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Biodegradation in Municipal Solid Waste landfills

 

Biodegradation in Landfills

 

The biodegradation of MSW takes place through the metabolic activity of microorganisms and results in changes in the mechanical and hydraulic properties of the waste as shown in Figure 11.

 

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Fig 11. Schematic of the processes taking place during MSW biodegradation and examples of the parameters measured (Fei et al., 2014)

 

Landfills represent a complex and unique anaerobic ecosystem and are involved in the global cycling of organic carbon. Landfills serve as a repository for biogenic carbon that is biodegraded to CH4 and potentially recovered for energy. Methane may be recovered from landfills for use as an energy source, as occurs at an estimated 445 U.S. landfills (LMOP, 2007). When CH4 is recovered for beneficial reuse, it results in avoided emissions from power plants that rely on fossil fuels for energy production. However, as a result of CH4 that is not collected, landfills are estimated to be the second-largest source of anthropogenic CH4 emissions in the United States (US EPA, 2008).

 

Although cellulose and hemicellulose are intrinsically biodegradable under anaerobic conditions, they do not degrade completely, as some are protected by lignin and are not bioavailable. In addition, waste decomposition in a landfill is by no means complete, as environmental conditions are often suboptimal. Estimates of the fraction of MSW components that do biodegrade under even the most favorable optimal conditions range from 5% carbon sequestration of office paper to 42% sequestration for newspaper (Barlaz, 1998).

 

The burial of solid waste in a landfill initiates a complex series of chemical and biological reactions that has been described in a series of phases (Barlaz et al., 1989a). The rate and characteristics of leachate produced and biogas generated from a landfill vary from one phase to another, and reflect the microbially mediated processes taking place inside the landfill (Figure 12).

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Figure 12. Phases of MSW degradation in a typical landfill (Pohland and Harper, 1986)

 

  • Phase I: Initial adjustment phase - This phase is associated with initial placement of solid waste and accumulation of moisture within landfills. An acclimation period (or initial lag time) is observed until sufficient moisture develops and supports an active microbial community. Preliminary changes in environmental components occur in order to create favourable conditions for biochemical decomposition.

 

  • Phase II: Transition phase - In the transition phase, the field capacity is sometimes exceeded, and a transformation from an aerobic to anaerobic environment occurs, as evidenced by the depletion of oxygen trapped within a landfill media. 

 

  • Phase III: Acid formation phase - The continuous hydrolysis (solubilization) of solid waste, followed by the microbial conversion of biodegradable organic content results in the production of intermediate short chain carboxylic acids at high concentrations throughout this phase. A decrease in pH values is often observed. Viable biomass growth associated with the acid formers (acidogenic bacteria), and rapid consumption of substrate and nutrients are the predominant features of this phase. The leachate contains a high chemical oxygen demand (COD) that is attributable to carboxylic acids. Because these acids are biodegradable, the highest BOD and COD concentrations in the leachate will be measured during this phase (Kjeldsen et al., 2003).

 

  • Phase IV: Methane fermentation phase - During Phase IV, intermediate acids are consumed by methanogenic bacteria and converted into methane and carbon dioxide. Sulphate and nitrate are reduced to sulphides and ammonia, respectively. The pH value is elevated, being controlled by the bicarbonate buffering system, and consequently supports the growth of methanogenic bacteria. Heavy metals are removed by complexation and precipitation. Carboxylic acid concentrations decrease with corresponding decreases in the leachate COD and BOD.

 

  • Phase V: Maturation phase - During the final state of landfill stabilization, nutrients and available substrate become limiting, and the biological activity shifts to relative dormancy. Gas production drops dramatically and leachate strength stays steady at much lower concentrations. Reappearance of oxygen and oxidized species may be observed slowly. However, the slow degradation of resistant organic fractions. In this phase the BOD/COD is relatively low because dissolved organic matter that is degradable is consumed as rapidly as it is produced.

 

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