US20090090240A1

US20090090240A1 - Biofiltration process and apparatus for odour or voc treatment

Info

Publication number: US20090090240A1
Application number: US12/246,171
Authority: US
Inventors: Hidayat Husain
Original assignee: BIOREM TECHNOLOGIES Inc
Current assignee: BIOREM TECHNOLOGIES Inc
Priority date: 2007-10-04
Filing date: 2008-10-06
Publication date: 2009-04-09
Also published as: US20140256021A1; US20090093042A1; US8772015B2

Abstract

A bioreactor and process is described which offers a transition or continuum between one or more of (a) from biotrickling filter conditions to biofilter conditions, (b) changing media characteristics, c) liquid or nutrient recirculation rates or frequency, (d) pH, (e) gas velocity, (f) retention time along the gas flow passage or (g) cross-sectional area. For example, media may be arranged in sections of a rectangle, with gas flow in a horizontal direction sequentially through the sections, and liquid flow in a vertical direction from top to bottom in one or more sections. Zonal control of process conditions may be provided as the gas passes from inlet to outlet and liquid is introduced at the top of one or more zones and flows down by gravity.

Description

This application claims the benefit of U.S. Provisional Patent Applications 60/977,493 (filed on Oct. 4, 2007), 60/979,605 (filed on Oct. 12, 2007), and 60/979,619 (filed on Oct. 12, 2007), each of which are incorporated herein in their entirety by this reference to them.

FIELD

This specification relates to the treatment of gases using biotrickling and biofiltration.

BACKGROUND

The following is not an admission that anything described below is citable as prior art or part of the common knowledge of persons skilled in the art.
Odours generated from diverse sources such as wastewater treatment plants, pet food processing, biosolids and municipal solid waste composting, rendering of animal fat, etc are a nuisance for the surrounding population, and plant operators must provide treatment. Technologies for treatment of these odours include bioscrubbers, biofilters, biotrickling filters, carbon adsorbers, and chemical scrubbers. Each of these solutions has its own advantages and disadvantages, and all of these result in significant capital and operating costs with no recovery of valuables. Sometimes the impurities are present at such a high level, or the exhaust concentrations limits are so low, that a number of these operations must be installed in series. In some cases, different impurities, that might inhibit biodegradation of each other, or require different process conditions, may force the use of the same unit process twice to permit operation under optimum conditions for each impurity to obtain acceptable removal.
Biofiltration has been used to treat odours. In an example, biofiltration involves a vessel with an organic or inorganic media, through which odorous gases are passed. The media is typically biologically active and inorganic media may be coated to make it biologically active. A biofilm grows on the surface of the media, and odour causing compounds such as hydrogen sulphide are oxidized into odourless or low odour compounds. As many of the odour causing compounds are difficult to biodegrade, long residence times of up to 1 minute are often required. This results in very large equipment, large space requirements and high costs. Organic media, such as compost which is typically used in biofilters, is easily degraded, resulting in channelling, high pressure drop, and poor treatment and must be replaced periodically.
Another method of treating odorous gases, for example hydrogen sulphide, is a biotrickling filter. This is analogous to the biotrickling filter used for liquid wastewater treatment, except the air contains the contaminants instead of water. These systems, while more compact than the biofiltration example described above, require gas collection and liquid recirculation. Since high treatment rates are the key objective, conditions are optimized for a selective contaminant, and may not be suitable for others. For example, for off-gases containing high concentration of hydrogen sulphide and low concentrations of other reduced sulphur compounds such as dimethyl sulphide, methyl mercaptan, dimethyl disulphide, and carbon disulphide, the filter may be operated at a pH in the range of 1.5 to 2 to obtain high treatment rates for hydrogen sulphide. But biological treatment of other reduced sulphur compounds is poor under these conditions and inadequate treatment is achieved. The process is particularly ineffective for biosolids system exhausts where a combination of hydrogen sulphide, reduced sulphur compounds, reduced nitrogen compounds and other volatile hydrocarbons may be present.
U.S. Pat. No. 6,790,653, U.S. Pat. No. 5,891,711, US patent Publication 2003/0027325, Gholamreza Moussavi et al (Journal of Hazardous Materials, 2006), Madjid Mohseni et al, (Journal of Chemical Technology and Biotechnology, 2005), U.S. Pat. No. 6,632,659, and, Huiqi Duan et al (Applied Microbiology Biotechnology, (2005), 67: 143-149) also describe treatment processes.

SUMMARY

The following summary is intended to introduce the reader to the more detailed discussion to follow. The summary is not intended to define or limit the claims.
A system and reactor are described that combine multiple steps or zones, optionally in an integrated system or reactor. The zones may vary relative to each other, or there may be variations within a zone, or both, in relation to one or more characteristics. The varying characteristic may be one or more of cross sectional area perpendicular to flow direction, media, or process type or parameters. For example, the reactor may have multiple process zones including two or more of a biotrickling zone, an intermittent biotrickling (transition) zone, and a biofilter zone in series in a horizontal flow configuration. Alternately, the reactor may have multiple process zones including two or more of a biotrickling zone, an intermediate biofilter zone, and a biofilter zone in series in a horizontal flow configuration.
A reactor and process are described which offer a transition or continuum between one or more of (a) a transition between two or more of biotrickling filter (BTF) conditions, intermittent biotrickling filter conditions, and biofilter (BF) conditions, (b) a transition between two or more of biotrickling filter conditions, intermediate biofilter conditions and biofilter conditions, (c) changing media characteristics, (d) liquid or nutrient solution recirculation rates or frequency, (e) pH, (f) gas velocity, (g) retention time along the gas flow passage or (h) cross-sectional area. For example, the media may be arranged in sections of a tank, with gas flow in a horizontal direction through the tank, and liquid flow in a vertical direction from top to bottom. Zonal control of process conditions may be provided as the gas passes from inlet to outlet and liquid is introduced at the top of one or more zones and flows down by gravity.
Optionally, one or more physical/chemical treatment processes are located either upstream of the biological process or downstream. This may further improve the range of contaminants that can be treated. The complementary processes may include ultraviolet treatment, non-thermal plasma, ozone or water, chemical or biological scrubbing on the inlet side of the biofiltration system or activated carbon or another adsorption process on the outlet side of the system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic see-through isometric drawing of an example of a reactor, with a top cover off;

FIG. 2 is a schematic drawing of an underside of an example of top cover suitable for use with the reactor of FIG. 1; and

FIG. 3 is a partial schematic see-through isometric drawing of another example of a reactor.

