Abstract:
Air laden with biodegradable volatile organic compounds is passed sequentially through a first filter bed containing a biologically inert filter media, a second filter bed containing a biologically active filter media, and a third filter bed containing a biologically inert filter media. Water is present in the biologically active filter media and the biologically inert filter media. Water that drains from the second filter bed is collected and supplied to the first filter bed. Water that drains from the third filter bed is collected and recirculated to the third filter bed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is filed as a continuation-in-part of copending patent application Ser. No. 09/938,478 filed Aug. 23, 2001 now U.S. Pat. No. 6,479,274 B1 and claims benefit thereof. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a biological filter apparatus with multiple filter units, and to a method of operating a biological filter apparatus. 
     The copending application, the entire disclosure of which is hereby incorporated by reference herein, discloses a biological filter apparatus that comprises a trickling filter unit and a biological filter unit. The tricking filter unit includes a biologically inert filter bed whereas the biological filter unit includes a filter bed that contains organic compost containing a population of microorganisms that have an affinity for volatile organic compounds (VOCs) and consume VOCs and nutrients present in the compost. Each filter unit includes a spray tube for spraying water onto the filter bed. The water passes downwards through the filter beds and is collected in sumps beneath the two filter beds respectively. The sumps of the two filter units are in fluid communication, and water is drawn from the sump of the trickling filter unit for spraying onto both the inert filter bed and the biologically active filter bed. Because the two sumps are in fluid communication, the water that is sprayed onto the inert filter bed contains compost that has eroded from the biologically active filter bed. Accordingly, the inert filter bed is inoculated with microorganisms from the biologically active filter bed. Air laden with VOCs passes upwards through the trickling filter unit and then passes upwards through the biologically active filter unit. The inoculum in the inert filter bed metabolizes water soluble VOCs, such as certain alcohols, present in the feed air stream, thus reducing the concentration of water soluble VOCs in the air stream before it reaches the biofilter. In this manner, the population of microorganisms in the biologically active filter bed becomes conditioned to remove the water insoluble VOCs. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the invention there is provided a filter apparatus for processing air laden with volatile organic compounds, the apparatus comprising a first filter unit defining a first filter chamber and having a first air inlet for admitting air into the first filter chamber and a first air outlet for exhausting air from the first filter chamber, a second filter unit defining a second filter chamber, a second air inlet for admitting air into the second filter chamber and a second air outlet for exhausting air from the second filter chamber, a third filter unit defining a third filter chamber, a third air inlet for admitting air into the third filter chamber and a third air outlet for exhausting air from the third filter chamber, the third inlet being connected to the second air outlet, a biologically inert filter media in the first filter chamber, 
     a biologically active filter media in the second filter chamber, and a biologically inert filter media in the third filter chamber, whereby air passing through the apparatus passes sequentially through the first biologically inert filter media, the biologically active filter media and the second biologically inert filter media. 
     In accordance with a second aspect of the invention there is provided a method of processing air laden with biodegradable VOCs, said method comprising (a) passing the air sequentially through first, second and third filter beds, the first and third filter beds each containing a biologically inert filter media and the second filter bed containing a biologically active filter media, (b) collecting water that drains from the first and second filter beds, (c) supplying water collected in step (b) to at least one of the first and second filter beds, (d) collecting water that drains from the third filter bed, and (e) supplying water collected in step (d) to the third filter bed. 
     In accordance with a third aspect of the invention there is provided a filter apparatus for processing air laden with volatile organic compounds, said apparatus comprising a first wall means defining first, second and third filter chambers, a first air inlet for admitting air to the first filter chamber, a first air outlet for exhausting air from the first filter chamber, a second air inlet, connected to said first air outlet, for admitting air leaving the first filter chamber to the second filter chamber, a second air outlet for exhausting air from the second filter chamber, a third air inlet, connected to said second air outlet, for admitting air leaving the second filter chamber to the third filter chamber, and a third air outlet for exhausting air from the third filter chamber, a biologically inert filter media in the first filter chamber, a biologically active filter media in the second filter chamber, a biologically inert filter media in the third filter chamber, a first water supply means for supplying water to an upper region of at least one of the first and second filter chambers, a second wall means defining a first sump for receiving water from a lower region of at least one of the first and second filter chambers, a first recirculation means for feeding water from the first sump to the first water supply means, a second water supply means for supplying water to an upper region of the third filter chamber, a third wall means defining a second sump for receiving water from a lower region of the third filter chamber, and a second recirculation means for feeding water from the second sump to the second water supply means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, the single figure of which is a schematic sectional view of a filter embodying the present invention. 
