Mitigation of Landfill Gas Emissions

Biological oxidation as a method for mitigation of LFG emission 77

and properties of the bed material; composition and amount of purified gas; and external con-

ditions, such as temperature and pressure. As a result of overlapping of the effects of various

factors; specific community of microorganisms, which is sensitive to environmental changes,

is being created. Knowledge of habitat requirements of specific groups of microorganisms

enables to control the direction of the development of community in the bed. It is aimed at

obtaining the greatest efficiency of gas removal from the mixture with a specific chemical com-

position. It is not possible to create conditions that will allow for maximum ability to remove

all biodegradable compounds in the bed. Habitat requirements of different groups of microor-

ganisms differ significantly. Therefore, it is necessary to focus on compounds which constitute

the most dangerous threat to the environment. Due to the high concentration of methane in

the landfill gas, in the case of biofilters used in landfills, the aim is to create the best conditions

for the growth of methanotrophic bacteria. An additional argument in favour of such activi-

ties is the fact that methanotrophs also possess the capability for cometabolic degradation

of many trace compounds contained in the biogas. Thus, the development of such bacteria

community allows for the conversion of a number of complex hydrocarbons into simpler com-

pounds, which may become available as substrates for other groups of microorganisms.

5.5.1 Parameters of filter bed material

Filter bed material plays the key role in the development of a consortium of microorganisms.

The material must be stable in terms of physical and chemical state; have an appropriate

granulometric composition, which determines the optimal water and air conditions and a

large specific surface area; be rich in macro and micronutrients; have no compounds toxic to

bacteria; provide easy access of substrates and discharge of metabolites. The material should

be homogeneous in order to avoid zones, which differ significantly according to the growth

of microorganisms or flow resistance for gases (Humer-Huber et al. 2008). It should also be

resistant to sedimentation.

Granulometric and mineral composition of the material decide about its physical properties.

They determine porosity, specific surface area, water capacity, and gas and water permeabil-

ity. Those properties determine the surface area available for colonization by microorganisms

and the conditions of water and air, which are significant due to the availability of water,

oxygen and substrates for microbial cells. Porosity determines the properties of water and

air by adjusting the gas diffusion profile of the filter bed material. The specific surface area

affects the development of a bacterial population (which form a biofilm on the solid phase),

sorption properties of the filter bed material, nutrients retention capacity (reducing their

vulnerability to leaching) and water; as well as increase of buffering properties with respect

to changes in pH. When it comes to the granulometric composition, it is also very important

in terms of ensuring the low pressure loss during the passage of gas through the bed.

Regarding the gas flow facility; the coarse mineral materials with significant share of the

sand and gravel fractions possess the most suitable conditions for gas migration. However,

their disadvantage is a small specific surface area and a lack of nutrients. It was indicated that

the mineral materials with medium and coarse sand have a greater ability to oxidize CH

4

than

materials with a grain size of gravels (Kightley et al. 1995, Pawłowska et al. 2003).

In order to create good conditions for the development of microorganisms and the flow of

gas through the bed, an appropriate balance between porosity and the surface of a material

is required. A large total porosity (>60%) and a content of macropores above 25% should be

considered optimal conditions (Huber-Humer et al. 2009). Various kinds of compost meet

such parameters. Moreover, due to their high content of organic matter, they constitute a

rich source of nutrients and have a pH value that is near to neutral; which is preferred for

many species of microorganisms responsible for the biodegradation of methane and other

compounds present in the biogas.

The importance of organic matter as a component of the filter bed material used in the

biofiltration of landfill gas is highlighted by the results of laboratory tests conducted in

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