Filters biofilters

Fig. 4.26. The trickling filter (biofilter). 1 — rotating distributor, 2 — filling space, 3 — grate bottom, 4 — air holes, 5 — bottom with outlet... 

Appropriate techniques for dedusting are cyclones combined with wet scrubbers or dry filters. Biofilters are also used and are discussed in Section 4.5.8.6. 

Other than the biofilter and culture chambers, recirculating systems typically also employ one or more settling chambers or mechanical filters to remove soHds such as unconsumed feed, feces, and mats of bacteria that slough from the bio filter into the water. Each recirculating system requires a mechanical means of moving water from component to component. That usually means mechanical pumping, though air-lifts can also be used. 

Air. Biofilters are an effective way of dealing with air from industrial processes that use halogenated solvents such chloromethane, dichioromethane, chloroethane, 1,2-dichloroethane and vinyl chloride, that support aerobic growth (26). Both compost-based dry systems and trickling filter wet systems are in use. Similar filters could be incorporated into pump-and-treat operations. 

Design and Construction The capacity and efficiency of biofilter operation are a direc t function of active surface area, filter void... 

The filter media should have buffering capacity in order to maintain a pH of at least 3. This is especially a concern when inorganic compounds are targeted for reduction by the biofilter. 

Kinetics The capacity and efficiency of biofilter operation is a function of active surface area, filter void space, target removal efficiency, gas species, gas concentration, and gas flow rate. A simphfied theoretical model described by S.P.P. Ottengraf et al. is schematically represented by in Fig. 25-18. The mass balance made around the hq-uid-phase biolayer can be described as follows ... 

Biofilters—Processes in which contaminants are biodegraded in a fixed-film bioreactor, typically consisting of a bed of high surface area filter media, such as GAC, that acts as a support matrix for a thin film consisting of microbes that are acclimated to the biodegradation of MTBE or other contaminants. 

Very low liquid flow rates correspond to the case of biofilter operation and higher rates to the case of a trickling filter. The formulation of this problem for the two ketones would best be done with a tanks in series dynamic model. An alternative iterative steady state solution is given in Snape et al. (1995) for the two-component case with recycle. 

Operate BIOFILT as a trickling filter by setting reasonably high values of liquid flow and investigate the influence of liquid flow rate. 

The ventilated air from livestock buildings comprises a potential source of complaints from people living in the vicinity. In order to avoid the complaints it may be necessary to reduce the odour in the ventilated air. This may be achieved using biofilters. Very comprehensive development and evaluation of such filters have been made by Zeisig (2,... 

The investment cost for a biofilter was about 100 DM per pig in South Germany. As regards a Swedish filter the investment cost was ca 50 DM per pig. 

The effect of biofilters has been studied by Kowalewsky (4). In these studies he used both Dragerrof as well as photometric and gas chromatographic measurements to study the reduction of certain odours such as ammonia and hydrogen sulphide. The measurements showed that the concentration of the gases studied decreased by, on average, 50%. When the moisture content of the filter was at an optimum level the reduction was 80%. 

The investment costs for a biofilter in Germany with 160 pigs were about 100 DM per pig (4). The operating cost, i.e., electricity, maintenance and repairs, is reported to be 3 DM per marketed pig. As regards the Swedish filter, the investments amount to slightly more than 60 DM per pig but details of operational costs are not yet available. 

The full potential use of biofilters for cleaning air from livestock buildings, treatment plants and emissions from chemical industry have yet to be exploited. The papers presented produced proof of the efficiency of these filters to remove odour emissions from odorous air streams. Research and Development work is required to reduce the costs of construction and to find materials to which will not deteriorate quickly. Work is required to reduce the large surface areas required at present. 

Develop cheaper construction materials for biofilters, to develop filters which require less land area. To find cheap, simple, self cleaning dust filters for biofilters. 

This is a biological air treatment technology that uses an air/water separator, trickling filter, and biofilter for removal of organic contaminants and to reduce odor. The technology is currently commercially available in Canada but is not available in the United States. The company has expressed interest in expanding operations to France. 

Waste gas is first drawn in through the air/water separator, used to remove impurities in the air such as water droplets, solid particles, etc. The effluent then travels to the trickling filter, which is a packed column of very porous polymer material. The use of a polymer as the packing material enhances mass transfers between the liquid and gas. The polymer can fix a large culmre of specific bacteria capable of degrading contaminants found in the liquid phase. The trickling filter is followed by a compost-based biofilter that removes residual contaminants. 

Soil biofilters are relatively large compared to filters using other media since soil pores are smaller and compounds have low permeability in soil. Soil biofilters also have limited depths due to problems associated with maintaining humidity in soil and minimizing pressure drop. Furthermore, soil sorption capacity is limited and residual contaminants are vented immediately to the atmosphere. 

Laboratory development of the in situ microbial filter has been completed. According to LLNL, the key engineering design parameters have been measured under controlled conditions, and scaled laboratory experiments have demonstrated the success of the approach. A field demonstration of the biofilter concept was conducted at a contaminated site in Chico, California. 

A methanotrophic biofilter is a biofilter in which methanotrophs are present. In a biofilter, a gas- or aqueous-phase contaminant stream is passed through a media on which the bacteria are growing. The media can be of several different materials, including compost, peat, soil material, or granular activated carbon. Specific strains of bacteria may be introduced into the filter and optimal conditions provided to preferentially degrade specific compounds. 

This technology has been demonstrated in bench-, pUot-, and full-scale applications. The Bohn Biofilter Bohn off-gas treatment (TO 130) is one example of a commercially available biofilter system that uses compost or soil as the filter medium. 

According to the vendor, the capital and operating costs associated with a PCC biofiltration system vary depending on site-specific factors. The capital cost of the system is directly related to the size of the reactor. The size of the reactor is dependent on the flow rate, chemical composition, and concentration. The operating costs often include electricity consumption, natural gas consumption, steam, maintenance cost, filter media replacement, water consumption, and media disposal. These operating costs are directly related to the design and size of the biofilter (D213161, pp. 1 2). 

Information on capital and operating cost for various biofilter systems installed in Germany and The Netherlands has been reported. These data suggest total operating costs of approximately 1.50 per 100,000 ft of off-gas, depending on the size of the filter. Cost figures obtained from systems installed in the United States of 0.30 to 0.60 per standard cubic feet do not include the replacement of the filter material and also reflect the generally lower cost of electricity in the United States. 

Gabriel, D., Cox, H.J.. Brown.. 1.. Biotrickling filters for POTWs air treatment Full-scale experience with a converted scrubber, Odors and Toxic Air Emissions, 2002. Cole, D.C., Evaluation and testing of an experimental biofilter medium, Master of Science Thesis in Marine Bio-resources, August, 1999. 

While our focus has been on the treatment of wastewater, another technique known as biofiltering is intended to treat contaminated air. The most common technique and the one we will cite in the case studies is a subclass known as trickling filters. The general engineering design involves a countercurrent flow of air and water not unlike a scrubber. The exchange or extraction of the pollutant from the air into the water is the dominant step in the process. At steady state, the water is the vehicle within which remediation takes place. Thus while the pollution is resident in the air initially, the remediation takes place from the water and thus relates the process to the discussion above. 

Air. Hydrocarbon vapors in air are readily treated with biofilters. These arc typically rather large devices with a very large surface area provided by bulky material such as a bark or straw compost. Tile contaminated air, perhaps from a soil vapor-extraction treatment or from a factory using hydrocarbon solvents, is blown through the filter, and organisms, usually indigenous to the filler material or provided by a soil or commercial inoculum, grow and consume the hydrocarbons. 

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