Patent Application
20080096268
The embodiments of the present invention shall be more clearly understood with reference to the following detailed description of the embodiments of the invention taken in conjunction with the accompanying drawings, in which:
The description which follows, and the embodiments described therein are provided by way of illustration of an example, or examples of particular embodiments of principles and aspects of the present invention. These examples are provided for the purposes of explanation and not of limitation, of those principles of the invention. In the description that follows, like parts are marked throughout the specification and the drawings with the same respective reference numerals.
In the following specification, the terms “biotrickling filter” and “biotrickling filter system” refer to a system having packing material through which a liquid is recirculated or trickled and wherein the contaminants in a waste gas stream urged to flow therethrough are biodegraded primarily by the action of microorganisms in a fixed biofilm formed on the packing material. The term “bioscrubber”, “bioscrubbing filter” and “bioscrubbing filter system” refer to a system having packing material through which a liquid is flowed and wherein the contaminants in a waste gas stream urged to flow therethrough are biodegraded primarily by the action of suspended microbial cultures in a liquid holding tank. The term “packing material” refers to the media or packing material used in the filter beds of such biotrickling filter systems or bioscrubbers. Furthermore, the term “contaminants” or “air contaminants” refer to chemical compounds present in waste gas streams and includes, but is not limited to, sulfur-based compounds, such as hydrogen sulfide (“H2S”), organic sulfides, reduced sulfur compounds, for instance, methyl mercaptan, dimethyl sulfide and dimethyl disulfide, and volatile organic compounds (“VOC”), such as aliphatic and aromatic compounds. Further, the terms “contaminated air stream” or “waste gas stream” refer to a flow of air/gas that contains contaminants.
Biotrickling Filter System
The packing bed 24 is carried above the sump 25 and has a vented support base 26 upon which rests a column 28 of packing material 30. The column 28 is disposed between the top and bottom ends 32 and 34 of the housing 22. A blower or fan 36 is provided to draw the contaminated air stream from a waste gas inlet 38 and through the packing bed 24. The waste gas inlet 38 is connected to housing 22 adjacent the bottom end 34 of the housing 22. A gas flow sensor 40 positioned within the waste gas inlet 38 measures the flow of contaminated air entering the biotrickling filter system 20.
In this embodiment, the biotrickling filter system 20 is operated as a counter-current system with the contaminated air stream being urged to flow upward through the packing bed 24 in a direction opposite to that of the trickling liquid. It will, however, be appreciated that in an alternative embodiment, the biotrickling filter could operate as a co-current system.
An outlet 42 located near the top end of the 32 of the housing 22 permits a cleaned air stream to exit the housing 22 following treatment in the packing bed 24. Optionally, a mist eliminator (not shown) positioned upstream of the outlet 42 could also be provided to remove fine liquid droplets from the cleaned air stream.
The sump 25 holds the trickling liquid which will be used in the biotrickling filter system 20. The trickling liquid may include effluent and/or freshwater and recirculated water and may be supplemented with a nutrient solution containing minerals such as nitrogen, phosphorus and potassium, and/or other additives such as liquid buffers for adjusting the pH of the packing material 30. In this embodiment, the effluent and nutrient solution are drawn into the sump 25 from dedicated effluent tank 44 and nutrient tank 46 using pumps 48 and 50, respectively. The effluent may be partially treated wastewater containing some residual nutrients. In other embodiments, the effluent may be provided by a supply line connected to a wastewater treatment plant. A liquid level sensor 52 measures the level of the trickling liquid in the sump 25. Some or all of the trickling fluid may be drained from the sump 25 through a first manually actuated drain line 54.
A liquid recirculation system 56 is connected to the sump 25. It includes a recirculation pump 58 and liquid distribution means 60. The recirculation pump 58 is operable to draw from the sump 25 the trickling liquid for delivery to the packing bed 24. The liquid distribution means 60 includes a feed line 62 with a plurality of nozzles 64 for continuously or intermittently spraying/trickling liquid onto the packing material 30. A liquid flow sensor 66 and a pH sensor 68 measure the flow rate and pH of the trickling liquid flowing through the feed line 62. Further provided, is a temperature sensor 70 to monitor the temperature of the packing material. Other sensors could also be provided for instance, to monitor the need for further nutrients. The liquid recirculation system 56 has a second drain line 72 for periodically purging the trickling fluid.
