The choice of filter material is of great importance in order to maintain the efficiency of a biofilter, in that it must (1) guarantee optimum environmental living conditions for the microorganisms; (2) constitute at the same time a nutritious reserve, a humidity reservoir, and a mechanical odorless support; and (3) provide the structural stability of the bed.
The choice of the material is strongly influenced by the need to minimize the overall volume necessary for the reactor, optimize the removal efficiency, keep energy consumption to a minimum, and minimize maintenance. In addition, the characteristics of the carrier material impact directly on the microbial growth and activity, thus in turn affecting biofilter performance. Biofilter beds have the advantage of immobilizing the microflora on the packing material, as a result of which these organisms, forming a bio-layer, are not drained from the system, as is often the case in freely dispersed systems.
Small particles of natural organic materials, such as compost, peat, soil, or mixtures of these materials with bark, leaves, wood chips, heather branches, humus earths, or brushwood (less than 10 mm in diameter), are widely used as packing media in biofilters because they provide a high specific surface area (from 300 to 1,000 m_1), favorable living conditions for the resident microbial population (ensured by high retention capacities of water and nutrients), and favorable immobilization for the microflora involved. In practice these packing materials have shown the common disadvantage of being strongly subject to aging phenomena, resulting in bed shrinkage. This phenomenon disturbs the homogeneous flow distribution of the gas and may cause a considerable increase in the pressure drop, a decrease in the specific area, and shorter filter lifetime. Moreover, the natural unhomogeneity of the structure of these natural materials prevents the uniform distribution of the gas flow, provoking short-circuiting channel development in the filter bed, with an increase in the flow resistance and a decrease in the biological degradation capacity of the bed.
In order to prevent these problems, inert materials, such as polystyrene spheres, lava particles, glass beads, porous clay, and ceramic, are usually added to the natural filling materials. This combination, improving the uniform distribution of the gas flow, lowers the head losses up to 100-150 mm of water gauge (8), thus contributing to reducing the power requirement necessary to convey the gas through the filter and, consequently, the operating costs. Moreover, the addition of porous materials (e.g., granular activated carbon) with high internal porosity and hydro-philic properties, increases the buffering capacity of the filter, (which is very favorable, particularly in the treatment of gas streams with strongly fluctuating pollutant concentrations). The selection of the most suitable material for a specific application depends on the nature of the pollutants and on the magnitude of the concentration fluctuations in the gaseous stream. In order to buffer the fluctuations in the concentration so that a constant supply of contaminants to the biofilter can be achieved, the adsorbent should have both good adsorbing power and reasonable desorbing properties. This combination provides the adsorbent with the capacity of adsorbing at high concentrations and desorbing at low concentrations and allows a significant reduction in the required filter volume (15).
Compared to soil, compost has the advantage of providing lower resistance to the gas flow and consequently contained pressure drop, which should not, in general, exceed 250 mm of water gauge (17). Peat is the material with highest water-retention capacity and constitutes the optimal substrate for the microorganisms; nevertheless, as it implies a higher pressure drop, it is often mixed with other materials in order to improve the structural stability.
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