Option 1: This option is incorrect as dark fermentation is not associated with the disposal of nuclear wastes. Dark fermentation is a biological process, whereas nuclear waste disposal involves different methods.

Option 2: This option is the correct answer. Dark fermentation is a method used to produce methane from organic wastes. It is a microbial process where bacteria break down organic matter in the absence of light and oxygen, producing methane gas. This process is commonly used in biogas production and waste management.

Option 3: This option is incorrect as dark fermentation does not specifically target reducing COD (Chemical Oxygen Demand) in the atmosphere. The main goal of dark fermentation is to produce biogas.

Option 4: This option is the correct answer. Dark fermentation can also be used to produce hydrogen gas as a fuel from wastewater. This is achieved by the breakdown of organic matter by bacteria, which generates hydrogen gas as a byproduct.

Overall, dark fermentation is primarily used for the production of methane and hydrogen gas from organic wastes, making options 2 and 4 the correct choices.