The biotrickling filter system 20 further includes a control system 74 that governs the operation of the system. The control system 74 communicates with (i.e. receives input signals from, and transmits output signals to,) the various sensors 40, 52, 66, 68 and 70 and other system equipment and can actuate the blower 36, the pumps 48, 50 and 58, the drain lines 54 and 72 and the liquid recirculation system 56 to ensure proper operation of the system.
Packing Material
Within the biotrickling filter system 20, the packing material 30 is provided to remove contaminants from the contaminated air stream received within the housing 22.
In this embodiment, the expanded glass granule 80 is a manufactured and shaped granule having a generally spherical shape. The expanded glass granule 80 may be sized between 4 mm and 100 mm. However, preferably it measures between 6 mm and 20 mm. In other embodiments, differently shaped granules could also be used to similar advantage. The expanded glass granulate product made commercially available by Dennert Poraver GmbH of Schlüsselfeld, Germany under the name PORAVER™ has been found to be suitable for use as the expanded glass granule. This product is manufactured from recycled glass and has been used in the past as a component of building materials such as plasters, mortars, adhesives and fillers. However, it will be appreciated that other granulate products exhibiting similar material properties and having different chemical compositions could also be employed to advantage.
The expanded glass granules 80 are randomly packed in the packing bed 24 to permit the configuration of the packing material 30 to be optimized for the particular shape of the packing bed 24. In this manner, improved packing efficiency may be achieved thereby leading to a more uniform distribution of the waste gas streams and the trickling liquid in the packing material 30. As a result, problems associated with gas channelling within the packing bed may be mitigated.
The relatively light-weight/low density characteristics of the packing material 30 tend to facilitate handling of the packing material when charging and discharging the packing material 30 in the packing bed 24 and during maintenance and servicing operations. In particular, the packing material 30 may be removed from the packing bed 24 to permit the excess biomass collected on the surface of the packing material to be washed off thereby allowing recycling of the packing material. In this way, the clogging problems typically associated with conventional biotrickling filter packing materials tend to be mitigated in the packing material 30. Alternatively, the packing material 30 may be easily removed from the packing bed for replacement using a vacuum device.
In addition, freight costs associated with the packing material tend to be lower than those associated with the heavier conventional biotrickling filter media thereby enhancing the cost effectiveness of the packing material 30.
Operation
The operation of the biotrickling filter system 20 will now be described in greater detail. The biotrickling filter system 20 is supplied with a waste gas stream from, for example, a wastewater treatment plant or a rendering plant. The contaminated air is drawn into the housing 22 through the waste gas inlet 38 and is urged to flow upwardly through the packing bed 24 by the operation of the blower 36. The recirculating pump 56 draws liquid from the sump 25, which liquid is sprayed onto the surface of the packing material 30 by the nozzles 64 of the liquid distribution means 60.
As the waste gas stream flows upwardly and the liquid trickles downwardly through the packing material 30, the contaminants are solubilized in the trickling liquid and undergo phase transfer from the gas phase to the liquid phase. In the biotrickling filter system of the present embodiment, the phase transfer of hydrogen sulfide tends to occur more rapidly in the packing material 30 than in conventional biotrickling filter media It is believed that the higher rate of phase transfer of hydrogen sulfide is due to its particular affinity for the expanded glass granule 80. This increased affinity for the expanded glass granule 80 may allow the biotrickling filter system 20 to achieve higher removal efficiencies (elimination capacities) for hydrogen sulfide than were previously obtained with biotrickling filter systems employing conventional packing material. An example of the removal efficiencies achieved for hydrogen sulfide (at various concentrations and at various EBRTs) using the biotrickling filter system having the packing material provided in accordance with an embodiment of the present invention, are shown in
Once the contaminants have transitioned to the liquid phase, the contaminants are adsorbed onto the biofilm formed on the surface of the expanded glass granule 80 and then degraded by the metabolic activities of the microorganisms. Carbon dioxide and water are produced as a result of the biological oxidation of VOCs. The sulfur-based compounds may break down into sulfites (SO32−), sulfates (SO42−) or sulfur (S). The water soluble sulfur compounds tend to be flushed out of the packing bed 24 by the recirculating trickling liquid. Once the contaminants are removed, the treated air stream is exhausted from the housing 22 through the outlet 42.