    
    
     DETAILED DESCRIPTION 
     The filter shown in the drawing comprises two trickling filter units  10 ,  10 ′ and a biological filter unit  12 . Except as specifically pointed out below, the trickling filter unit  10 ′ is essentially the same as the trickling filter unit  10  and accordingly it is not necessary to describe the structure of the trickling filter unit  10 ′ in detail. Unprimed reference numerals are used to designate elements of the trickling filter unit  10  and the corresponding primed reference numerals are used to designate the corresponding elements of the trickling filter unit  10 ′. 
     The trickling filter unit  10  includes a housing  14  resting on a floor  16 . The housing  14  has an inlet opening  18  and an air supply duct  20  connects a space that contains air laden with VOCs to the inlet opening  18 . The housing  14  has an outlet opening  22  connected to a duct  24 . 
     The housing  14  contains a support grid (not specifically shown) above the inlet opening  18 . A biologically inert filter bed  26  composed of a cross flow filter media rests on the support grid. The cross flow media is made of corrugated sheets of synthetic polymer material. The corrugated sheets are bonded together in face-to-face relationship with the channels of alternate sheets inclined to each other at about 60°. One suitable form of material is commercially available from Brentwood Industries, Inc. of Reading, Pa The cross flow media is oriented with the sheets generally vertical so that the channels pass upwards within the filter bed  26  and are inclined at about 30° to vertical. 
     The biological filter unit  12  includes a housing  28  resting on the floor  16  and having an inlet opening  30  at about the same height as the inlet opening  18  of the housing  14 . The duct  24  is connected to the inlet opening  30 . The housing  28  has an outlet opening  34  which is connected by a duct  36  to the inlet opening  18 ′ of the housing  14 ′. The outlet opening  22 ′ of the housing  14 ′ is connected to the suction side of a blower  38 . 
     The housing  28  is provided with a support grid (not specifically shown) above the inlet opening  30  and a filter bed  32  rests on the support grid. The filter bed  32  is a composite bed that includes a lower bed  32 A of biologically inert cross flow media and an upper bed  32 B of biologically active filter media. Preferably, the biologically active filter media is composed of compost balls made in the manner described in co-pending patent application Ser. No. 09/938,466 filed Aug. 23, 2001, the entire disclosure of which is hereby incorporated by reference herein for all purposes. 
     The inlet opening  18  of the housing  14  and the inlet opening  30  of the housing  28  are positioned at a significant height above the floor  16 . The housing  14  is connected to the housing  28  by a tie line  40  which is only slightly above the floor  16  and is well below the inlet openings  18  and  30 . Accordingly, the housing  14  provides a scrubber sump below the inlet opening  18  for collecting water that drains from the filter bed  26  and the housing  28  provides a filter sump below the inlet opening  30  for collecting water that drains from the filter bed  32 , the two sumps being connected by the tie line  40 . The scrubber sump is provided with a thermostatically-controlled electrical resistance heater  42  for heating the water in that sump. 
     A spray tube  46  is mounted in the housing  14  above the filter bed  26  and is provided with nozzles  48  for spraying water onto the filter bed  26 . Similarly, a spray tube  52  is provided in the housing  28  above the filter bed  32  and is provided with nozzles  54  for spraying water on the filter bed  32 . A water pump  58  has its suction side connected to the scrubber sump and its pressure side connected to the spray tubes  46  and  52  by way of a proportioning valve  60  which distributes the flow provided by the pump between the spray tubes  46  and  52 . A level detector  64  detects whether the free surface of liquid in the scrubber sump is below a nominal maximum level and controls a valve  66  connecting a source of water under pressure to the scrubber sump. 
     The housing  14 ′ provides a polisher sump below the inlet opening  18 ′ for collecting water that drains from the filter bed  26 ′. A pump  58 ′ has its suction side connected to the polisher sump and its pressure side connected to the spray tube  46 ′. 
     In operation of the apparatus shown in FIG. 1, the blower  38  induces a flow of air laden with VOCs through the duct  20 , the housing  14 , the duct  24 , the housing  28 , the duct  36  and the housing  14 ′, the pump  58  draws warm water from the scrubber sump and sprays the water onto the two filter beds  26  and  32 , and the pump  58 ′ draws water from the polisher sump and sprays the water onto the filter bed  26 ′. The warm water trickling down through the filter bed  26  scrubs the air flowing upward through the filter bed  26 . This action conditions the upward flow of air by warming and humidifying it. Moreover, water soluble VOCs, such as certain alcohols and ketones, dissolve in the water and are thereby removed from the air stream. Some of the water sprayed onto the filter bed  26  is retained in depressions or pockets formed in the flanks of the corrugations of the cross flow media. VOCs present in the water are metabolized by the inoculum in the cross flow media. 
