How does methane form in old waste dumps?

In the context of hazardous-waste and solid-waste management, methane is produced in old waste dumps through the process of anaerobic digestion.12

When organic matter—such as food scraps and other biodegradable waste—is buried in a landfill, it is subjected to a low-oxygen environment. In the absence of oxygen, various microorganisms break down this organic material in a multi-stage process. During the final stage, methanogenic (methane-producing) bacteria convert organic acids into methane ($C{H}_4$) and carbon dioxide ($C{O}_2$).32

This natural decomposition occurs over many years, resulting in the generation of landfill gas. Because methane is a potent greenhouse gas and an explosive hazard, modern sanitary landfills are engineered to collect this gas through interconnected pipe systems to prevent it from accumulating or escaping into the atmosphere. In contrast, older, uncontrolled dumps often lack these collection systems, allowing methane to migrate through the soil and pose ongoing environmental and safety risks.13