Air filter by biological filtration method

What is biofilter?

Biofiltration is a relatively new treatment for pollution. This is an attractive method for handling bad smell gases and volatile organic compounds with low concentrations.

The common shape of a biological filtration system is like a large box, some systems can be as big as a basketball court, some systems can be as small as a yard yard (0.76 m3). The main principle of the treatment system is to allow bacteria to come into contact with pollutants in the exhaust gas. This exhaust filter system is a place to store filter materials and breeding grounds for microorganisms. In this system, microorganisms will form a biofilm, a thin and moist membrane surrounding the filter material. During the filtration process, exhaust gas is pumped slowly through the filter system, pollutants in the exhaust gas will be absorbed by the filter materials. The polluting gases will be adsorbed by biofilms, where microorganisms will decompose them to create energy and byproducts of CO2 and H ₂ O according to the following equation:

Organic pollutants + O ₂ ---> CO ₂ + H ₂ O + heat + biomass

History

While the use of biological filtration systems has not been popular in the US, hundreds of biofiltration systems have been successfully and effectively applied in Europe (Netherlands, New Zealand, Germany) and Japan. Copy. Previous biofiltration systems are often designed to handle the odors of wastewater treatment systems, recycling plants, composting processes. Later, it is commonly used in the treatment of volatile organic compounds and other organic compounds.

Here are some historical milestones of the development of biofilter systems:

1. 1923 Bioremediation is recommended for the treatment of foul-smelling gases.
2. 1955 The biological treatment method is applied to treat bad odor gases in low concentrations in Germany.
3. 1960 Biofilters are used to treat polluted gases in Germany and the United States.
4. 1970 Biofiltration system achieved high results in Germany.
5. 1980 Biofilter systems are used to treat toxic gases and volatile organic compounds of industries.
6. 1990 Currently, more than 500 biofilter systems are operating in Germany, the Netherlands and widespread in the United States.

   The treatment of odors has been carried out since the 1950s and at that time people often use a soil filter system, or a drip biofilter. Smelly gases are usually hydrogen sulphite or mercaptant and other sulfur compounds. The recent application of volatile organic substances has become popular in the last decade and is now under further investigation. For example, some studies have now demonstrated that biological filtration systems can be used to treat aromatic organic compounds and fatty compounds, alcohols, aldehydes, organic acids, acrylate, Carbolic acid, amines and ammonia.

   Describe the process

   The biofiltration system provides an environment for microorganisms to grow and decompose foul gases and organic pollutants in the exhaust gas. The filtration system consists of a closed chamber containing microorganisms and absorbs steam, keeping them in the filter material. The filtered material is designed to provide a high water absorption capacity, high durability, and less degrading air flow pressure through it.

   

   The biofilter has a filter material (6000 ft2 area) at Monsanto factory

   Smaller, more popular systems with multiple layers of filter materials are shown in the following figure:

   

   These filter material units, called "biocube" (Biocube) designed by EG&G Corporation, are about 7 ft high and about 6 ft in diameter. The use of many layers of this type of filter material prevents the filtered materials from being repressed and traverses airflows into escape routes through the filter material. Moreover, it also facilitates maintenance or replacement of filter materials.

   During biofiltration, pollutants are moistened and then pumped into a chamber below the filter material. When the gas passes through the filter material, pollutants are absorbed and decomposed. The purified exhaust gas is released into the atmosphere from the top of the filtration system. Most of the current biofilter systems have a capacity to treat volatile odor and volatile organic matter greater than 90%. However, the limitation of this method is to only handle emissions with low pollutant concentrations (<1000ppm) and processed air flow within 300-500 ft3 / ft2-hour limits.

   Filter materials

   The layer of moisture filtration materials creates a chemical and physical condition favorable for the conversion of pollutants from the gas phase to the liquid phase and the biodegradation of these pollutants by biofilm. The mechanism of biological filtration involves the absorption, absorption and decomposition by microorganisms. Microorganisms in biofilms continuously absorb and metabolize pollutants, turning them into the final products of water, carbon dioxide and salts.

   Typical filter material is a mixture of composting substrates, soil, heather (heather), plastic and wood by-products. Filter materials are intended to provide a large surface area to absorb and adsorb contaminants. In addition, it also serves to provide nutrients to microorganisms. Some types of filter materials do not meet the nutritional needs of microorganisms, so we must calibrate by adding protein and phosphorus compounds.

   Filter materials usually have a life expectancy of 5-7 years before being replaced.