DETAILED DESCRIPTION

Various apparatuses or processes will be described below, including an example of each claimed invention. No example described below limits any claimed invention and any claimed invention may cover processes or apparatuses that are not described below. The claimed inventions are not limited to apparatuses or processes having all of the features of any one apparatus or process described below or to features common to multiple or all of the apparatuses or processes described below. It is possible that an apparatus or process described below is not an example of any claimed invention. Rights to file continuing applications are reserved in any invention disclosed in an apparatus or process that is not claimed in this document. Any one or more features of any one or more examples can be combined with any one or more features of any one or more other examples.
An integrated process, system, and reactor with gradual or other transitions may treat exhaust gases containing volatile impurities or one or more gases that may cause unpleasant odours or may be hazardous in nature. Examples of gases that might be treated include but are not limited to reduced sulphur compounds such as hydrogen sulphide, methyl mercaptan, carbon disulphide, dimethyl sulphide and dimethyl disulphide; volatile organic carbons such as aromatic hydrocarbons, esters, aliphatic hydrocarbons and chlorinated hydrocarbons; ammonium compounds such as ammonia, trimethyl amine, indole and skatole; hazardous air pollutants such as methanol and formaldehyde and other volatile compounds. There may be synergistic treatment of different contaminants in a single vessel using more than one unit process.
Gas flow in a horizontal direction in a reactor, with multiple zones with, for example, different process conditions and/or types of media may help to do one or more of the following: increase the ability of the system to handle gases with dust and other impurities, may avoid use of humidifier, provide greater ability to manage biofilm, enable operation with minimum drainage for retention of microorganisms for treating minor, yet highly odorous contaminants such as dimethyl sulphide, and may minimize footprint and gas side pressure drop. Liquid flow from top to bottom perpendicular to the gas flow may permit establishment of different process zones along the gas path. Progressively finer media may be used from inlet side to the outlet side to provide roughing treatment at the inlet where concentrations are high, and high level of treatment on the outlet side where concentrations are very low. For example, a coarse media may be used in the inlet section where biotrickling filter conditions predominate, and a fine media may be used on the outside for the biofilter. Optionally or additionally, an inert media may be used closer to the inlet for removal of high concentrations of specific contaminants, and a biologically active media may be used on the outlet side for enhanced removal of trace contaminants. In examples wherein an intermediate biofilter zone and a biofilter zone are provided, the media used may be different between the intermediate biofilter zone and the biofilter zone. For example, a coarse, biologically active media may be used in the intermediate biofilter zone where the highest contaminant concentrations exist and a finer biologically active media may be used in the biofilter zone where very contaminant concentrations may exist and plugging by biofilm may not be a concern. When high levels of VOCs are present, media with high void volume and surface may be used, and may be installed in a manner to facilitate manual cleaning of the excess biofilm. When dictated by the types of contaminants present, the biofilter section may be further divided into zone with different types and sizes of biologically active media.
In some examples, the cross section of each zone is constant, for example in a rectangular reactor. In other examples, the cross section varies between each zone, for example in a cylindrical reactor with concentric zones.
A process or system may provide a gradual transition from a biotrickling filtration process or zone to a biofiltration process or zone. This may help deal with a range of impurities that cannot be treated as effectively by a single process or zone, and to increase the efficacy of use of reactor volume.
A process or reactor may provide liquid flow from top to bottom in one or more zones. There may be a gradual transition in the nature, flow rate and/or frequency of trickle flow. For example, continuous trickle rate with recycle may be used on the inlet side and infrequent or no, irrigation may be used on the outlet side. Also, the trickle liquid may be collected and recirculated on the inlet side to control pH, and fresh water may be used infrequently on the outside. Within the different biofilter zones, different rates and frequencies of irrigation may be used. Typically, a biotrickling zone is in communication with the inlet, for example adjacent and horizontally spaced from the inlet, to remove bulk of the high concentration impurities such as hydrogen sulphide present in sewage plant exhausts at an optimum pH of 1.5-2.0. This may be followed by an intermittent biotrickling zone in communication with the biotrickling zone, for example adjacent and horizontally spaced from the biotrickling zone where trickle rate may be lower or intermittent to provide the ability to remove the balance of hydrogen sulphide and ammonia, while effectively flushing out the sulphuric acid and nitric acid by-products, and maintaining a near neutral pH to encourage growth of heterotrophic species for the removal of contaminants such as methyl mercaptan, dimethyl sulphide, and volatile organic compounds including methanol, dibutyl acetate, xylene, toluene, methyl mercaptan and others. A biofilter zone may be provided in communication with the intermittent biotrickling zone, for example adjacent and horizontally spaced form the biofilter zone, where a very small amount of irrigation may be used to preserve bacteria able to degrade dimethyl sulphide, one of the more odorous compounds, which is typically present in very small concentrations, and high irrigation rates may prevent its treatment by washing off nascent microorganisms. Alternately, rather than an intermittent biotrickling zone, an intermediate biofilter zone may be provided in communication with the biotrickling zone and the biofilter zone. The intermediate biofilter zone may have a higher irrigation rate to flush out byproducts, and the biofilter zone may have very low or no irrigation to preserve biomass to help provide very high level of treatment. Similar strategy is used to enhance treatment of difficult to degrade VOCs such as alfa pinene, commonly present in exhausts for plywood and oriented strand board operations.
A pre-treatment step such as aqueous or chemical scrubbing may be used upstream of the biological processes for contaminants that are not water soluble or not easy to degrade using biological process, or for removal of particulate matter, or for removal of condensable impurities such as organic compounds present in many hot gas streams.
Photo-oxidation using ultraviolet light may be provided upstream of the biological processes for contaminants that are hydrophobic or difficult to biodegrade, to convert these to hydrophilic, or more easily biodegradable molecules.
Gases may be pre-treated using ozone gas upstream for contaminants that are highly hydrophobic or not easy to remove using biological process. This may help break these down into simpler to biodegrade compounds.
Gases may be pre-treated using non-thermal plasma upstream for contaminants that are highly hydrophobic or not easy to remove using biological process. This may help break these down into simpler to biodegrade compounds.
A post-treatment polishing step may be used such as activated carbon in the same vessel or in a separate vessel as the biological processes. This may help achieve a high level of treatment.
Alternately, gases may be post-treated using non thermal plasma.
Trickle water may be recycled to an inlet side biotrickling section to maintain a relatively uniform low pH condition for rapid hydrogen sulphide oxidation. Once through liquid flow may be used to maintain near neutral conditions in the intermittent biotrickling zone and the biofiltration zones, for example to oxidize reduced sulphur compounds.
Once through trickle liquid may be used when highly soluble gases such as ammonia are present in the inlet gas stream to use it as a combination of biotrickling filter and scrubber.
A fine water mist may be sprayed in the inlet plenum when a soluble contaminant such as ammonia is present in the inlet gas. The mist is then removed on the media in a biotrickling zone and is discharged with the once through trickle liquid.
A venturi scrubber may be installed upstream of the biotrickling zone to remove particulate matter and condensable matter.
A perforated inside air plenum may be provided to feed air to the media bed, and perforated outside tank wall to discharge treated air, and a solid floor and tank ceiling to ensure that the gas flow is horizontal.
A reactor may be operated under negative pressure to provide full containment of the gases.
A reactor may be operated under positive pressure to simplify exhaust air discharge and to reduce system cost by eliminating the outlet plenum.
A method and a system for the treatment of a gas, for example a gas that contains odour causing chemicals as present in exhaust gases from different types of operations, may consist of one or more of the following steps:

- 1. Receiving odorous or toxic exhausts, either under suction or positive pressure.
- 2. Providing a generally horizontal air flow/generally vertical liquid flow odour treatment system, with conditions similar to biotrickling filter (i.e. a biotrickling zone) upstream and conditions similar to biofilter (i.e. a biofilter zone) downstream, and a transition intermittent biotrickling zone in between. This transition is achieved by selecting an inert media for the biotrickling zone and the intermittent biotrickling zone, and biologically active media for the biofilter zone, by recirculating the liquid to the upstream section continuously and providing once through, intermittent irrigation on the downstream end, and at a gradually decreasing flow and frequency in the intermittent zone. Alternately, the treatment system may include a biotrickling zone upstream, and biofilter zone downstream, and an intermediate biofilter zone between the biotrickling zone and the biofilter zone. The biotrickling zone may have an inert media, and the intermediate biofilter zone and the biofilter zone may have a biologically active media. The intermediate biofilter zone and the biofilter zone may differ in irrigation rates, and may include different media. For example, the intermediate biofilter zone may contain coarser biologically active media, and a high irrigation rate, and the biofilter zone may have a finer media and a low irrigation rate. This may avoid biofilm formation and plugging in the intermediate biofilter zone while flushing out byproducts, whereas plugging may not be a concern in the biofilter zone. For example, the intermediate biofilter zone may be irrigated at a rate of 0.01-0.1 ft3 of water/ft3 of media per day, total, delivered in equal proportions for 1 to 24 times per day, and the biofilter zone may be irrigated at a rate of 0.001-0.01 ft3 of water/ft3 of media per day, total, delivered in equal proportions for 0 to 3 times per day. More specifically, the intermediate biofilter zone may be irrigated at a rate of 0.03-0.07 ft3 of water/ft3 of media per day, total, delivered in equal proportions for 1 to 24 times per day, and the biofilter zone may be irrigated at a rate of 0.003-0.007 ft3 of water/ft3 of media per day, total, delivered in equal proportions for 0 to 3 times per day.
- 3. Introducing the gases into the central gas distribution manifold of the concentric treatment device.
- 4. Transferring gas from the central manifold to the biotreatment system in a horizontal direction, where treatment of hydrogen sulphide, reduced sulphur compounds, volatile organic carbon compounds, and other gases takes place at a high rate.
- 5. Transferring gases from the biotrickling zone to the intermittent biotrickling zone to the biofilter zone in a horizontal direction, where further treatment of residual contaminants takes place. Alternately, transferring gases from the biotricking zone to the intermediate biofilter zone to the biofilter zone in a horizontal direction, where further treatment of residual contaminants takes place.
- 6. Discharging gases to the outside from perforated biofilter wall, or from a manifold placed outside of the biofilter to collect the gas prior to discharge through a stack.
- 7. Introducing liquid at the top of the biotrickling zone continuously, and distributing it evenly across the gas flow path, but differentially along the gas flow path, using spray nozzles or perforated pipes
- 8. Introducing liquid at the top of the intermittent biotrickling zone intermittently, or the intermediate biofilter zone intermittently, and distributing it evenly across the gas flow path, but differentially along the gas flow path, using spray nozzles or perforated pipes. The frequency of liquid recirculation may depend on many factors, including level of contaminants, pH of the drainage solution, etc.
- 9. Introducing irrigation liquid at the top of the biofilter zone periodically. This may also depend on levels of contaminants. In some cases, where very high level of treatment of trace components is desired, very infrequent irrigation may be provided.
- 10. Collecting drainage from biofilter and biotrickling zones in a sump and recycling the drainage to the top of the biotrickling zone using a recirculating pump. Excess water may be discharged from the sump to maintain desirable pH conditions.

