Examples of important reactions for four groups of bacteria involved in anaerobic waste degradation:

 

Fermentative processes

C₆H₁₂O₆ + 2H₂O → 2CH₃COOH + 4H₂ + 2CO₂

C₆H₁₂O₆ → CH₃C₂H₄COOH + 2H₂ + 2CO₂

C₆H₁₂O₆ → 2CH₃CH₂OH + 2CO₂

 

Acetogenic processes

CH₃CH₂COOH + 2H₂O  → CH₃COOH + CO₂ + 3H₂

CH₃C₂H₄COOH  + 2H₂O →   2CH₃COOH + 2H₂

CH₃CH₂OH + H₂O →   CH₃COOH + 2H₂

C₆H₅COOH + 6H₂O → 3CH₃COOH + CO₂ + 3H₂

 

Methanogenic processes

4H₂ + CO₂   → CH₄ + 2H₂O

CH₃COOH  → CH₄ + CO₂     

HCOOH + 3H₂ → CH₄ + 2H₂O

CH₃OH + H₂ → CH₄ + H₂O

 

Sulphate-reducing processes

4H₂ + SO₄ ^2– + H⁺ → HS^- + 4H₂O

CH₃COOH + SO₄ ^2–   → CO₂ + HS^- + HCO₃^- + H₂O

2CH₃C₂H₄COOH + SO₄ ^2– + H⁺  →  4CH₃COOH + HS^-

 

HCOOH, formic acid; CH₃COOH, acetic acid; CH₃CH₂COOH, propionic acid; CH₃C₂H₄COOH, butyric acid; C₆H₁₂O₆, glucose; CH₃OH, methanol; CH₃CH₂OH, ethanol; C₆H₅COOH, benzoic acid; CH₄, methane; CO₂, carbon dioxide; H₂, hydrogen; SO₄^2- , sulphate; HS^-, hydrogen sulphide; HCO₃^-, hydrogen carbonate; H⁺, proton; H₂O, water.

 

 

Factors affecting degradation

•  Oxygen, O₂

•  Hydrogen, H₂

•  pH and alkalinity: pH 6 – 8 is preferred

•  Nutrients

•  Inhibitors

•  TemperaturePotential problem:

•  Water content

•  Sulphate

If methanogenesis is slow, acid formation may be too vigorous and H₂ may become inhibitory and pH too low, which again will inhibit methanogenesis.

 

 

 

Gas generation

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Gas issues in landfill management

• Gas generated in the landfill (LFG) is a result of a mass transfer process

• The main components of the landfill gas (CH₄, CO₂) are a result of biological processes, while the trace components, which are numerous, are generated by biological processes as well as by volatilization

• Gas control strategies may involve control of the waste input, control of the landfill reactor, and control of gas discharges to the environment

• LFG can be used for different purposes, such as burning to produce heat, supplying gas engines or turbines producing electricity , and upgrading to natural gas quality by removing CO₂ and trace gases

 

 

Gas-generating processes in landfills

• In landfills receiving organic waste, the microbial conversion of organic carbon to methane and carbon dioxide is the dominating gas-generating process responsible for the main gas components and the gas flux also transporting the trace components out of the landfill

• The predominant part of the landfilled waste becomes anaerobic after disposal, and microbial processes degrade the organic waste, eventually converting the solid organic carbon to methane (CH₄) and carbon dioxide (CO₂)

• To understand the overall process attention must be paid to the microbial consortium performing the carbon conversion, the basic parameters influencing the process, and how actual landfilling technology affects the methane formation

• Methane formation is usually seen as beneficial in terms of improving both the leachate quality and the efficiency of the landfill gas utilization

Landfill gas components

• Although leachate quality indicates that a landfill is in the acid phase, the landfill may already exhibit active gas production with methane concentrations >50%

• The main components of landfill gas after relatively short times after disposal (weeks to months) are ~55% of methane and ~45% of carbon dioxide

• Higher concentrations can be observed to a larger degree in older landfills

• A change of landfill gas composition inside the landfill can take place when oxygen enters the landfill; this has been observed only during gas extraction whwn air has been sucked into the fill  

 

 

Trace components

• The type and concentration of the trace components depend on the composition of the landfilled wastes

• The trace components may cause damage to the technical equipment used for gas extraction and utilization and have an adverse effect on the environment, especially the atmosphere, and on the health of human beings and animals

• Owing to their generation, a differentiation into two steps of trace components can be made:

•   trace components generated during anaerobic degradation in the landfill

•   trace components generated by man (anthropogenic trace components) and deposited together with the wastes

 

 

Landfill operation affecting degradation of organic waste

•  Waste composition: Only degradable organic waste will produce significant quantities of methane

•  Sewage sludge

•  Buffer addition

•  Shredding

•  Compaction

•  Soil cover

•  Recirculation of leachate

•  Precomposting of bottom layer

 

 

Landfill gas management in Asia

Gas generation

Gas quantities

LFG utilisation

Alternatives to utilisation

Development in organic matter degradation in a landfill by changes in landfill gas composition