and that it can be improved if the AD is carried out in co-digestion with sludge from wastewater treatment plants. The lack of data on AD of fennel - onion mixture has necessitated a thorough study that went beyond the mere technical aspects of the standard parameterization of an AD process. The first part of the experiment involved the analytical determination of the chemical and physico-chemical parameters of the materials collected from an industry involved in the processing of this kind of vegetable. The second part focused on the evaluation of the performance parameters connected to biogas and methane yield for the aforementioned mixture in the proportion of 80 % F and 20% O. Since it appeared this an unprecedented case of AD tests were carried out on both substrates to assess the existence of catalytic effects and / or inhibitors that could can impair or enhance the stability and yield of the process. Tests were also performed in the presence of other biomass substrates in order to verify the possibility of getting higher methane yields. This requirement is necessary when materials and / or mixtures whose the behavior in an anaerobic environment is unknown are used. It should be emphasized that mono- digestion, with the exception of few cases, has always to be considered extremely critical and unstable, so that it has been often suggested to find , also occasionally, other waste feedstock such as those believed to be the most suitable such as manure, slaughter wastes, etc..., or other plant substrates such as straw, shredded sorghum, triticale and corn. These types of substrates, as well as to increase the percentage yield of methane in the biogas, are known for providing supplementary nutrients and micronutrients and overcome the toxic effects or inhibitors arising from particular substrates present in the mixture. The study herewith described compared to standard tests of biogas yield, covered aspects still unknown for these two specific substrates.