     The conditioned air that leaves the housing  14  through the outlet opening  22  passes through the duct  24  and enters the housing  28  through the inlet opening  30 . The air flows upwards through the lower filter bed  32 A, which serves to distribute the flow over the cross-sectional area of the housing  28 , and the air then flows upwards through the upper filter bed  32 B. Microorganisms present in the filter bed  32 B capture and consume biodegradable VOCs present in the air entering the filter bed  32 B and convert them to non-toxic form, typically carbon dioxide and water. Accordingly, the biofilter unit  12  removes biodegradable VOCs from the feed air flow and supplies an outlet air flow having a substantially lower concentration of biodegradable VOCs than the feed air flow. 
     The filter bed  32 B is maintained in favorable condition for the population of microorganisms by the warm water that is sprayed onto the filter bed from the spray tube  46 . 
     The pump  58  sprays water onto the filter bed  26  and the filter bed  32  at a rate that exceeds the rate of evaporation from the respective beds, so that water drains from the filter beds  26  and  32  into the scrubber sump and the filter sump respectively. Since the sumps are connected by the tie line  40 , when water is drawn from the scrubber sump, replacement water flows into the scrubber sump from the filter sump. 
     Considerable evidence suggests that in normal operation of a biofilter to remove water insoluble VOCs from an air stream, the biofilter will emit water soluble VOCs into the air stream. Accordingly, although operation of the biological filter unit  12  removes water insoluble VOCs from the air stream, it may in fact increase the concentration of water soluble VOCs in the air stream. The trickling filter unit  10 ′ is employed to remove water soluble VOCs from the air stream leaving the biological filter unit  12 . 
     The solubility in water of a water soluble VOC depends on the concentration of that water soluble VOC in the water. Since the water that accumulates in the scrubber sump contains water soluble VOCs removed from the inflowing air stream, it is desirable that the water supplied to the spray tube  46 ′ not be drawn from the scrubber sump. Accordingly, the polisher sump, from which the pump  58 ′ draws water for supply to the spray tube  46 ′, is not connected to the scrubber sump and the water that circulates in the filter unit  10 ′ is effectively isolated from the water circulating the units  10  and  12 . 
     Even though the water circulating in the trickling filter unit  10 ′ is effectively isolated from the water circulating in the filter units  10  and  12 , microorganisms that are carried over from the filter unit  12  in the air stream, as well as microorganisms present in the ambient air, provide a sufficient population of microorganisms in the filter unit  10 ′ for effective biodegradation of water soluble VOCs. 
     During normal operation of the filter shown in the drawing, the concentration of soluble VOCs in the feed air stream to the filter unit  12  is fairly low and accordingly the population of microorganisms in the filter bed  32  becomes habituated to removal of water insoluble VOCs. In the event of a surge in concentration of water soluble VOCs, the air stream flowing from the filter unit  12  will contain an increased concentration of water soluble VOCs, and these excess water soluble VOCs will be removed by the filter unit  10 ′. 
     Since the purpose of the filter unit  10 ′ is to remove excess water soluble VOCs that are not removed by the filter unit  10  and any water soluble VOCs that are generated in operation of the filter unit  12 , the filter unit  10 ′ may be smaller than the filter unit  10 . 
     As in the case of the copending application, the filter beds  26  and  32 B may be installed in a common housing. Although it would in principle be possible to install the filter bed  26 ′ in the same housing as the filter beds  26  and  32 B, so that the air flow passes upwardly through the housing and traverses the three filter beds sequentially, and to collect water that drains from the filter bed  26 ′ before it falls onto the filter bed  32 B, is it preferred that the filter bed  26 ′ be installed in a separate housing from the filter beds  26  and  32 B. 
     The drawing illustrates the trickling filter unit  10  connected to the ducts  20 ,  24  so that the feed air stream flows upwards through the filter bed  26 , counter to the water trickling downwards through the filter bed. The counter flow of the air and water results in efficient humidification of the air stream. It is in fact possible that the quantity of water taken up by the air stream and carried over to the filter unit  12  would saturate the biological filter media and impair the operation of the filter unit  12 . In order to guard against this possibility, it may be preferred to connect the ducts  20 ,  24  to the trickling filter unit  10  so that the feed air stream flows downwards through the filter bed  26 . 
     It will be appreciated that the invention is not restricted to the particular embodiment that has been described, and that variations may be made therein without departing from the scope of the invention as defined in the appended claims and equivalents thereof. Unless the context indicates otherwise, a reference in a claim to the number of instances of an element, be it a reference to one instance or more than one instance, requires at least the stated number of instances of the element but is not intended to exclude from the scope of the claim a structure or method having more instances of that element than stated.