The coarse granular configuration of the expanded glass granule 80 as well as its characteristic low density/light weight tends to permit easy washing of the packing material to remove not only the products of the contaminant degradation but also any excessive biomass which may have accumulated on the surface of the packing material 30. The problems associated with high gas flow resistance and clogging encountered in known biotrickling filter media tend to be minimized in the packing material 30. Accordingly, the packing material may be recycled, regenerated and reused with relative ease thus tending to impart to it a relatively long service life.
The residue trickling liquid from the packing bed 24 collects in the sump 25. Effluent and nutrients may be drawn from tanks 44 and 46 and pumped into the sump 25 wherein they mix with the residue trickling liquid. The mixture is conveyed back to the liquid recirculation system 56 for reuse. Occasionally, a portion of the liquid along with small amounts of biomass and dissolved pollutant are discharged or purged from the sump 25 through the drain line 72.
During operation of the biotrickling filter system 20, the control system 74 monitors the operational parameters of the system to ensure the optimal operating conditions are maintained within the packing bed 24. For instance, if the pH value measured by the pH sensor 68 falls outside of the desired range, some of the trickling fluid could be purged and effluent or potable water could be added through the liquid recirculation system 56 to adjust the pH. Alternatively, the pH may be adjusted chemically with the addition of a liquid buffer. In another example, if the liquid level sensor 52 detects that the liquid level in the sump 25 is too high or too low, the control system 74 may cause the appropriate remedial action to be taken (i.e. excess liquid may be purged through the drain line 72 or the sump 25 may be recharged with liquid from the effluent tank 44).
With its light weight/low density characteristics, the packing material 30 allows for greater flexibility in the design of biotrickling filter systems. More specifically, the packing material 30 can be used to lighten the overall weight of a biotrickling filter system thereby lessening the need for more structural support (i.e. larger and heavier foundations). In addition, in biotrickling filter systems that employ the packing material 30, the height of the column in the packing bed may be increased to permit greater bed depth and higher inlet gas velocities. This may allow the installation footprint of the biotrickling filter system to be reduced for even greater versatility.
The removal kinetics of various contaminants using a biotrickling filter system having the packing material 30 in accordance with an embodiment of the present invention have been examined through performance data obtained in laboratory during initial pilot studies. The findings obtained from the different studies are described as follows:
Using a biotrickling filter system constructed and operated in accordance with the principles of the present invention, high H2S removal efficiency at high inlet concentrations in low empty bed residence times (EBRT) has been consistently obtained. More specifically, the biotrickling filter system has achieved 93% removal of 100 ppm of H2S in 6 seconds EBRT and 99% removal in 12 seconds EBRT. At higher concentrations of H2S, the biotrickling filter system tended to perform very well. The biotrickling filter system successfully removed greater than 99% of 200 ppm of H2S in 16 seconds EBRT. In comparison, the high performance polyurethane foam media currently used by the assignee of the present application, BIOREM Technologies Inc. of Guelph, Ontario in its biotrickling filter systems made commercially available under the name MYTILUS™, is capable of removing only 81% of 100 ppm of H2S in 6 seconds EBRT and 99% in 15 seconds EBRT. At a concentration of 200 ppm of H2S, the polyurethane foam packing material used in the MYTILUS™ biotrickling filter system was able to achieve only 75% removal efficiency.
Performance data for the removal of H2S at concentrations of up to 200 ppm with the biotrickling filter packing media provided in accordance with the principles of the present embodiment (identified as “LWE”) and with the known packing material (identified as “PUF”) currently used in the MYTILUS™ biotrickling filter system are compared in Table 1 below:
As will be appreciated, the biotrickling filter packing material provided in accordance with the principles of the present invention exhibits improved removal efficiencies.
