Environmental Engineering Reference
In-Depth Information
Emcon Associates (1980) stated that MSW density is about 650 Kg/m
3
, a wider MSW density
range of 387 to 1662 Kg/m
3
was suggested by Landva et al. (1990).
12. Nutrients and trace metals
Microorganisms in the landfill require various nutrients for their activity, such as nitrogen
and phosphorous, as well as traces of heavy metals like zinc, iron, copper, potassium,
calcium, cobalt and molybdenite. Rees (1980) and Christensen et al. (1996) found from
existing literature that all the necessary nutrients and traces of heavy metals are available in
most landfills, but heterogeneous insufficient mixing of the wastes may result in nutrient
limited environments. The optimal ratios needed in order to enhance the biodegradation are
100:0.44:0.08 for organic matter expressed as chemical oxygen demand (COD), nitrogen and
phosphorous (McCarty 1964).
13. Acidity
The acidity of the landfill site influences the activity of the various microorganisms and
therefore determines the rate of biodegradation. The pH of leachate produced from a landfill
can have a significant effect on the stabilization of methane production. The pH of a typical
landfill site would initially be neutral, followed by acidic phases, Stages II and III, where
organic acids are produced from waste degradation by the acetogenic micro-organisms, and
the pH falls to as low as 4. The resultant organic acids provide the nutrients for the
methanogenic bacteria and as the acids are consumed, the pH rises. The fermentative and
acetogenic microorganisms have a wider range of pH compared to methanogenic bacteria.
The ideal methanogenic bacteria activity occurs in environmental conditions within a pH
range of 6.8 to 8.0 (Warith 2003). Any drop in the pH value below 6.8 will slow down the
activity and growth of methanogenic microorganisms. In a well-established methanogenic
media, if the methanogenic activity
is inhibited by other factors [O
2
, H
2
, etc.], the conversion
of acetic acid to methane and CO
2
decreases and leads to an accumulation of the acids,
thereby decreasing the pH which in turn may stop the generation of methane (Christensen et
al. 1996).
14. Inhibitors
There are a number of elements or compounds that can inhibit the biodegradation of
MSW (methane production) besides O
2
, H
2
,
pH (acidity) and high concentrations of heavy
metals. These inhibitors are carbon dioxide, sulphate, and high concentrations of cations such
as sodium, potassium, calcium, magnesium, and ammonium. The CO
2
acts as an inhibitor by
raising the redox potential which has an effect on the acetic acid conversion to methane
(Christensen et al. 1996).
Rees (1980) reported that high sulphate concentrations inhibit the methanogenic bacteria
for two reasons: reduction of SO
4
2-
to S
2-
, which is toxic; and competition for common
substrate between methanogenic and sulphate reducing bacteria.
These cations in low concentrations are required for biodegradation, but in high
concentrations they inhibit the methanogenic bacteria. To the author's knowledge, there have
been no studies to evaluate the impact of Chloride on the degradation of MSW.
