Comparing cost data quoted in the literature for biofiltration systems is very difficult because of the use of different currencies, the fluctuations of their reciprocal rates, the cost variability with time, the different units employed, and so on. Nevertheless, there is no doubt of biofilter economic convenience when compared with other traditional abatement techniques, due to the following advantages (8): minimum operational input, reduced production and disposal of by-products, easy start-up procedures, operational stability at steady state, low operative temperature, low cost of materials, simple building technologies, simple control systems, and low power requirements.
From the comparison of different biofilters in use in Europe (1,37,81-83), total operating and maintenance costs ranging from $0.2 to 0.5 per 1,000 m3 of waste gas can be estimated, which includes depreciation and interest, water consumption, replacement of filter materials and personnel, wastewater treatment, and energy costs for compressors. Costs reductions showing a half of these amounts for United States installations do not include expenses for support replacement and reflect the lower cost of electricity (1).
Capital costs are often negatively influenced by several factors, among which high pollutant concentration in the waste gas, recalcitrant nature ofthe components, necessity of gas pretreatment, reduced waste-gas availability, lack of space near the polluting source, and transport. Since the transport costs are strongly dependent on the country and the nature of the site, a cost comparison is only possible if this factor is not considered. On these bases, capital costs for open single-bed filters are in the range between $300 and $1,000 per m2 of filter material in Europe (84,85) and between $800 and $1,000 per m2 in the United States (1). These costs would increase up to $3,800-5,700 per m2 if lack of space would force the build-up of the filter on the plant cover, because of the additional costs of personnel, piping, and compression (88), and up to $1,000-5,000 per m2 if particular meteorological situations (snow, frequent atmospheric precipitation, etc.) require an upper covering or enclosed systems (1). Multiple-bed systems seem to be twice as expensive as the single-bed ones.
The filling material may consist, according to the pollutants, of heather, peat, and earth, often mixed with other inert materials necessary to minimize head losses. Because of these variables and the market situation, costs may fluctuate remarkably from $80 to $700 per m3 (85,88).
Taking odor abatement as a basis for comparison of different techniques, biofiltration is by far the most convenient process in terms of space availability, resulting in, as a function of waste-gas composition, total cost savings of 15-30%, 45-70%, 50-75%, and 80% with respect to bio-scrubbing, chemical scrubbing, adsorption, and ozonation, respectively (17,85,87). The success of biofiltration appears to be the result of the very low incidence of running costs to total costs as compared to other techniques. Extending the above comparison also to incineration, it has been estimated that biofiltration allows total cost savings of 8075%, and investment and operational cost savings of 4075% and 70-80%, respectively (17,83).
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