FIGS. 1 and 3 show alternate examples of a reactor 100. The reactors may be designed to treat gases from, for example, a municipal waste water plant having up to 50 ppmv of H₂S, although it may be used or modified for other applications. Each reactor comprises a biotrickling zone 102, an intermittent biotrickling zone 104, and a biofilter zone 106. However, in alternate examples, only a biotrickling zone and a biofilter zone may be provided. Further, in alternate examples, a biotrickling zone, an intermediate biofilter zone, and a biofilter zone may be provided. In such examples, the reactor would be similar to reactor 100, however the intermediate biofilter zone would include biologically active media, rather than inert media. Further, in alternate examples, a biotrickling zone, an intermittent biotrickling zone, an intermediate biofilter zone, and a biofilter zone may be provided. The term “biotrickling zone” refers to a zone operated like a biotrickling filter. The term “intermittent biotrickling zone” refers to biotrickling filtration zone with intermittent, but frequent irrigation to remove contaminants such as hydrogen sulphide and ammonia, that produce acidic by products. The term “biofilter zone” refers to a zone which operates like a biofilter, with infrequent irrigation at a low rate to preserve biomass and to reduce losses of heterotrophic bacteria.
Referring to FIG. 1, in the example shown, the reactor 100 has a central gas distribution manifold or plenum 108 with two identical biofiltration systems 110 a, 110 b, on opposed sides of the central plenum 108. The first system 110 a is adjacent and horizontally spaced from a first side 120 a of the plenum 108, and the second system is adjacent and horizontally spaced from the second side 110 b of the plenum 108. In alternate examples, the reactor may only comprise one biofiltration system.
Contaminated air is supplied to the central plenum 108, which may be, for example, sized for a gas velocity in the range of 500 to 2500 ft/minute. A perforated wall 112 permits the gas to pass to the next zone of the reactor 100, which is a biotrickling zone 102. The biotrickling zone 102 is in communication with and horizontally spaced from the plenum 108. A coarse, inert packing material may be in zone 102. For example, the packing material may include a lightweight media made of expanded glass, for example, PORAVER™ granules as made by Poraver North America, Barrie, Ontario, Canada. To make a biologically active media, the granules may be coated. Various media which may be used are described in U.S. patent application Ser. No. 10/687,761 filed Oct. 20, 2003, U.S. patent application Ser. No. 11/583,783 filed Oct. 20, 2006, U.S. patent application Ser. No. 11/542,107 filed Oct. 4, 2006 and U.S. patent application Ser. No. 12/245,327 filed on Oct. 3, 2008, all of which are incorporated herein in their entirety by this reference to them. Such lightweight media is generally non-compacting, less dense than many other types of non-compacting media, and so allows a tall reactor to be constructed, for example 4 meters high or more.
Velocity in zone 102 may range from 5 ft/minute 30 ft/minute. Total empty bed residence time (EBRT) may range from 2 seconds to 30 seconds.
Trickle liquid is introduced via line 212 from the top 214 of zone 102 using perforated pipes or spray nozzles 216, which may be provided on a top cover 218 of the reactor 100 (shown in FIG. 2), and the air flows across the liquid in a generally horizontal direction, indicated by arrows A. Further, baffles 218 may be provided to separate the zones. Treatment of high concentration of contaminants such as hydrogen sulphide takes place in zone 102 at a high rate. However, as the gas travels along the length of the reactor, in the direction indicated by arrows A, the pH conditions are changed by adjusting the trickle liquid characteristics, rates, and frequency.
The intermittent biotrickling zone 104 is horizontally spaced from and in communication with the biotrickling zone 102. In the intermittent biotrickling zone 104, conversion of residual hydrogen sulphide to sulphuric acid and water is generally progressed and ammonia to nitric acid and water are nearly completed. Significant conversion of reduced sulphur compounds and other volatile organic matter also starts in zone 104. The intermittent biotrickling zone may also include inert media.
In communication with and horizontally spaced from the intermittent biotrickling zone 104 is the biofilter zone 106. In the biofilter zone 106, which preferably contains a biologically active media, the residual reduced sulphur compounds and volatile organic compounds are treated. This media may be of a finer size than the biotrickling filter media to provide high surface area intensity, and may contain additives such as activated carbon, pH buffering agents, nutrients and compost to support biological growth. Biotrickling filter media may be generally 8-16 mm, as sized by screens with openings of those sizes, whereas biofilter media may be generally 4-8 mm size.
The reactor 100 shown in FIG. 1 may result in a substantial reduction in empty bed retention time as compared to both biotrickling filter and a biofilter, where a combination of these processes is used for treatment of gases containing hydrogen sulphide and reduced sulphur compounds. As used herein, empty bed retention time (EBRT) refers to the contact time between the gases and the biofilter media. For example, in an off gas stream from a sewage pumping station, the biotrickling zone 102 is designed for an empty bed retention time of 1-10 seconds, depending on the nature and concentration of hydrogen sulphide. Similarly, the intermittent biotrickling zone 104 is designed for a retention time of 1-10 seconds, depending on the level of hydrogen sulphide and ammonia. Biofilter zone 106 is then designed for an empty bed retention time of 5-30 seconds to treat reduced sulphur compounds. In this invention, because of the overlap between the biotrickling zone 102 and the biofilter zone 106, and because of gradual change in process conditions, parallel treatment can occur and it is estimated that the biotrickling zone 102 EBRT can be reduced by up to 30%.
In some examples, the inlet plenum 108 can be retrofitted with a pre-treatment step, depending on the nature of contaminants present. For example, for a water-soluble impurity such as ammonia, which is not easily degraded through bioreaction, the inlet plenum 108 may be packed with media with very high void space and water may be used to scrub out the impurity. In another example, ultraviolet treatment may be provided for difficult to biodegrade compounds such as chlorinated hydrocarbons and aromatics. In such cases, the ultraviolet system may be located in the inlet plenum 108 or outside in a separate treatment system. In another example, ozone may be used to oxidize impurities such as chlorinated hydrocarbons, which are difficult to biodegrade, and can be oxidized to easily biodegrade molecules. Ozone would likely be added in a separate reactor because of safety and materials considerations. In another example, non thermal plasma may be used to pretreat the gases.
The generally horizontal configuration of FIG. 1, wherein gas passes generally horizontally through each of the zones, may have many benefits:
1. It may be possible to use only 3-5 ft media bed depth along the gas flow path without increasing foot print by increasing the height of the bed. The pressure drop can be reduced, providing buffer for pressure drop build up due to sulphur accumulation in the biofilter zone under high H₂S concentration episodes.
2. The biotrickling filter zone 102, using 8-16 mm LWE media, may be completely integrated with the biofilter zone 106 at little additional cost.
3. Flexibility is available to use different irrigation strategies along the air flow path. This may improve performance for methyl mercaptan, DMS, amines and other highly odorous but low concentration compounds. Tools for this include use of very low irrigation rates in the biofilter zone 106 for biomass preservation, and pH management.
Integration of the biotrickling zone with biofilter zone may make it possible to:
1. Use even lower biotrickling filter and biofilter zone EBRTs for applications with high H₂S feed even if high total odour reduction is required;
2. Reduce or eliminate humidification as a biotrickling filter, along with recirculation solution temperature management, can be used for +95% humidification. Heating may further improve the biotrickling zone H₂S tolerance by increasing reaction rates. Note that an evaporative pre-humidifier may be required for tropical climates;
3. Feeds may be treated with up to 200 pppmv (or even higher—if recirculation temperature management is feasible) H₂S using 8-16 LWE;
4. The biofilter zone may be protected from sulphur deposition;
5. The need for biofilter irrigation may be reduced, minimized, or eliminated; and,
6. If present, ammonia may be adsorbed in the biotrickling zone using high trickle and purge rates.
In some examples, media settling may be reduced by selecting media that is highly uniform, for example with a uniformity coefficient of about 2 or less. Uniformity coefficient is the ratio of the size of particle that has 60 percent of the material by weight finer than itself, to the size of the particle that has 10 percent (by weight) finer than itself. This restricts movement of particles and so reduces settling of particles which might otherwise result in non-uniform air flow and treatment. Loose carbon fines may be washed off initially to the bottom and may drain away. The low media density may permit pre-filling the tank at the fabricator prior to shipping, resulting in reduced degradation during handling. Also, to minimize settling, a non-compressible media may be used. Examples of such media include lava rock, expanded glass beads, or structured media such as HD QPac, manufactured by Lantic Packing. A biotrickling filter upstream of the biofilter may humidify air completely, and may reduce the importance of irrigation and uniform moisture distribution across the air flow path.
To ensure that the biotrickling filter zone humidifies air to a greater than 90% relative humidity, a sump, with a pump 220 and a heater may be provided (shown in FIG. 2). The degree of heating is expected to be minimal compared with heating required during winter to prevent freezing. Trickle liquid may be introduced through either buried porous pipes or through nozzles. There may be hydraulic separation between the biotrickling zone and biofilter zone to minimize leakage of biotrickling solution into the biofilter zone, for example using baffles 218. Similarly, the biofilter zone may be divided into two sections (i.e. an intermediate biofilter zone and a biofilter zone) permit use of different media, or different irrigation schedules. For tropical climates, evaporative cooling/humidification may be more energy efficient, and a separate humidifier may be provided.
Sample dimensions and other parameters are provided in Table 3.

