Page 341 - Materials Chemistry, Second Edition

CAT3525_C10.qxd 1/31/2005 12:00 PM Page 312
312 Waste Management Practices: Municipal, Hazardous, and Industrial
are reduced to N and H S (see Equations 10.5 and 10.6). The extent of anaerobic conditions can
2
2
be monitored by measuring the oxidation and reduction potential of the waste. Reducing conditions
sufficient to support the reduction of nitrate and sulfate occur at about 50 to 100 mV.
1 1 1 1 1 7
NO {CH O} H → N CO H O (10.5)
2
2
2
2
3
5 4 5 10 4 20

SO 4 2 2{CH O} 2H → H S 2CO 2H O (10.6)
2
2
2
2
In Phase II, the pH of landfill liquids decreases due to the formation of organic acids and the
effect of the elevated concentrations of CO within the voids, which may partly dissolve and form
2
carbonic acid, H CO (Tchobanoglous et al., 1993).
2
3
CO H O → H CO 3 (10.7)
2
2
2
Phase III: Second anaerobic phase. In Phase III, also known as the acid phase, anaerobic
microbial activity is accelerated with the concomitant production of copious amounts of organic
acids and modest amounts of H gas. The process is the result of enzyme-mediated hydrolysis of
2
high-molecular-weight compounds such as lipids, polysaccharides, and proteins into smaller com-
pounds. Microbial populations subsequently convert these compounds into a number of organic
acids such as acetic acid (CH COOH), butyric acid (CH CH CH COOH), and lactic acid
3
2
2
3
(CH CH(OH)COOH), and small concentrations of fulvic acid and other complex organic acids.
3
Carbon dioxide is the principal gas generated during Phase III.
The pH of landfill liquids drops to ~5 due to the presence of the organic acids and the relatively
high concentrations of CO within the void spaces. There is no methane production during this period,
2
as methanogenic (methane-producing) bacteria cannot tolerate acidic conditions. The biochemical oxy-
gen demand (BOD ), the chemical oxygen demand (COD), and the conductivity of the leachate
5
increase significantly during Phase III due to the dissolution of the organic acids in the leachate. Also,
because of the low pH values, metals and other inorganic constituents are solubilized during this phase.
Phase IV: Methane fermentation. In Phase IV, a second consortium of anaerobic microorgan-
isms becomes prominent, which converts the organic acids and H gas formed previously into CH 4
2
and CO . The microorganisms responsible for this conversion are strict anaerobes and are labeled
2
methanogenic. In Phase IV, both methane and some acid formation proceed simultaneously. Many
of the acids have already decomposed, however, so the pH rises and stabilizes at about 6.8 to 8
(Tchobanoglous et al., 1993). Consequently, metals which were previously soluble now precipitate.
The concentration of BOD , the COD, and the conductivity also decline.
5
Phase V: Maturation. Phase V occurs after the readily available biodegradable organic material
has been converted into CH and CO (Phase IV). The rate of gas generation declines significantly
2
4
because most of the available nutrients have been removed with the leachate during the previous
phases and the substrates that remain in the landfill are only slowly biodegradable. The principal
landfill gases evolved are CH and CO . During the maturation phase, landfill liquids often contain
4
2
humic and fulvic acids, which are complex and highly stable compounds.
The duration of each of the phases outlined above vary as a function of the distribution of the
organic components in the landfill cell, the availability of nutrients, the moisture content of the
waste, and the degree of initial compaction. For example, the generation of landfill gas will be lim-
ited if sufficient moisture is not available. Increasing the density of the material in the landfill may
prevent adequate water movement to all parts of the waste thereby reducing the rate of biological
reactions and subsequent gas production. Data on the distribution of gases occurring in a newly
closed landfill cell as a function of time are shown in Table 10.12.
The volume of the gases released during anaerobic decomposition can be estimated in a num-
ber of ways. For example, if the individual organic constituents found in MSW (excepting plastics)
are represented with a generalized formula of the form C H O N , then the total volume of gas can
b
a
d
c