Using the biotrickling filter system constructed and operated in accordance with the principles of the present invention, an elimination capacity of greater than 100 g/m3/h has been obtained for hydrogen sulfide.
It will thus be appreciated that the physical, material and biological characteristics of the packing material 30 as described above enable the packing material to perform better than other known packing materials. Whereas some conventional packing materials are able to achieve satisfactory removal rates for hydrogen sulfide by improving biodegradation of the contaminants, the packing material 30 is designed to encourage both phase transfer and enhance biodegradation of the contaminants. As a result, the packing material is able to remove hydrogen sulfide from waste gas streams with superior efficiency.
In the foregoing embodiment, use of the expanded glass granules 80 as the packing material in a biotrickling filter, was described. However, it will be appreciated that, due to its particular physical, material and biological characteristics (described above), the expanded glass granule 80 may also be suitable for use in a bioscrubbing filter system. Referring to
The bioscrubber 90 is generally similar to the biotrickling filter system 20 in that it includes a housing 96 that encloses a packing bed 98 and a sump or liquid reservoir 100. However, in this embodiment, the size of the sump 100 has been substantially increased to create a liquid phase reactor 102 in which suspended microbial cultures may biodegrade the solubilized contaminants.
The packing bed 98 is generally similar in structure and construction to the packing bed 24 of the biotrickling filter system 20 except that in the bioscrubber 90, the packing bed 98 is sized smaller than the packing bed 24. The packing bed 98 defines the air phase reactor 94 in which the contaminants undergo mass transfer from the gas to the liquid phase. As in the biotrickling filter system 20, a plurality of expanded glass granules 80 are randomly packed in the packing bed 98. While expanded glass granules 80 measuring between 4 mm and 100 mm may be used in the packing bed 98, preferably, the size of the granules 80 is in the range of 6 mm to 20 mm.
A blower or fan 104 is provided to draw the contaminated air stream from a waste gas inlet 106 and through the packing bed 98. A gas flow sensor 108 positioned within the waste gas inlet 106 measures the flow of contaminated air entering the bioscrubber 90.
In this embodiment, the bioscrubber 90 is operated as a counter-current system with the contaminated air stream being urged to flow upward through the packing bed 98 in a direction opposite to that of the trickling liquid. It will, however, be appreciated that in an alternative embodiment, the bioscrubber could operate as a co-current system.
An outlet 110 located near the top of the housing 96 permits a cleaned air stream to exit the housing 96 following treatment in the packing bed 98. Optionally, a mist eliminator (not shown) positioned upstream of the outlet 110 could also be provided to remove fine liquid droplets from the cleaned air stream.
The sump 100 holds the trickling liquid which will be used in the bioscrubber 90 and contains suspended growth microbial cultures. The trickling liquid may include effluent and/or freshwater and recirculated water and may be supplemented with a nutrient solution containing minerals such as nitrogen, phosphorus and potassium, and/or other additives such as liquid buffers for adjusting the pH of the packing material 92. In this embodiment, the effluent and nutrient solution are drawn into the sump 100 from dedicated effluent tank 112 and nutrient tank 114 using pumps 116 and 118, respectively. The effluent may be partially treated wastewater containing some residual nutrients. In other embodiments, the effluent may be provided by an effluent supply line connected to a wastewater treatment plant. A liquid level sensor 120 measures the level of the liquid in the sump 100. Some or all of the trickling fluid may be drained from the sump 100 through a first manually actuated drain line 122. Aeration means 144 are operable to deliver oxygen to the liquid stored in the sump to encourage aerobic biodegradation of the contaminants within the liquid phase reactor 94. The aeration means 144 includes an air compressor 146 operatively connected to an air diffuser 148 located within the sump 100.