TABLE 3

Design of Sample Horizontal Biofilter

Inlet gas hydrogen sulphide concentration	50 ppm average,
	peaking to 100 ppm
Inlet gas organic sulphur compounds	0.5-10 ppm
Inlet gas odour	10,000 to 100,000
	D/T
Hydrogen sulfide removal	99%
Organic sulphur compounds removal	>90%
Total odour removal	>90%
Tank height, ft	6
Tank width, ft	5
Tank length, ft	6
BTF width, ft	1
Air Flow, cfm	440
BTF Design
Media	Expanded glass
	beads, 8-16 mm
	diameter or
	structured media
	such as HD QPac by
	Lantic Products
BTF X-sectional area, ft2	27.5
BTF length, ft	1
Velocity, fpm	16
BTF volume, ft3	27.5
BTF Residence time, sec	3.75
BTF media PD, in Water column	1.5
BTF trickle rate	0.7 to 1.4 gpm/ft2
	of reactor surface
BTF trickle liquid pH	1.5-2
BF Zone 1 (BF 1) Design
Media	Biosorbens XLD
BF 1 height, ft	5.5
BF 1 Width, ft	5
BF1 Cross-sectional area, ft2	27.5
Velocity, fpm	16
Pressure drop, in WC/ft	0.23
EBRT, s	7.5
BF 1 media Volume, ft3	55
BF 1 Length, ft	2
Irrigation rate, ft3 of water/ft2 of media surface/day	0.05
Irrigation application frequency	3 times/day
BF Zone 2 (BF 2) Design
Media	Biosorbens XLD
BF 2 height, ft	5.5
BF 2 Width, ft	5
BF2 Cross-sectional area, ft2	27.5
Velocity, fpm	16
Pressure drop, in WC/ft	0.23
EBRT, s	75
BF 2 media Volume, ft3	55
BF 2 Length, ft	2
Irrigation rate, ft3 of water/ft2 of media surface/day	0.005
Irrigation application frequency	1 times/day

An alternate example of a reactor 300 is shown in FIG. 3. The reactor 300 operates in a similar fashion to reactor 100, however, only a biotrickling zone 302 and a biofilter zone 306 are provided. Further, the reactor 300 defines a generally vertical cylinder, and the zones are concentric, with outside-in gas flow. Dividers, for example plastic mesh dividers, may be provided between the zones. The reactor 300 includes an outer concentric zone defining a gas inlet plenum 308. A perforated cylindrical wall 312 permits the gas to pass inwardly from the inlet plenum 308 into biotrickling zone 302, which is in communication with and horizontally spaced from the inlet plenum 308. The biotrickling zone has a low pH and is provided with coarse LWE media for elemental sulphur control. Drainage is collected from the biotrickling zone 302 in a sump 320 and is recycled to the top of the biotrickling zone via line 322 using a recirculating pump 324. Excess water may be discharged from the sump via line 326 to maintain desirable pH conditions (e.g. between 1.5 and 2). From the biotrickling zone 302, the gas passes inwardly to the a biofilter zone 306, which is in communication with and horizontally spaced from the biotrickling zone 302. The biofilter zone may have a neutral pH and fine LWE media for high removal of total reduced sulphur compounds. The biofilter zone 306 may be provided with once-through secondary effluent or tap water, via line 328, for neutral pH conditions. If the water is re-circulated, the pH may be controlled to near neutral conditions. From the biofilter zone 306, the treated gas passes into a central outlet manifold 318.

