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containing O, N, or S

functional groups containing O, N, or S

functional groups

Source: H. Bohn, 1992, Consider biofiltration for decominating gases, Chem. Eng. Prog. (April). Notes: *Aliphatics degrade faster than aromatics such as xylene, toluene, benzene, and styrene. tSuch as TCE, TCA, carbon tetrachloride, and pentachlorophenol.

Figure 5.22.3 is a schematic diagram of a biofiltration system. A biofiltration system uses microorganisms immobilized in the form of a biofilm layer on an adsorptive filter substrate such as compost, peat, or soil. As a contaminated vapor stream passes through the filtered bed, pollutants transfer from the vapor to the liquid biolayer and oxidize. More sophisticated enclosed units allow for the control of temperature, bed moisture content, and pH to optimize degradation efficiency. At an economically viable vapor residence time (1 to 1.5 min), biofilters can be used for treating vapor containing about 1500 ¡g/l of biodegradable VOCs.

Clean Air

Clean Air

Biofilters For Wastewater Treatment

FIG. 5.22.3 Biofilter schematic diagram. (Reprinted, with permission, from A.P. Togna and B.R. Folsom, 1992, Removal of styrene from air using bench-scale biofilter and biotrickling filter reactors, Paper No. 92-116.04, 85th Annual Air & Waste Management Association Meeting and Exhibition, Kansas City, June 21-26.)

Biofilter

Humidifier

FIG. 5.22.3 Biofilter schematic diagram. (Reprinted, with permission, from A.P. Togna and B.R. Folsom, 1992, Removal of styrene from air using bench-scale biofilter and biotrickling filter reactors, Paper No. 92-116.04, 85th Annual Air & Waste Management Association Meeting and Exhibition, Kansas City, June 21-26.)

Figure 5.22.4 is a schematic diagram of a biotrickling filter for treating VOCs (Hartman and Tramper 1991). Biotrickling filters are similar to biofilters but contain conventional packing instead of compost and operate with recirculating liquid flowing over the packing. Only the recirculating liquid is initially inoculated with a microorganism, but a biofilm layer establishes itself after start up. The automatic addition of acid or base monitors and controls the pH of the recirculating liquid.

The pH within a biofilter is controlled only by the addition of a solid buffer agent to the packing material at the start of the operation. Once this buffering capacity is exhausted, the filtered bed is removed and replaced with fresh material. For the biodegradation of halogenated contaminants, biofilter bed replacement can be frequent. Therefore, biotrickling filters are more effective than biofilters for the treatment of readily biodegradable halogenated contaminants such as methylene chloride.

Biotrickling filters, possibly because of higher internal biomass concentrations, offer greater performance than biofilters at higher contaminant loadings. At a 0.5-min vapor residence time, the maximum concentration of styrene that can be degraded with 90% efficiency using biotrick-ling filters is two times greater than what can be degraded using biofilters.

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Organic Gardeners Composting

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