## 3 Developed Biogas Upgrading Technologies  
The developed technologies for biogas upgrading include adsorption, absorption (chemical and physical), MS, and CS (Figure 4) [61]. Adsorption technologies employ an adsorbent that selectively captures CO\({}_{2}\) from biogas, allowing methane to exit the system. According to how the adsorbent is regenerated, three different technologies exist: Pressure Swing Adsorption (PSA) (with Vacuum Swing Adsorption (VSA) being a special type), Temperature Swing Adsorption (TSA), and Electrical Swing Adsorption (ESA) [62]. Absorption technologies are divided into physical and chemical types [63]. Physical absorption is based on the different solubility values for the gases in a liquid solution [51]. There are two types: Water Scrubbing (WS) and Organic Physical Scrubbing (OPS). WS is the most-used technology for biogas upgrading [64]. In WS, the water absorbs the CO\({}_{2}\) since its solubility in water at 25 \({}^{\circ}\)C is 26 times higher than methane's [65]. WS is a simple process in comparison to other technologies. Its major drawback is a high water volume requirement [20]. In OPS, the scrubbing liquid is an organic solvent, such as methanol, N-methyl pyrrolidone [61], or dimethyl ethers of polyethylene glycol [66]. Their affinity for CO\({}_{2}\) and H\({}_{2}\)S is five times higher than water's, requiring smaller flows of the scrubbing liquid compared to WS [67]. Chemical absorption (referred to as amine scrubbing directly by some authors) is similar to physical absorption, but there is a reversible chemical reaction between the absorbed substances and the solvent [68]. There are two types: amine scrubbing (AS) and inorganic solvent scrubbing (ISS), with ISS still in the developing phase. MS takes advantage of membranes to separate the components of biogas, allowing the CO\({}_{2}\) to pass through while retaining the methane [69]. There are two types: gas-gas separation (gas phase on both sides of the membrane) and gas-liquid separation (a liquid absorbs the molecules that pass through) [51]. The fibrous membrane materials used in the gas separation technique can be categorized as inorganic membranes, polymeric membranes, and mixed-composition membranes [70]. The membranes used in biogas upgrading are high-cost, thus requiring research to optimize both the economy and efficiency of the process. CS is the least established among the developed technologies, but since several facilities are in operation and it is commercially available, it cannot be considered an emerging technology. It consists of physically removing the CO\({}_{2}\) and other contaminants by taking advantage of their different condensation temperatures [71]. For the substances to be separated individually, cooling and compression are performed in different steps [72]. The main advantage of CS is the possibility of obtaining liquefied biomethane, which is equivalent to liquefied NG [51]. The frozen CO\({}_{2}\) generated during the upgrading process has industrial applications, and by liquefying the biomethane at 15 bars and \(-125\,^{\circ}\)C, energy consumption can be minimized [73]. However, the high cost of separating carbon dioxide remains a disadvantage compared to other techniques [74]. The raw biogas contains approximately 35% CO\({}_{2}\), which can be used to produce dry ice at a temperature of \(-78.5\,^{\circ}\)C. Dry ice is  
Figure 4: Developed technologies for biogas upgrading [60; 61]. an attractive alternative to conventional ice in many industrial applications, and it can generate additional revenue [20]. Some characteristics of each developed biogas upgrading technology are presented in Tables 2 and 3. CS is also included, but, since it is not as mature as the other technologies [75], its reported characteristics differ more among references, and a fair comparison cannot be assessed. There is not a single best technical solution valid for every case; each technology presents advantages and disadvantages.