EXAMPLE 1

A biofiltration system was tested with a 4-8 mm expanded glass coated media for treatment of hydrogen sulphide bearing air. A 3-ft deep bed in an 8 inch diameter column was tested to simulate the conditions in a low bed depth biofilter. Conditions were as a follows:
Apparent air velocity: 11.5 ft/min
Bed depth: 3 ft
Inlet H₂S concentration: 31.2 ppmv
Temperature: 81 F
Outlet and mid point concentrations were measured. Results are as follows:
Mid point, representing 7.5 second empty bed residence time: 4.7 ppm v
Outlet, representing 15 second empty bed residence time: 0 ppm v
This demonstrates that at low air velocity and shallow bed depth, similar to that proposed for the concentric design, very high removal efficiencies are achievable in a biofilter.

EXAMPLE 2

A biotrickling filtration system was tested with an 8-12 mm expanded glass uncoated media for hydrogen sulphide treatment. The objective was to test velocities that would typically exist in the reactor with BTF. Operating conditions and results are as follows:
Apparent air velocity: 23 ft/min
Bed depth: 3 ft
Inlet H2S concentration: 99-100.9 ppm v
Temperature: 74.3
Outlet and mid point concentrations were measured. Results are as follows:
Mid point, representing 4 s EBRT: 9.3 ppm v
Top point, representing 8 s EBRT: 4.2 ppm v
This indicates very high performance at typical EBRT and velocity in the BTF section of the reactor. It would appear that a 4 s EBRT in BTF, followed by 15 s EBRT in BF will provide very high removal of H2S and other reduce sulphur compounds.

EXAMPLE 3

The biofiltration system of Example 1, using process conditions of Example 1, but with no hydrogen sulphide and 5 ppm of dimethyl sulphide was tested after about three weeks of acclimation. DMS was added for an 8-hour period. DMS is considered to be one of the most difficult compounds to remove. 51% removal was achieved.
A system or process as described above may result is a high level of treatment in an efficient manner, such as for gases containing a mixture of hydrogen sulphide and reduced sulphur compounds, and where acid generated by hydrogen sulphide treatment might otherwise prevent biological degradation of reduced sulphur compounds such as methyl mercaptan and dimethyl sulphide. System pressure drop may be low, while providing high velocity at the points of high reaction rates.

EXAMPLE 4

A horizontal system with a biotrickling zone, an intermediate biofilter zone and a biofilter zone, all arranged in series, with air passing from inlet plenum to the biotrickling zone to the intermediate biofilter zone and the biofilter zone, and finally to an outlet plenum was tested. Dimensions of the horizontal flow system are as follows:
Operating Conditions:

- Overall media Bed Dimensions: 5.5 ft [1.7 m] high×5 ft [1.7 m] wide by 5 ft [1.7 m] deep
- Biotrickling zone bed depth: 1 ft [0.3 m]
- Intermediate Biofilter Zone depth: 2 ft [0.61 m]
- Biofilter Zone 2 depth: 2 ft [0.61 m]
- Biotrickling zone Media: HD Q-Pac Manufactured by Lantic Products Inc. Agoura Hills, Calif. Media volume: 27.5 ft3 [0.78 m3]. Media surface: 5 ft2 [0.46 m2]
- Intermediate biofliter zone Media: Biosorbens XLD, manufactured by Biorem Technologies Inc., Guelph, Ontario. Media volume: 55 ft3 [1.56 m3]. Media surface: 10 ft2 [0.93 m2]
- Biofilter zone Media: Biosorbens XLD, manufactured by Biorem Technologies Inc., Guelph, Ontario. Media volume: 55 ft3 [1.56 m3]. Media surface: 10 ft2 [0.93 m2]
- Biotrickling zone trickle rate: 0.7 gpm/ft2 [28.5 L/minute/m2] of media surface.
- Biotrickling zone trickle flow rate: 3.5 gpm [13.2 L/minute]
- Intermediate Biofilter Zone irrigation rate: 0.044 ft3/ft2/d [0.013 m3/m2/d]. Irrigation average flow rate: 0.44 ft3/d [0.012 m3/d]. Instantaneous flow: 9.2 gpm [34.8 L/minute]
- Biofilter Zone irrigation Rate: 0.013 ft3/ft2/d [0.004 m3/m2/d]. Irrigation average flow rate: 0.13 ft3/d [0.004 m3/d]. Instantaneous flow rate: 2.8 gpm [10.6 L/minute]
- Air source: Sewage pump station and sludge storage and stabilization
- Air flow rate: 412 cubic feet/minute [11.7 m3/minute]
- Biotrickling zone empty bed residence time (volume of reactor without media/air flow rate): 4 seconds
- Intermediate biofilter zone empty bed residence time: 8 seconds
- Biofilter zone empty bed residence time: 8 seconds

Procedure:
Contaminated air was passed through the media bed, which had already been acclimated over an approximately 40 days of operation. Inlet and outlet samples were taken and analyzed for by St. Croix Sensory using gas chromatography. Detailed information on the laboratory and methodology is as follows:


COLUMBIA ANALYTICAL SERVICES, INC.
RESULTS OF ANALYSIS
Page 1 of 1

Client:	St Croix Sensory, Incorporated
Client Sample ID:	TRS OUTLET 1	CAS Project ID:	P0800552
Client Project ID:	BioRem/No. Hatch Is./1849	CAS Sample ID:	P0800552-003
Test Code:	ASTM D 5504-01	Date Collected:	Mar. 4, 2008
Instrument ID:	Agilent6890A/GC13/SCD	Time Collected:	NA
Analyst:	Zheng Wang/Chris Comett	Date Received:	Mar. 5, 2008
Sampling Media:	1.0 L Tedlar Bag	Date Analyzed:	Mar. 5, 2008
Test Notes:		Time Analyzed:	12:33
		Volume(s) Analyzed:	1.0 ml(s)

Inlet sample results are as follows:


		Result	MRL	Result	MRL
CAS #	Compound	μg/m_—	μg/m_—	ppbV	ppbV

7783-06-4	Hydrogen Sulfide	39,000	14	28,000	10
463-58-1	Carbonyl Sulfide	230	25	94	10
74-93-1	Methyl Mercaptan	1,100	20	540	10
75-08-1	Ethyl Mercaptan	ND	25	ND	10
75-18-3	Dimethyl Sulfide	78	25	31	10

