Hydrogen sulfide should also be removed before the gas can be used for generation of electricityor heat. The degree of treatment, however, depends on the desired gas utilization process. The presence of hydrogen sulfide in biogas can lead to serious problems on process equipments, due to its toxic and corrosive nature, besides leading to sulfur oxide emissions during combustion. From an environmental and health view-point, hydrogen sulfide is a flammable and highly toxic gas, heavier than air. Its odour threshold is about 0.00047 ppmv