The International Information Center for Geotechnical Engineers

Biodegradation in Municipal Solid Waste landfills

Factors Influencing Landfill Settlement

 Influencing factors can be viewed as :

(i) factors defining the condition of the waste within a landfill cell, i.e. internal to the landfill (the boxed factors), and (ii) factors or site operations and controls or initial conditions which influence the internal conditions. 

Figure 7 shows a range of factors influencing landfill settlement feeding into a simplified suite of primary and secondary settlement mechanisms.

 Fig7

Fig. 7 Mechanisms and factors influencing landfill settlement (McDougall, 2011)

 Immediate compression is an abiotic mechanical process that occurs rapidly in response to an increase in stress. Mechanical creep involves the slow time-dependent abiotic yielding and reorientation of MSW constituents under constant stress, whereas biocompression involves abiotic mechanical creep coupled with biotic decomposition of the MSW organic fraction. Biocompression is primarily associated with anaerobic decomposition of the biodegradable organic fraction of the waste, which depends on the organic content, pH, moisture content, and temperature (Table 1).

 

Table 1. Environmental factors which most significantly impact upon MSW degradation in landfills (Yuen et al., 1994)

Influencing factors

Criteria/ Comments

Reference

Moisture

Optimum : 60 % and above

Pohland (1986), Rees (1980)

pH

Optimum for methanogenesis:

6 to 8

6.4 to 7.2

Ehrig (1983)

Farquhar & Rovers (1973)

Temperature

Optimum for methanogenesis:

40°C

41°C

45 (34 – 38°C)

 

Rees (1980)

Hartz et al (1982)

Mata-Alvarez et al (1986)

 

An idealized long-term settlement curve observed from laboratory large-scale simulators is illustrated in Figure 8 and can be divided into the following phases (Fei et al. 2013):

Phase 1 - Transitional phase (P1): It consists of primarily physical mechanisms such as particle reorientation and movement, raveling, creep, as well as compression or softening of waste constituents as moisture is introduced. The introduction of liquids gradually supports microorganism growth, but the microbial populations are still low and their activities do not contribute significantly to settlement.

Phase 2 - Active biodegradation phase (P2): Microorganisms actively consume waste and a peak in their activity is observed. The intensive and wide-spread microbial activities lead to biodegradation of MSW. As a result, rapid and significant settlement occurs. During this phase, and as the waste state changes,mechanisms such as creep and raveling may also contribute to settlement, but probably not significantly.

Phase 3 - Residual phase (P3): Availability and accessibility of organic MSW decrease steadily during the active biodegradation phase. When the majority of the substrate for microorganisms is depleted, biodegradation of MSW become stagnant and a reduction in amount and rate of settlement is observed, indicating stabilization of MSW. Residual biodegradation and the mechanical creep become the main contributions to the observed settlement.

 

Fig8

Fig. 8 Idealized long-term settlement curve with three phases: transitional phase (P1), active biodegradation phase (P2) and residual phase (P3). (Modified from Fei et. al, 2013)

Active biodegradation phase, P2, has the largest contribution to the long-term settlement and hence, is studied in detail in the following sections.

 

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