Outlet sample results are as follows:


		Result	MRL	Result	MRL
CAS #	Compound	μg/m_—	μg/m_—	ppbV	ppbV

7783-06-4	Hydrogen Sulfide	14	7.0	10	5.0
463-58-1	Carbonyl Sulfide	220	12	88	5.0
74-93-1	Methyl Mercaptan	10	9.8	5.3	5.0
75-08-1	Ethyl Mercaptan	ND	13	ND	5.0
75-18-3	Dimethyl Sulfide	35	13	14	5.0
75-15-0	Carbon Disulfide	14	7.8	4.5	2.5
75-33-2	Isopropyl Mercaptan	ND	16	ND	5.0
75-66-1	tert-Butyl Mercaptan	ND	18	ND	5.0
107-03-9	n-Propyl Mercaptan	ND	16	ND	5.0
624-89-5	Ethyl Methyl Sulfide	ND	16	ND	5.0
110-02-1	Thiophene	ND	17	ND	5.0
513-44-0	Isobutyl Mercaptan	ND	18	ND	5.0
352-93-2	Diethyl Sulfide	ND	18	ND	5.0
109-79-5	n-Butyl Mercaptan	ND	18	ND	5.0
624-92-0	Dimethyl Disulfide	25	9.6	6.6	2.5

Removal of total reduced sulphur compounds (TRS), including hydrogen sulfide was 99.6%. Removal of organic sulphur compounds (OSC) was 95.5%.
In addition, odour was tested using ASTM E679 & EN13725 methods for St. Croix Sensory. Results are presented below. Very low outlet values of 320 and 600 D/T were obtained and average odour reduction exceeded 98%.
The removal rates obtained in a total EBRT of 20 seconds are surprisingly high when compared with state of art technologies involving a separate biotrickling filter and biofilter, which may use total EBRT of 45 seconds or higher to achieve similar results. It is believed that these results were achieved due to the process configuration and the multistage process disclosed in this invention.

EXAMPLE 5

A horizontal system with a biotrickling zone, an intermediate biofilter zone and a biofilter zone, all arranged in series, with air passing from inlet plenum to the biotrickling zone to the intermediate biofilter zone and biofilter zone, and finally to an outlet plenum was tested. Dimensions of the horizontal flow system are as follows:

- Location: Preston, Ontario sewage treatment plant
- Operating Conditions:
- Overall media Bed Dimensions: 5.5 ft [1.7 m] high×5 ft [1.7 m] wide by 5 ft [1.7 m] deep
- Biotrickling filter bed depth: 1 ft [0.3 m]
- Intermediate Biofilter zone depth: 2 ft [0.61 m]
- Biofilter Zone depth: 2 ft [0.61 m]
- Biotrickling Media: HD Q-Pac Manufactured by Lantic Products Inc. Agoura Hills, Calif. Media volume: 27.5 ft3 [0.78 m3]. Media surface: 5 ft2 [0.46 m2]
- Intermediate Biofilter zone Media: Biosorbens XLD, manufactured by Biorem Technologies Inc., Guelph, Ontario. Media volume: 55 ft3 [1.56 m3]. Media surface: 10 ft2 [0.93 m2]
- Biofilter Zone Media: Biosorbens XLD, manufactured by Biorem Technologies Inc., Guelph, Ontario. Media volume: 55 ft3 [1.56 m3]. Media surface: 10 ft2 [0.93 m2]
- Biotrickling filter trickle rate: 0.7 gpm/ft2 [28.5 L/m/m2] of media surface.
- Biotrickling filter trickle flow rate: 3.5 gpm [13.2 L/m]
- Intermediate Biofilter Zone irrigation rate: 0.044 ft3/ft2/d [0.013 m3/m2/d]. Irrigation average flow rate: 0.44 ft3/d [0.012 m3/d]. Instantaneous flow: 9.2 gpm [34.8 L/m]
- Biofilter Zone irrigation Rate: 0.013 ft3/ft2/d [0.004 m3/m2/d]. Irrigation average flow rate: 0.13 ft3/d [0.004 m3/d]. Instantaneous flow rate: 2.8 gpm [10.6 L/m]
- Air source: Sewage head works, primary clarifier, intermediate pump station
- Airflow rate: 660 cubic feet/minute [18.7 m3/min]
- Biotrickling filter empty bed residence time (volume of reactor without media/air flow rate): 2.5 seconds
- Intermediate Biofilter Zone empty bed residence time: 5 seconds
- Biofilter Zone empty bed residence time: 5 seconds

Procedure:

Contaminated air was passed through the media bed, which had already been acclimated over an approximately 30 days of operation. Inlet and outlet samples were taken and analyzed using a gas chromatograph with SCD sulphur detector. Results are as follows:


Inlet, ppm by	Outlet, ppm by	Percent
volume	volume	removal

Hydrogen	31.7	Non Detect	100%
sulphide
Methyl mercaptan	0.4	Non Detect	100%
Total reduced	32.1	Non Detect	100%
sulphur
compounds

The process has succeeded in achieving complete removal of hydrogen sulphide and organic sulphur compounds in less than 12.5 s. This is a surprisingly high performance compared with state of art technology where this gas stream may require up to 10 second of biotrickling filtration and 30 second biofiltration to achieve similar performance. It validates the concept of synergistic performance, in particular the use of horizontal flow, which permits providing variable process conditions along the length of gas flow to achieve complete removal.

