Gas Bio-filtration Techniques

Gas Bio-filtration

• A relatively new technology that purifies contaminated air.
• It works against volatile organic and inorganic compounds (VOCs and VICs).
• Also effective against aromatic compounds (BTEX), toxic and odorous substances produced at water treatment and composting plants and odor-producing processes.
• It reduces or removes the need for additional treatment for the end products.
• Popular due to low cost, simple operation, and removal efficiency.

Bio-filtration Effectiveness

• Bio-filtration can reduce hydrogen sulfide (H2S) emissions by up to 95%.
• Ammonia emissions are reduced by up to 65%.
• Bio-filtration is used for odor control in livestock and poultry systems.

Bio-Filter Basics

• A bio-filter consists of a layer of organic material, typically with compost and wood chips.
• The material supports microbes.
• Odorous air is forced through the medium.
• Microbes convert the air to carbon dioxide and water.
• Bio-filters work via adsorption, followed by biodegradation.

Bio-filtration Mechanisms

• Contaminant removal involves multiple steps.
• Contaminant transport occurs to the liquid phase, then to the bacterial cell in the biofilm.
• A transport then occurs across the cell membrane, and compound biodegradation takes place.
• Two mechanisms involved are sorption and biodegradation.

VOC Control Devices

• VOCs from industrial and domestic activities contribute to air pollution.
• Conventional methods (combustion, adsorption) require much energy and cause secondary pollution.
• Biodegradation by microorganisms offers a preferred method for air pollution control of VOCs.
• Types of VOCs include alcohols, ketones, aldehydes and organic acids which are rapidly degraded. -Hydrocarbons, phenols, and organic solvents are slowly degraded.
• Poly aromatic and polyhalogenated hydrocarbons are very slowly degraded.
• Bio-filters, bio-scrubbers, and bio-trickling filters are VOC control devices.

Bio-Filter Components

• Bio-filters use a porous medium (compost and wood chips) to support a film of microorganisms.
• Polluted air is first passed through a humidifier.
• Air is then passed through the bio-filter.
• 90% of VOCs in the air are biodegraded.

Bio-filtration

• Bio-filtration relies on immobilized biomass, an immobile water phase, and single reactors.
• It's advantageous due to simplicity, suitability for soluble pollutants (hydrocarbons), and use for xenobiotic treatment.
• High gas phase surface area, easy operation and startup, low operation costs, and 90% VOC removal make it superior.
• Disadvantages include poor control of operating conditions (pH & T), slow adaptation to gas concentration changes, and large area required.

Bio-Scrubbing

• Bio-scrubbing utilizes suspended biomass, a mobile water phase, and two reactors.
• It offers better control of reaction conditions, compact equipment, and low pressure drop.
• Disadvantages: low surface area for mass transfer, washout of slow-growing microorganisms, disposal of excess sludge, difficult startup process, extra air supply needs at high degradation rates, and high investment, maintenance and operation costs.

Bio-trickling Filtration

• Bio-trickle filters offer a new development and more efficient VOC biodegradation.
• They work on the principles of adsorption, followed by biodegradation.
• Pollutant gases are adsorbed on a solid surface.
• Gases are subjected to biodegradation through a stream of medium with desired microorganisms.
• Bio-trickle filters are still experimental.
• Bio-trickling filtration uses immobilized biomass, mobile water phase, and a single reactor.
• Better retention of slow growing microorganisms.
• Disadvantages: low surface area for mass transfer, disposal of excess sludge, difficult start up procedure, and high operation costs.

Microorganism Immobilization at Filter Bedding

• Carrier binding method binds cells with physical adsorption, ionic or covalent bonds to water-insoluble carriers.
• Polysaccharides like cellulose and dextron, proteins like gelatin and albumin are carrier materials
• Synthetic polymers like ion-exchange resin and PVC and inorganic materials like brick, sand and porous glass are also carrier materials.
• Cross linking method uses bi or multifunctional reagents to immobilize cells
• Reagents include glutaraldehyde, toluene diisocyanate for joining microorganisms to each other to create a large structure.
• Entrapment encapsulates cells into polymer matrices with smaller pores than the cells.
• Matrices used for entrapment are collagen, gelatin, agar, alginate, carrageenan, cellulose, polyacrimide, epoxy resin, polyester, polystyrene, & polyurethane.
• Microencapsulation immobilizes microorganisms in wrapped droplets with a thin membrane.
• The cells can move freely, use substrates that penetrate the membrane, and the microcapsules are mainly constructed of nylon and cellulose nitrate typically 10 to 100 μm.
• Membrane Separation isolates microorganisms from fluid using membrane sheets that allow substrate penetration.
• Porous ultra-filtration membranes (.0002 to 0.1 μm) are used.

Media Characteristics for Bio-filters

• Peat: Average porosity, good moisture & nutrient capacity, and useful life.
• Soil (heavy loam): Poor porosity, good moisture & nutrient capacity and good useful life.
• Compost: Average porosity, retains good moisture & nutrient capacity with good useful life.
• Wood chips (3in): Good porosity, average moisture/nutrient capacity and useful life.
• Straw: Good porosity, average moisture capacity, poor nutrient capacity and useful life.

Influencing Factors in Bio-filter Performance

• Air time in bio-filter and moisture content are factors.
• Design includes the size of filter bed.
• The fan must be selected appropriately.
• Type of bio-filter media is also important in the design.

Advantages

• Lower capital and operation costs, lower chemical usage, and no combustion source.
• Units designed to fit industrial settings.
• Treatis odors, toxic compounds, and VOCs; >90% efficiency for low contamination.
• Different media, microorganisms and operational conditions are possible, for multiple emission points.

Disadvantages

• Ineffective for compounds with low adsorption and degradation rates, particularly chlorinated VOCs.
• Large bio-filters or area is needed to treat chemical sources with high emissions.
• Fluctuating emission sources can harm bio-filter performance and microbial population.
• Long acclimation needed (weeks or months), especially for VOC treatment.