Environmental Engineering Reference
In-Depth Information
reaction product of the hydrogen sulphide and metal ions in solution. The presence of the
organic acids generates a very acidic solution which can have a pH level of 4 or even less
(Moss 1997).
Stage IV. Methanogenesis
The methanogenesis stage is the main landfill gas generation stage, with the gas
composition of typical landfill gas generated at approximately 60% methane and 40% carbon
dioxide. The reactions are relatively slow and take many years for completion. The conditions
maintain the anaerobic, oxygen-depleted environment of Stages II and III. Low levels of
hydrogen are required to promote organisms, the methanogens, which generate carbon
dioxide and methane from the organic acids, and their derivatives such as acetates and
formates, generated in the earlier stages. Methane may also form from the direct micro-
organism conversion of hydrogen and carbon dioxide to form methane and water. Hydrogen
concentrations produced during Stages II and III therefore fall to low levels during this fourth
stage. There are two classes of microorganisms which are active in the methanogenic stage,
the mesophilic bacteria which are active in the temperature range 30-35 °C and the
thermophilic bacteria active in the range 45-65 °C. Therefore, landfill gas can be generated
during the methanogenic stage over a temperature range of 30-65 °C, with an optimum
temperature range of gas generation between 30 and 45 °C. In fact, most landfill sites fall
within this temperature range with an average range for UK landfill sites of between 30 and
35 °C. Where temperatures in the mass of waste drop significantly, for example, to below 15
°C in cold weather in shallow sites, then the rate of biological degradation falls off. The
organic acids formed during Stages II and III are degraded by the methanogenic micro-
organisms, and as the acid concentration becomes depleted, the pH rises to about pH 7-8
during the methanogenesis stage. Ideal conditions for the methanogenic micro-organisms are
a pH range from 6.8 to 7.5, but there is some activity between pH 5 and pH 9. Stage IV is the
longest stage of waste degradation, but may not commence until 6 months to several years
after the waste is placed in the landfill, depending on the level of water content and water
circulation. Significant concentrations of methane are generated after between 3 and 12
months, depending on the development of the anaerobic micro-organisms and waste
degradation products. Landfill gas will continue to be generated for periods of between 15
years and 30 years after final deposition of the waste, depending on waste and site
characteristics (Landfill Gas Development Guidelines 1996). However, low levels of landfill
gas may be generated up to 100 years after waste emplacement.
Stage V. Oxidation
The final stage of waste degradation results from the end of the degradation reactions, as
the acids are used up in the production of the landfill gas methane and carbon dioxide. New
aerobic micro-organisms slowly replace the anaerobic forms and re-establish aerobic
conditions. Aerobic micro-organisms which convert residual methane to carbon dioxide and
water may become established.
Figure 6 shows the changes in composition of landfill gas and leachate as the five stages
of waste degradation progress with time. Initial formation of hydrogen and carbon dioxide in
the hydrolysis/aerobic degradation, hydrolysis and fermentation and acetogenesis stages is
followed by the main landfill gas generation stage, the methanogenesis stage. The
characteristic landfill gas composition is methane and carbon dioxide with other minor
