Methanogenic phase

In relation to PVC additives, the study concluded that heavy metal additives are more likely to be released under acidogenic conditions (compared with plasticizers, for example, which are released mainly during the anaerobic and methanogenic phases of landfill development). With respect to landfill emissions, it was concluded that organotin compormds carmot be directly attributed to the presence of PVC in landfills. 

In sewer networks without considerable amounts of sediments, the anaerobic processes are dominated by the acidogenic production of VFA and C02 and by sulfate reduction (hydrogen sulfide production). The methanogenic phase can normally be excluded as being of minor importance. These facts were verified by Tanaka and Hvitved-Jacobsen (1999) and Tanaka (1998) under sewer conditions in a number of laboratory experiments and in the field. 

For instance, low pH values (pH 6) and high chemical and biochemical oxygen demand (COD-20 000 and BODs-lO 000) are typical for the acetic phase. Moderate to high pH values (pH 8) and relatively low values of chemical and biochemical oxygen demand (COD-3 000 and BOD5-200) are typical for the methanogenic phase (C. Gomes, 2005). 

Once established, the methanogenic bacteria will help to control the pH at or near neutrality such that acetogenic phases (pH S - 6) and methanogenic phases (pH 6 - 8) of landfill can be cleariy discerned. At this stage, landfill gas production will increase as a result of the bacterial activity, with the result that carbon compounds that might otherwise contribute to leadiate TOC may be converted to landfill gas with a resultant decrease in this parameter within the MSW leachate (Table 6). The leachate hazard will therefore be reduced with a consequent increase in the landfill gas hazard. 

The composition of landfill leachate may be expected to vary with time, and from site to site, as a consequence of variable water infiltration rates and amounts, differing ages of sites, differing waste composition and local environmental conditions. The processes of leachate generation within the site have already been discussed, and the change in leachate composition as the waste moves from the acetogenic phase to the methanogenic phase have been described briefly. 

It is difficult, because of the above, to describe a typical landfill leachate, although most leachates will contain characteristic components which change as the waste moves from the acetogenic phase to the methanogenic phase. However the data in Table 6 are. within the confines of the above, considered to be typical... 

Acidogenic phase Methanogenic phase Post-Methanogenic phase... 

Table 11.3. Leachate composition in terms of average values and ranges for parameters with differences between acid and methanogenic phase (Ehrig 1988) and average values for parameters with no observed differences between acid and methanogenic phase (Ehrig 1983)...

Another group of archaebacteria is methanogens, which produce methane from carbon dioxide emd hydrogen. These bacteria are killed by oxygen and therefore occur only in anaerobic environment. In the earliest stage of evolution, when there was no oxygen atmosphere (more than 2 billion years ago), these bacteria could have existed everywhere. Their cell walls are periodic open structures ("mesh" phases) of self-assembled S-proteins, discussed in section 4.13. 

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