## 1 Introduction
H\({}_{2}\)S is a toxic gas corrosive to metal equipment, poisonous for fuel cells [23], and harmful to human health even at concentrations of 10-100 ppm [8]. H\({}_{2}\)S and its SOx combustion products are dangerous not only for humans but also for the environment as they are the main precursors of acid rain [23]. H\({}_{2}\)S abatement and removal can be achieved using various technological strategies [21; 24] that can be broadly divided into three groups: in situ desulfurization processes, ex situ biogas post-treatment, and feedstock pre-treatment. In situ H\({}_{2}\)S removal is performed through the precipitation of elemental sulfur localized within the digester using micro-aeration and iron dosing [25]. At the industrial level, micro-aeration showed higher desulfurization efficiencies (68-99%) than iron dosing (35-72%). Both techniques seem to be cheaper than most ex situ desulfurization methods [26] although the economic feasibility is essentially indicative due to a lack of data [25]. However, the main disadvantage of these techniques is finding the right operating conditions since an excessive dosage of iron and oxygen can lead to disturbances or inhibition of the AD process. Ex situ biogas post-treatment is the most used desulfurization technique and is performed using (i) absorption technologies according to H\({}_{2}\)S different solubility in water and organic solvents with respect to CH\({}_{4}\) (removal efficiency: 17-100%); (ii) adsorption-based system using H\({}_{2}\)S adsorbents at high pressure and temperature and with high specific area and porosity, such as iron oxide-based material, activated carbon, and zeolites (break-through capacity: from several mg to about 10 g of adsorbed H\({}_{2}\)S per each g of adsorbent material); (iii) biofililtration and biotrickling filtration performing biological H2S oxidation by microorganisms grown on organic media or immobilized on chemically inert materials (removal efficiency: 60-100% and 85-100% for biofiltration and biotrickling filtration, respectively); (iv) polymeric membranes filtration (output H2S > 50 ppm\({}_{v}\)), (v) system based on the set of several ex situ techniques, such as physical, chemical, and biological scrubbing (removal efficiency: 80-99%). The purpose of the feedstock pre-treatment is to remove, via liquid-solid separation processes, the sulfur contained in the organic substrate before it enters the digester in order to prevent H2S formation during the AD process. By using inorganic iron salts, hydrogen peroxide, calcium oxide, and hydroxide, it is possible to precipitate soluble sulfur compounds. However, since the critical step of this technique is the separation of precipitated sulfur derivatives from the organic material before it enters the digester, feedstock pre-treatment is restricted to liquid substrates such as wastewater. Aromatic and halogenated hydrocarbons, alkanes, terpenes, organosilicon derivatives, oxygenated and sulfur compounds represent most of the non-CH4 VOCs produced by industrial FW AD that can cause serious harm to both the environment and human health [27]. Due to their different chemical-physical characteristics, biogas VOCs can also cause various problems to the equipment and pipeline. Halogen derivatives cause severe metal corrosion [16]; terpenes affect the performance of adsorption-based and membrane-based biogas upgrading [7]; siloxanes are harmful to fuel cells [23] and, after combustion, cause SiO2 deposition on equipment lowering its life-time and performance [28]. The most used and cheaper industrial method to remove non-CH4 VOCs is based on their adsorption on activated carbon [28], while other physical-chemical technologies include chemical oxidation in the liquid phase [29], absorption into organic solvents [30] or strong acids, and deep chilling [16]. Depending on the adopted technique, the VOC removal efficiency can reach up to 100%. NH3 is a toxic gas [8] present in biogas produced by FW AD.