2.3 Landfill Leachate
2.3.1 Leachate Generation
Leachate is
created by liquid percolating through waste, with the chemical
composition of the waste and the biochemical processes within it playing
a role. As the liquid migrates through the waste, it encounters
pathogenic micro-organisms and extracts solutes and suspended solids
from the waste, thus becoming contaminated (Christensen, 2001).
Increased levels of leachate occur with increased precipitation, such as
during the wetter seasons (Chiang, 1995).
The level of
contamination in the Leachate is influenced by the type of waste it
moves through, and the level of biodegradation of the waste. Leachate
from biodegradable waste may hold a significant quantity of natural
substances, including alkali nitrogen and chlorinated natural and
inorganic salts. All these substances are poisonous to a number of
organic entities, particularly to sea life, and can result in harm to
human health (Renou, 2008).
As water travels through the waste, it
collects contaminants in a few different ways. Contaminants could be
absorbed into the water by disintegration or suspension (Lin 2000). As
natural materials in the waste disintegrate and decompose due to biotic
activity, metabolic intermediates and by-products can be absorbed.
Moreover, (Li, 1999) noticed that these by-products can lead to metals
being dissolved due to the lowered pH (Li, 1999).
The composition of
municipal solid waste will be changed physically- chemically and
biologically after landfilling directly, due to weathering conditions.
When rainfalls it will combine with waste and generate a high
contaminant liquid called leachate. The waste in the landfill undergoes
changes from a short acidic phase (aerobic) to much longer acidogenic
and
methanogenic phases (anaerobic decomposition) (Tonni et al., 2005).
The leachate generation process consists of several procedures which are illustrated below:-
1. Precipitation falls on the landfill and some of it becomes runoff.
2. Some of the precipitation infiltrates the surface (uncovered refuse, intermediate cover, or final cover).
3. Some of the infiltration evaporates from the surface and (or) transpires through the vegetative cover if it exists.
4.
The remainder of the infiltration, remaining after evaporation, and the
storage water moves downward was forming percolate arid which
eventually becomes leachate as it reaches the base of the landfill.
5. Percolation may be increased by infiltration of groundwater.
2.3.2 Leachate formation
Landfill leachate formation depends on several phases which are related by changes in the solid waste composition. These phases are hydrolysis and acidification (phase I), Initial methane generation (phase II), Stable methane (phase III), Slow-down (phase IV), and Final storage (phase V).
In Phase I, the decomposition of solid waste by bacteria begin through aerobic process which requires oxygen. In the Phase II, the anaerobic process starts, while the amount of oxygen decreases. The presences of organic acids reduce the pH in the leachate. Also through this stage the methanogenic bacteria starts to grow slowly. Furthermore, the consumption of simple organic compounds helps to produce methane and carbon dioxide within the landfill.
Waste decomposition can take many years. The transition from phase II to phase III may not be completed for decades. Phase III is called the acid phase due to the production of acids during the phase. This stage includes many activities such as the beginning of methane production, growth of methanogenic bacteria which is responsible for degradation of soluble organic compounds in the leachate. Furthermore, the biochemical oxygen demand (BOD5), chemical oxygen demand (COD) and conductivity of leachate will increase due to the dissolution of the organic acids in the leachate. Many inorganic constituents, in particular heavy metals, will be solubilized during this phase due to the low pH of the landfill leachate (Christensen & Kjeldsen, 1989).
