1. Landfill Gas Generation: Introduction

Gases found in landfills include ammonia ($NH_3$), carbon dioxide ($CO_2$), carbon monoxide ($CO$), hydrogen ($H_2$), hydrogen sulfide ($H_2S$), methane ($CH_4$), nitrogen ($N_2$), and oxygen ($O_2$). The typical percentage distribution of the gases found in the landfill is reported in Table 1. Methane and carbon dioxide are the principal gases produced from the anaerobic decomposition of the biodegradable organic waste components in MSW.

In addition, a number of trace gases will also be found in landfill gas. The type and concentration of the trace gases will depend to a large extent on the past history of the landfill. Issues related to the generation, control of migration, and utilization of landfill gas are considered in the following discussion.

Table 1: Typical Constituents in Landfill Gas

2. Generation of the Principal Landfill Gases

The generation of principal landfill gases is thought to occur in five more or less sequential phases, as illustrated in the Figure below.

Phase I. Initial Adjustment Phase I is the initial adjustment phase, in which the organic biodegradable components in municipal solid waste begin to undergo bacterial decomposition soon after they are placed in a landfill. In phase I, biological decomposition occurs under aerobic conditions because a certain amount of air is trapped within the landfill.

Phase II. Transition Phase In phase II, identified as the transition phase, oxygen is depleted and anaerobic conditions begin to develop.

Phase III. Acid Phase In phase III, the bacterial activity initiated in phase II is accelerated with the production of significant amounts of organic acids and lesser amounts of hydrogen gas. The first step in the three step process involves the enzyme-mediated transformation (hydrolysis) of higher molecular mass compounds (e.g., lipids, organic polymers, and proteins) into compounds suitable for use by microorganisms as a source of energy and cell carbon. The second step in the process (acidogenesis) involves the bacterial conversion of the compounds resulting from the first step into lower molecular weight intermediate compounds, as typified by acetic acid ($CH_3COOH$) and small concentrations of fulvic and other more complex organic acids. Carbon dioxide ($CO_2$) is the principal gas generated during phase III.

Phase IV. Methane Fermentation Phase In phase IV, a second group of microorganisms that convert the acetic acid and hydrogen gas formed by the acid formers in the acid phase to methane ($CH_4$) and $CO_2$ becomes more predominant. Because the acids and the hydrogen gas produced by the acid formers have been converted to $CH_4$ and $CO_2$ in phase IV, the pH within the landfill will rise to more neutral values in the range of 6.8 to 8.

Phase V. Maturation Phase Phase V occurs after the readily available biodegradable organic material has been converted to $CH_4$ and $CO_2$ in phase IV. As moisture continues to migrate through the waste, portions of the biodegradable material that were previously unavailable will be converted.

3. Control of Landfill Gas Migration

When methane is present in the air in concentrations between 5 and 15 percent, it is explosive. Because only limited amounts of oxygen are present in a landfill when methane concentrations reach this critical level, there is little danger that the landfill will explode.

However, methane mixtures in the explosive range can be formed if landfill gas migrates off site and is mixed with air. The lateral migration of methane and other gases can be controlled by impermeable cutoff walls or barriers or by the provision of a ventilation system such as gravel filled trenches around the perimeter of the landfill. Gravel packed perforated pipe wells or collectors may also be used to collect and diffuse the gas to the atmosphere.