Question

When calculating landfill methane generation using the LandGEM model, does the methane production rate increase or decrease over time once waste placement stops?

Accepted Answer

When waste placement stops at a landfill, the methane production rate decreases over time. The LandGEM model calculates this using first-order decay kinetics, which assumes that the biological degradation of organic waste into landfill gas occurs at a rate proportional to the amount of waste remaining in the landfill. Because no new waste is being added to replenish the organic material, the total mass of biodegradable waste consistently declines as it is converted into methane and carbon dioxide. In the LandGEM equation, this process is governed by the methane generation rate constant, denoted as k, which represents how quickly the waste decomposes. A higher k value indicates faster decomposition and a steeper decline in methane production, while a lower k value indicates slower decomposition and a more gradual decline. Once the landfill is closed and no new waste is placed, the model mathematically reflects this lack of incoming organic matter by showing a steady, asymptotic drop in the annual methane generation rate as the available substrate for methane-producing bacteria is consumed.

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When calculating landfill methane generation using the LandGEM model, does the methane production rate increase or decrease over time once waste placement stops?