## 1 Introduction  
To protect the climate and natural environment, world policies are increasingly pushing toward the transition from fossil fuels-based technologies to energy carriers obtained using biomasses and renewable sources. At the same time, the need to avoid high environmental impacts deriving from both resource depletion and pollution of waste landfilling is increasingly felt. Technologies for the conversion of waste to energy and fuels can be applied to valorize and transform municipal solid waste [1] and, in particular, food waste (FW) [2] into steam, electricity, liquid, and gaseous fuels through thermochemical and biochemical processes. Although there are currently several technologies available for FW management [2], recycling FW through anaerobic digestion (AD) produces both an energy vector (biogas) [3] and an organic fertilizer (digestate) generating lower greenhouse gas emission than landfill and incineration [4]. Unlike thermal treatment methods, such as FW incmatter is lost as CO\({}_{2}\), FW AD is considered a better recycling process by both European [5] and the United States [6] standards. Biogas produced by AD is composed of methane (CH\({}_{4}\), 45-70%), carbon dioxide (CO\({}_{2}\), 30-55%), and trace contaminants such as hydrogen sulfide (H\({}_{2}\)S), volatile organic compounds (VOCs), ammonia (NH\({}_{3}\)), nitrogen (N\({}_{2}\)), oxygen (O\({}_{2}\)), and water vapor. Biogas bulk composition and trace compound content are mainly linked to the type and origin of the organic matter used in the AD process [7; 8]. Biogas can have multiple applications [9]: it can be directly applied in boilers and central heat and power (CHP) engines to generate heat, mechanical work, and electric energy or used in fuel cells. By moisture and trace contaminant removal (biogas cleaning pre-treatment) followed by CO\({}_{2}\) separation (biogas upgrading) [10; 11], biogas can also be converted to biomethane (CH\({}_{4}\), 97-99%), which can be utilized as an alternative to natural gas, fed into distribution grid for domestic and industrial usage, or employed as a gaseous or liquid fuel for motor power vehicles. It has been reported that biomethane grid injection is the most profitable solution if connection costs are low [12] and upgrading biogas to biomethane for the transport sector has less impact on the environment than directly burning it in a CHP unit [13]. The need to remove trace contaminants from biogas [14] is linked to several factors: (i) high levels of compounds harmful to human health and the environment, such as non-burnt hydrocarbons, polycyclic aromatic hydrocarbons, VOCs, NOx, SOx, CO, and CO\({}_{2}\), are released in exhaust gases [15; 16; 17] during biogas combustion; (ii) corrosion of pipeline infrastructure and damage of internal combustion engine components occur due to the activity of NH\({}_{3}\), siloxanes, H\({}_{2}\)S and its Sox, and H\({}_{2}\)SO\({}_{4}\) oxidation products [18]; (iii) biogas impurities dilute CH\({}_{4}\) content lowering its lower calorific value [16]; (iv) biogas trace compounds can affect proper functioning of the fuel cells degrading significantly any catalytic process [7; 18]; (v) trace contaminants have to comply with the limits set by the European standards for biomethane use in transport [19] and for injection in the natural gas network [20]; (vi) some biogas upgrading technologies, such as pressure swing adsorption and membrane separation, require the removal of compounds such as H\({}_{2}\)S and VOCs, which could poison the material constituting the separation system [16]. The most important and studied biogas trace contaminant is H\({}_{2}\)S. H\({}_{2}\)S formation in FW AD is related to the biodegradation of sulfur-containing compounds, such as inorganic SO\({}_{4}\)\({}^{2-}\) and proteins containing methionine and cysteine, through known biological mechanisms [21]. Structures and functions of the microbial community responsible for H\({}_{2}\)S formation from FW AD are also known [22].