Abstract:
Air laden with biodegradable volatile organic compounds is passed sequentially through a first filter bed containing a biologically inert filter media and a second filter bed containing a biologically active 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.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a biological filter apparatus and to a method of operating a biological filter apparatus. 
     U.S. Pat. No. 5,518,920 discloses a filter unit for treating biodegradable volatile organic compounds (VOCs). The filter unit includes a filter bed of organic compost containing a population of microorganisms that have an affinity for VOCs and consume VOCs and nutrients present in the compost. Air laden with the volatile organic compounds passes upwards through the filter bed and the microorganisms biologically convert the VOCs to non-toxic form, typically carbon dioxide and water. The filter bed is kept in a warm and moist condition by periodically spraying water onto the filter bed from above and by warming the air that flows into the filter bed. In this manner, favorable conditions for establishing and maintaining the population of microorganisms in the filter bed are provided. 
     Although the filter described in U.S. Pat. No. 5,518,920 is effective, there is nevertheless room for improvement in performance of the filter. 
     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, said apparatus comprising a first wall means defining a first filter chamber and a second filter chamber, 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, and a second air outlet for exhausting air from the second filter chamber, a biologically inert filter media in the first filter chamber, a biologically active filter media in the second filter chamber, a water supply means for supplying water to an upper region of the first filter chamber, a second wall means defining a sump for receiving water from a lower region of the second filter chamber, and a recirculation means for feeding water from the sump to the water supply means, whereby water received from the lower region of the second filter chamber is supplied to the upper region of the first filter chamber for wetting the biologically inert filter media. 
     In accordance with a second aspect of the invention there is provided a method of operating a filter for processing air laden with biodegradable volatile organic compounds, said method comprising (a) passing the air sequentially through first and second filter beds, the first filter bed containing a biologically inert filter media and the second filter bed containing a biologically active filter media, water being present in said biologically active filter media and said biologically inert filter media, (b) collecting water that drains from the second filter bed, and (c) supplying water collected in step (b) to the first filter bed. 
    
    
     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, in which 
     FIG. 1 is a schematic sectional view of a first filter in accordance with the invention, 
     FIG. 2 is a similar view of a second filter in accordance with the invention, and 
     FIG. 3 is a similar view of a third filter in accordance with the invention. 
    
    
     In the several figures of the drawings, like reference numerals are used to designate like or corresponding elements. 
     DETAILED DESCRIPTION 
     The filter shown in FIG. 1 comprises a trickling filter unit  10  and a biological filter unit  12 . The trickling filter unit 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 . Preferably, the inlet opening  13  is rectangular and its width is substantially greater than its height so that the inlet air flow is distributed across the width of the housing  14 . A fan out transition fitting is employed to connect the circular section supply duct  20  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, Penn. 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 . Preferably, the inlet opening  30  is rectangular and its width is substantially greater than its height and a transition fitting is used to connect the circular section duct  24  to the inlet opening  30 . 
     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 housing  28  has an outlet opening  34  which is connected by a duct  36  to the suction side of a blower  38 . 
     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 the scrubber 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. 
     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  and the duct  36  and the pump  58  draws warm water from the scrubber sump and sprays the water onto the two filter beds  26  and  32 . 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, are removed from the vapor phase and dissolved in the water. 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. 
     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  54  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. 
     The downward flow of water through the filter bed  32 B results in some erosion of compost particles from the compost balls and transportation of compost particles suspended in the water flow into the filter sump. In addition, some microorganisms are removed from the compost by the water flow, and the water flow leaches nutrients from the compost. The water that drains from the filter bed  32  into the filter sump thus forms a soup or tea in the filter sump and scrubber sump. The tea that is retained in the filter bed  26  provides a significant population of microorganisms in the trickling filter. These microorganisms digest the VOCs that are trapped by the water. In the event that the conditions in the trickling filter unit  10  are not optimum for breakdown of VOCs dissolved in the water, the recirculation of the water ultimately conveys the VOCs to the biofilter unit  12  where there is a greater population of microorganisms and biological breakdown can take place with greater efficacy. 
     Transfer of compost particles and nutrients from the biofilter unit to the trickling filter unit provides favorable conditions for the biological population of the trickling filter unit. 
     In the event that the water level in the scrubber sump falls below the nominal minimum level set by the level detector  64 , the level detector opens the valve  66 , admitting replenishment water to the sump. 
     It will be appreciated from the foregoing that the filter shown in FIG. 1 provides at least two distinct environments for biological breakdown of VOCs present in the feed air, namely the filter bed  26  and the filter bed  32 B. In addition, VOCs that are dissolved in the water that drains from the filter bed  26  or  32  into the common sump are subject to breakdown by microorganisms present in the sump. 
     In a modification of the filter shown in FIG. 1, the inlet opening  18  to the housing  14  is below the free surface of water in the scrubber sump and accordingly the feed air entering the housing  14  bubbles through the tea in the scrubber sump. This then provides the possibility of removing VOCs that are not water soluble in the scrubber sump, in addition to removing VOCs from the air by biological action in the filter bed  26  and in the filter bed  32 B. However, this modification is subject to the disadvantage that more energy is required to operate the blower  38  because of the back pressure presented by the tea to flow of air from the inlet opening. 
     The biological filter unit  12  is more expensive to manufacture than the trickling filter unit  10 . Accordingly, the trickling filter unit  10  proves a relatively low-cost means for reducing the load on the biological filter unit. 
     The filter shown in FIG. 2 comprises a tank  80  and the two housings  14 ,  28  extending upward from the tank  80 . The inlet opening  18  is at the upper end of the housing  14 . When the blower  38  is operated, it induces a flow of air through the duct  20 , the housing  14 , the tank  80 , the housing  28  and the duct  36  and discharges to atmosphere. 
     The pump  58  draws warm water from the tank  80  and sprays the water onto the two filter beds  26  and  32 . The manner of operation of the filter shown in FIG. 2 will be understood from the foregoing description of the manner of operation of the filter shown in FIG.  1 . It will be observed that in the case of FIG. 2, the air flows downwards through the trickling filter unit. 
     The filter shown in FIG. 3 comprises a housing  90  that is provided with lower and upper support grids  92  and  94 . The lower grid  92  supports the biologically inert cross flow filter bed  26  and the upper grid  94  supports the biologically active filter bed  32 B. The filter bed  26  serves not only to scrub and condition the air supplied to the filter bed  32 B but also to distribute the air uniformly over the cross section of the housing. In the case of FIG. 3, water sprayed onto the filter bed  32 B drains onto the filter bed  26  and supplies particles of compost, microorganisms and dissolved nutrients to the trickling filter. 
     The manner of operation of the filter shown in FIG. 3 will be understood from the foregoing description of the manner of operation of the filter shown in FIG.  1 . 
     It is important to establish and maintain favorable conditions for the microorganisms in the filter beds with respect to both warmth and moisture. In the case of the described embodiments, this is accomplished by spraying warm water on the filter beds. It will be appreciated that other methods of supplying adequate warmth and moisture are available, including, for example, injecting steam beneath the filter bed as shown in Provisional Application No. 60/245,763. 
     FIG. 1 shows an electrical resistance heater for heating the tea in the scrubber sump, but it will be appreciated that any other suitable means may be used for heating the tea, such as a thermally conductive tube conducting a flow of hot water or steam in heat-exchange relationship with the tea. 
     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.