EXAMPLE 6

A horizontal gas flow biotrickling filtration system was tested with an 8-12 mm expanded glass uncoated media for hydrogen sulphide treatment. The objective was to test performance with a non-compressible media using velocities that would typically exist in a horizontal BTF, but at a much higher concentration than was considered acceptable in a vertical biotrickling filter with this media due to high pressure drops. The reactor consisted of four equal sections that were independently monitored using continuous Odalog® hydrogen sulphide monitors. Dimensions of each reactor section (Sections 1 to 4) were as follows:
Length of bed: 8 inches (0.203 m)
Height of bed: 8 inches (0.203 m)
Width of bed: 8 inches (0.203 m)
Operating conditions were as follows:
Air flow: 9 ft3/minute (15.3 m3/h)
Apparent air velocity: 20.25 ft/min (0.103 m/s)
Recirculating Trickle rate: 1.4 gpm/ft2 (57 L/m2) of bed surface area)
Inlet H2S concentration: 200 ppm v
Temperature: 20-38 C
Recirculating liquid pH: 1.5-2
EBRT were as follows:
Section 1: 2 seconds
Section 2: 2 seconds
Section 3: 2 seconds
Section 4: 2 seconds
The system exhibited improved performance throughout the period with periodic fluctuations due to operational issues such as exhaustion of hydrogen supply for short periods, or mechanical breakdown of nutrient feed system. The following table presents data for two days, as these are considered typical after the initial acclimation period.
Horizontal BTF performance for two days under stable conditions


Day and	Temp			Section 1	Section 2	Section 3	Section 4
Time	Deg. C.	Parameter	Inlet	Out	Out	Out	Out

		EBRT, s		2	4	6	8
1 15:40	28	H₂S	199.1	40.8	8.7	5.7	3.6
2 16:40	25	Concentrations,	205.2	44.9	3.8	0.8	0.1
		ppmv
1 15:40	28	Removal		80%	96%	97%	98%
2 16:40	25	Efficiency		78%	98%	100%	100%
1 15:40	28	Elimination		402.13	241.84	163.77	124.16
2 16:40	25	Capacity, g		407.21	255.81	173.08	130.25
		H₂S/m³of
		media/h
1 16:00	28	Pressure Drop,		0.67	0.61	0.67	0.53
2 16:00	25	kPa/m of Bed		0.66	0.47	0.49	0.56

These results are highly surprising as very high removal efficiencies and elimination capacities were achieved at 2 and 4 seconds EBRT at very acceptable pressure drop. A stand-alone vertical BTF will typically be designed for 12 seconds, and up to 20 seconds to provide 99% removal, and to guarantee this performance under fluctuating load conditions. A horizontal BTF, which is integrally connected to a BF, need provide only 4 second EBRT even at 200 ppmv inlet concentration to achieve complete hydrogen sulphide removal, as any remaining contaminant will be removed in the BF. This will result in a very small BTF section and a small total reactor volume.
While the above description provides examples of one or more processes or apparatuses, it will be appreciated that other processes or apparatuses may be within the scope of the accompanying claims.

Claims

1. A process comprising

a) passing a gas to be treated through two or more adjacent zones, the zones differing in one or more of (a) extent of biotrickling filter conditions or biofilter conditions, (b) changing media characteristics, c) liquid or nutrient recirculation rates or frequency, (d) pH, (e) gas velocity, (f) retention time along the gas flow passage (g) extent of irrigation if any or (h) cross-sectional area.

2. The process of claim 1 further comprising a step of flowing a liquid vertically through at least one of the zones.

3. The process of claim 2 wherein the liquid comprises water, nutrients, and acid.

4. The process of claim 2 wherein the liquid is water collected from the bottom of at least one of the zones.

5. The process of claim 1 further comprising

a) providing zonal control of process conditions as the gas passes from an inlet to an outlet and liquid is introduced at the top of one or more of the zones and flows down by gravity.

6. A process comprising,

a) providing a gas to a reactor;

b) passing the gas in a generally horizontal direction through a biotrickling zone of the reactor

c) passing the gas in a generally horizontal direction through a biofilter zone of the reactor.

7. The process of claim 6 further comprising

d) passing the gas in a generally horizontal direction through an intermittent biotrickling or biofilter zone positioned between the biotrickling zone and the biofilter zone of the reactor.

8. The process of claim 6, further comprising:

d) selecting an inert media for the biotrickling zone and a biologically active media for the biofilter zone.

9. The process of claim 6, further comprising:

d) recirculating liquid through the biotrickling zone continuously, and;

e) providing once through, and intermittent irrigation of the biofilter zone.

10. The process of claim 9, further comprising

f) passing the gas in a generally horizontal direction through an intermittent biotrickling zone positioned between the biotrickling zone and the biofilter zone of the reactor

g) providing once-through and intermittent irrigation to the intermittent biotrickling zone, wherein the irrigation provided to the intermittent biotrickling zone is provided at a higher flow or frequency than the irrigation provided to the biofilter zone.

11. The process of claim 6, further comprising introducing a liquid at a top of the biotrickling zone, and distributing the liquid evenly across a media provided in the biotrickling zone.

12. The process of claim 9 further comprising introducing irrigation liquid at a top of the intermittent biotrickling zone or the biofilter zone periodically.

13. A reactor comprising

a) a gas inlet

b) a biotrickling zone in communication with the inlet;

c) a biofilter zone in communication with the biotrickling zone and spaced horizontally from the biotrickling zone; and

d) a gas outlet in communication with the biofilter zone.

14. The reactor of claim 13, further comprising an intermittent biotrickling zone in communication with and spaced horizontally from the biotrickling zone and in communication with and spaced horizontally from the biofilter zone.

15. The reactor of claim 14, wherein the intermittent biotrickling zone is provided between and adjacent the biotrickling zone and the biofilter zone.

16. The reactor of claim 13, wherein the reactor is generally rectangular.

17. The reactor of claim 13, further comprising an inlet gas distribution manifold.

18. The reactor of claim 17, wherein the biotrickling zone and biofilter zone are provided adjacent a first side of the inlet gas distribution manifold, and the reactor further comprises:

a) A second biotrickling zone provided adjacent a second opposed side of the inlet gas distribution manifold; and

b) A second biofilter zone in communication with and spaced horizontally from the second biotrickling zone.

19. The reactor of claim 13, further comprising a sump positioned to collect draining liquid from the biotrickling zone and the biofilter zone, and a recycle pump in communication with the sump and configured to provide the drainage liquid to the biotrickling zone.

20. The reactor of claim 13, further comprising a an intermediate biofilter zone in communication with and spaced horizontally from the biotrickling zone and in communication with and spaced horizontally from the biofilter zone