## 3 Results and Discussion
### Chemical Scrubber  
After the micro-oxygenation process, the biogas coming out from the digester was conveyed to a wet chemical scrubber. The biogas passed through the scrubber in a counter-current manner and H\({}_{2}\)S was oxidized by a nebulized 40% FeCl\({}_{3}\) solution at pH = 8.6. pH was maintained at a constant value by automatically dosing a 30% NaOH solution.  
A series of tests were carried out to determine the proper quantity of FeCl\({}_{3}\) to use and evaluate the H\({}_{2}\)S abatement efficiency. The tests were performed with a constant biogas flow of 935 \(\pm\) 27 Sm\({}^{3}\)/h entering the chemical scrubber. The relative results are reported in Table 4.  
The H\({}_{2}\)S removal yield rose to about 87% with an increasing amount of FeCl\({}_{3}\) till 1.5 L/h (entries 1-4). A further increase in the quantity of used FeCl\({}_{3}\) did not generate a significant rise in removal efficiency (entries 5-6). This trend could be attributed to the fixed contact time and surface between biogas and FeCl\({}_{3}\) solution, determined by scrubber dimensions and biogas pressure. Thus, raising the amount of FeCl\({}_{3}\) did not increase time and surface contact between the substrate and the oxidizing agent and, consequently, did not enhance H\({}_{2}\)S elimination efficiency.  
Keeping the amount of used FeCl\({}_{3}\) constant (1.5 L/h), the correlation between H\({}_{2}\)S abatement yield and pH variation was also studied (entries 7-8). At a pH of 8.3 and 9.0, H\({}_{2}\)S removal of 80.7% and 90.3%, respectively, was estimated. Varying the pH from 8.6 (entry 4) to 9.0 (entry 8) led to a higher H\({}_{2}\)S removal efficiency but also to an excessive consumption of NaOH solution, which was economically disadvantageous. The enhanced removal efficacy is due to the ability of alkaline solutions to further absorb traces of biogas H\({}_{2}\)S [39]. The spent Fe\({}^{2+}\) solution discharged from the scrubber was sent to an oxidation tank. Here, an air flow was bubbled through the liquid and the Fe\({}^{3+}\) solution was restored in order to be sent back to the chemical scrubber. The oxidation tank was automated and synchronized with the scrubber to supply it with the quantity of the re-oxidized Fesolution necessary for the abatement of biogas H2S. In this manner, a lower consumption of oxidant agents and a considerable reduction of costs were obtained.  
Biogas VOCs were also subjected to partial abatement in the chemical scrubber (Table 5) due to both the oxidation process and solubility in water.  
In particular, total VOCs contained in biogas flow in the period between December and May and mainly represented by hydrophobic terpen suffered a concentration reduction of 9.5%. During the rest of the year, total VOCs were characterized by a higher amount of hydrophilic compounds, so they underwent a concentration lowering of 36.2%. Likewise, a partial decrease in HCl and NH3 concentration down to 8 mg/Nm3 and 4 mg/Nm3, respectively, was observed.  
In the chemical scrubber, the water solution acted as an absorbent not only for H2S but also for CO2[40]. Consequently, the average CO2 concentration in biogas was reduced by about 2% when going out from the scrubber, thus increasing the CH4 percentage. CH4 and CO2 average concentrations in biogas respectively passed from 59.4% and 40.1% before the scrubber to 60.3% and 39.2% after the scrubber.