**4. Biogas purification/cleaning**

The quality of biogas fuel for use in power generation and other applications can be improved using a variety of techniques. By guaranteeing constant fermentation through continuous supply of feedstock of the right quality and digestion at the ideal parameters of C/N ratio, PH, dilution, and digestor temperature, biogas output can be optimized. A little amount of oxygen or air can be injected into the headspace of a storage fermenter to help hydrogen sulfide (H2S) be oxidized by microorganisms, leading to the removal of a significant portion of the sulfate from the gaseous phase. This technique is the most affordable and popular way to desulfurize biogas, removing up to 95% of the sulfur [27, 102].

The second method involves filtering the water with iron oxide and activated carbon as part of an external chemical treatment. Iron hydroxide is used in a reversible procedure that regenerates the filter by adding oxygen [99]. As an alternative, adsorption materials like iron-rich soils or leftovers from the steel or aluminum industries can be used. The governing equation is given by the relation.

$$\text{Fe}(OH)\_2 + H\_2\text{S} \rightarrow \text{FeS} + 2\text{CH}\_2\text{O} \tag{1}$$

Activated carbon filters are used as standard components are provided for the system [77, 103].

Raw biogas is generally corrosive making it necessary to carefully select and maintain equipment material and to treat raw biogas. The corrosive nature of biogas is because of the presence of traces of hydrogen sulfide. The internal combustion engine and micro-turbine applications recommend allowable H2S levels in the biogas, of 10 ppm or 0.0001% by volume for internal combustion engines and 5000 ppm. For microturbine. Different countries have also imposed different quality standards e.g., in Brazil the standard emission for sulfates according to CONAMA (the national environmental agency) is limited to 1.0 mg of S L<sup>1</sup> . Air pollutants containing sulfur include SO2, SO3, H2S, and sulfates. Ammonia is also a corrosive element in raw biogas and can be released in the form of nitrogen oxides as a product of combustion. Nitrogen oxides are also harmful to human health and to the environment [24, 25].
