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
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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