Thesis - faculty.ait.ac.th - Asian Institute of Technology

2.1 Introduction
Chapter 2
Literature Review
A landfill is any form of waste land, ranging from an uncontrolled rubbish "dump" to
a full "containment" site engineered with high standards to protect the environment. The
landfill is the most economical form of solid waste disposal as adverse environmental
effects and other risks and inconveniences are minimized, thereby allowing waste to
decompose under controlled conditions until it eventually transforms into relatively inert,
stabilized material (Robinson and Maris, 1983). Most landfills can be operated
satisfactorily for at least some period in their lifetime in this manner and in absence of any
significant negative environmental impact.
Unfortunately, in warmer climates, the increase in leachate production after
precipitation is rapid (Lema, et al., 1988) due to rainfall exceeding the amount which can
be effectively evaporated during winter or rainy seasons. Hence, leachate generation needs
to be controlled and effective leachate treatment options have to be identified in order to
avoid negative impacts caused by the leachate.
A common practice in controlling leachate generation is to control the water
infiltration in the landfill by waste compaction as it reduces the infiltration rate. Further, by
designing water proof covers and growing plants on the soil covers of the waste,
infiltration can be minimized. Figure 2.1 presents a typical engineered landfill. The landfill
leachate characteristic is controlled by solid waste characteristics, moisture content, pH,
redox potential, temperature, etc. The presence of moisture is necessary for the biological
conversions within the landfill and for landfill stabilisation, which occurs when there is
insufficient moisture. Degradation processes within the landfill are also temperature
dependent. The pH and redox potential set the conditions for the different phases of
degradation and biological processes within the landfill. Thus, the microbial composition
within the landfill effectively contributes to the landfill stabilization.
After the initial period of waste placement in a landfill, microbial processes proceed
under anoxic conditions. Hydrolytic and fermentative microbial processes solubilize the
waste components during the acid fermentation phase producing organic acids, alcohols,
ammonia, carbon dioxide and other low molecular weight compounds as major products.
This process occurs at a low pH (typically around 5) and is enhanced by the presence of
moisture within the landfill. After several months, the methane fermentation stage occurs.
Methanogenic leachate is neutral in pH and possesses moderate organic compounds which
are not easily degradable and are fermented to yield methane, carbon dioxide and other
gaseous end products (Harmsen, 1983; Farquhar, 1989).
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