Read the passage and answer the following questions :

Sewage, which is a major component of urban wastewater, contains a significant amount of human excreta, organic matter, and pathogenic microbes. Disposing of such a large quantity of sewage directly into natural water bodies like rivers and streams is not feasible due to its polluting nature. Therefore, sewage undergoes treatment in sewage treatment plants (STPs) to reduce its pollution levels before disposal. The treatment process involves two main steps. The primary treatment focuses on physically removing both large and small particles from the sewage through processes like filtration and sedimentation. Initially, floating debris is removed through sequential filtration. The secondary treatment, also known as biological treatment, involves passing the primary effluent into large aeration tanks. These tanks are mechanically agitated and air is pumped into them, creating an environment suitable for the growth of beneficial aerobic microbes. These microbes form mesh-like structures called flocs, which consist of bacteria associated with fungal filaments. As these microbes grow, they consume a significant portion of the organic matter present in the effluent. This process leads to a reduction in the Biochemical Oxygen Demand (BOD) of the effluent. BOD is a measure of the organic matter content in the water and indicates its pollution potential. The sewage water is treated until the BOD is sufficiently reduced. Overall, sewage treatment plants play a crucial role in reducing the pollution level of sewage by removing particles and organic matter, thereby ensuring that the treated water can be safely disposed of without causing harm to the environment or public health.

Which among the following is not a composition of biogas ?

Oxygen