The bioscrubber 90 further includes a liquid recirculation system 124 that is generally similar to the liquid recirculation system 56 of the biotrickling filter system 20. It includes a recirculation pump 126 and liquid distribution means 128. The recirculation pump 126 is operable to draw from the sump 100 the trickling liquid for delivery to the packing bed 98. The liquid distribution means 128 includes a feed line 130 with a plurality of nozzles 132 for continuously or intermittently spraying/trickling liquid onto the packing material 92. A liquid flow sensor 134 and a pH sensor 136 measure the flow rate and pH of the trickling liquid flowing through the feed line 130. Also provided, is a temperature sensor 138 to monitor the temperature of the packing material. Other sensors may also be provided, for instance, to monitor the need for additional nutrients. The liquid recirculation system 124 has a second drain line 140 for periodically purging the trickling fluid.
A control system 142 generally similar to control system 74 governs the operation of the bioscrubber 90. The control system 142 communicates with (i.e. receives input signals from, and transmits output signals to,) the various sensors 108, 120, 134, 136 and 138 and other system equipment and can actuate the blower 104, the pumps 116, 118 and 126, the drain lines 122 and 140 and the liquid recirculation system 124 to ensure proper operation of the system.
The operation of the bioscrubber 90 will now be described in greater detail. The bioscrubber 90 is supplied with a waste gas stream from, for example, a wastewater treatment plant or a rendering plant. The contaminated air is drawn into the housing 96 through the waste gas inlet 106 and is urged to flow upwardly through the packing bed 98 by the operation of the blower 104. The recirculating pump 126 draws liquid from the sump 100, which liquid is sprayed onto the surface of the packing material 92 by the nozzles 132 of the liquid distribution means 128.
As the waste gas stream flows upwardly and the liquid trickles downwardly through the packing material 92, the contaminants are solubilized in the trickling liquid and undergo phase transfer from the gas phase to the liquid phase. In the bioscrubber of the present embodiment, the phase transfer of hydrogen sulfide will tend to occur more rapidly in the packing material 92 than in conventional bioscrubber media. It is believed that the higher rate of phase transfer of hydrogen sulfide is due to its particular affinity for the expanded glass granule 80. This increased affinity for the expanded glass granule 80 may allow the bioscrubber to achieve higher removal efficiencies (elimination capacities) for hydrogen sulfide than were previously obtained with bioscrubbers employing conventional packing material.
Once the contaminants have transitioned to the liquid phase, the contaminants are absorbed in the trickling liquid flowing through the air phase reactor 94. The trickling fluid exiting the packing bed 98 is collected in the liquid phase reactor 102 (sump 100) wherein microorganisms suspended in the liquid biodegrade the contaminants.
Carbon dioxide and water are produced as a result of the biological oxidation of VOCs. The sulfur-based compounds may break down into sulfites (SO32−), sulfates (SO42−) or sulfur (S). The water soluble sulfur compounds tend to be flushed out of the packing bed 98 by the recirculating trickling liquid. Once the contaminants are removed, the treated air stream is exhausted from the housing 96 through the outlet 110.
The residue trickling liquid from the packing bed 98 collects in the sump 100. Effluent and nutrients may be drawn from tanks 112 and 114 and pumped into the sump reservoir 100 wherein they mix with the residue trickling liquid. The mixture is conveyed back to the liquid recirculation system 124 for reuse. Occasionally, a portion of the liquid along with small amounts of biomass and dissolved pollutant are discharged or purged from the sump reservoir 100 through first or second drain lines 122, 140.
During operation of the biotrickling filter system 20, the control system 142 monitors the operational parameters of the system to ensure the optimal operating conditions are maintained within the packing bed 98. For instance, if the pH value measured by the pH sensor 136 falls outside the desired range, an appropriate chemical solution, such as a liquid buffer, may be added through the liquid recirculation system 124. In another example, if the liquid level sensor 120 detects that the liquid level in the sump 100 is too high or too low, the control system 142 may cause the appropriate remedial action to be taken (i.e. excess liquid may be purged through the drain line 122 or the sump 100 may be recharged with liquid from the effluent tank 112 or effluent supply line).
Although the foregoing description and accompanying drawings relate to specific preferred embodiments of the present invention as presently contemplated by the inventor(s), it will be understood that various changes, modifications and adaptations, may be made without departing from the spirit of the invention.
