Page 74 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 3 Biomass Characteristics 53

cell (Figure 3.2) so that leached liquids can be collected and treated instead
of leaking into groundwater. The containment cells are covered with clay or
earth to avoid exposure to wind and rain.
An increasing number of municipalities are separating biodegradable
wastes and are subjecting them to digestion for degradation. This avoids dis-
posal of leachate and reduces the volume of waste. Two types of waste deg-
radation are used: aerobic digestion and anaerobic digestion.
1. Aerobic digestion: This process takes place in the presence of air and so
does not produce fuel gas. Here, the leachate is removed from the bottom
layer of the landfill and pumped back into the landfill, where it flows
over the waste repeatedly. Air added to the landfill enables microorgan-
isms to work faster to degrade the wastes into compost, carbon dioxide,
and water. Since it does not produce methane, aerobic digestion is most
widely used where there is no additional need for landfill gas.
2. Anaerobic digestion: This process does not use air and hence produces the
fuel gas methane. Here, the land-filled solids are sealed against contact
with the atmospheric oxygen. The leachate is collected and pumped back
into the landfill as in aerobic digestion (Figure 3.2). Additional liquids
may be added to the leachate to help biodegradation of the waste. In the
absence of oxygen, the waste is broken down into methane, carbon diox-
ide, and digestate (or solid residues). Methanogenesis bacteria like thermo-

philes (45 65 C), mesophiles (20 45 C), and psychophiles (0 20 C)
facilitate this process (Probstein and Hicks, 2006). These biodegradation
reactions are mildly exothermic. The process is represented by Eq. (3.2):
C 6 H 12 O 6 ðrepresenting wastesÞ 1 bacteria 5 3CO 2 1 3CH 4 1 digestate (3.2)

Leachate collection
system
Methane gas
recovery
system
Clay cap
Trash

Landfill
liner

FIGURE 3.2 Anaerobic digestion of biodegradable waste.