   Points to consider when deciding on filter materials:

   Moisturizing ability to create biofilm

7. Has a large surface area that facilitates the absorption and development of microorganisms
8. Contains nutrients to supply to microorganisms
9. Create low air resistance (reduce the level of pressure drop and energy required for pumps)
10. Other physical properties such as physical stability and easy manipulation.

    Some design parameters

    Acreage

    Area is a parameter that is of top interest in the design of biological filtration systems. To handle air flow of about 30 ft3 / min, a biological filtration system may need an area of ​​25 ft2. For larger gas flows, we need larger areas and maybe equal to the size of a basketball court as mentioned above.

    Chemical composition and content of pollutants in emissions

    Analyze the chemical composition and its content in the flue gas needed to determine if biological filtration is appropriate. Biofiltration systems work well when pollutants (insoluble in water) have low concentrations (

    Length of stay

    The length of stay is the period of time when microorganisms are exposed to the flue gas stream and are calculated by the following formula:
    RT = Total volume of pores of the filter material / emissions flow
    The longer the stay, the higher the performance. However, during the design process we need to minimize the length of stay so that the system can handle a larger traffic. Typically, the length of stay of biological filtration systems varies from 30 seconds to 1 minute.

    Humidity

    The moisture content of the flue gas stream to be treated is important because it maintains the moisture required for biofilms. Therefore, the flue gas flow is usually pumped through a humidification system before being pumped into the biofilter system to ensure that the moisture content of the exhaust flow into the biological filtration system is greater than 95%.

    PH control

    The byproducts of biodegradation are organic acids. In order to maintain the system pH in a range suitable for active microorganisms, we need to add pH buffer solutions.

    Filter materials

    Filter materials may include peat, heather, compost, granular coal or other suitable materials. In general, these materials must be able to provide nutrients to microorganisms and do not cause excessive airflow pressure. In addition, the moisture content of the filter materials must be maintained at 30-60% for the population of microorganisms to grow. Therefore, in addition to the equipment to moisten the exhaust gas, people often install sprinkler system for the filter material layer.

    Reduce the pressure

    The pressure drop of air flow when passing through the filter material should be minimized. If the filter material layer causes a big impediment to the gas source, we need to consume more energy for the air blower, which increases the processing cost. The ability to react to the gas source depends on the moisture content and the thickness of the filter material. Increased humidity, reduced material layer is the cause of increasing resistance to gas resources. For typical systems the pressure drop level is in the range of 1 -10 hPa.

    Maintenance

    When starting to operate, the system needs to be cared once a week. After the system has operated stably and solved all possible problems. The frequency of visits can be reduced to 1 / half or monthly.

    Advantages and disadvantages of biological filtration system

    Advantages

11. The main advantage is low cost, low operating price, low use of chemicals.
12. Flexible design, thus can adapt to all types of industries and the area of ​​the enterprise.
13. Flexible biofiltration system for handling odors, volatile organic compounds and toxins. Processing efficiency is usually greater than 90% for emissions with concentrations of pollutants
14. Various types of filter materials, microorganisms and operating conditions can be applied to meet the treatment needs.

     

    Defect

15. The biofiltration system cannot handle pollutants with low adsorption capacity and slow biodegradability such as volatile organic compounds containing chlorine.
16. Pollution sources with high chemical concentrations require large systems and large areas to install biofilter systems.
17. The source of pollution has a high degree of release of pollutants that will affect the microbiota as well as their processing efficiency.
18. Time for microorganisms to adapt to the environment and form biofilm can last for weeks to months, especially for the treatment of volatile organic matter.

     

    Commercialization

    There have been more than 50 biofiltration systems using compost as filter materials that have been commercially installed in Europe and the US over the past 15 years.

    The use of biological filtration systems to handle volatile organic substances has been applied in the following industries:

19. Chemical and petrochemical technology
20. Oil and gas technology
21. Synthetic plastic technology
22. Technology for producing paint and ink
23. Pharmaceutical technology
24. Treatment of substances and waste water
25. Treatment of soil and groundwater

     

    The use of biological filtration systems to treat odors has so far been applied in the following industries:

26. Handling sewage water
27. Treatment of slaughtering substances and wastewater
28. Recycling technologies
29. Gelatin and glue factories
30. Technology for processing meat and agricultural products
31. Technology for producing cigarettes, cocoa, sugar
32. Technology for producing spices and artificial smells.

     

    Selvi B. Anit & Robert J. Artuz
    Translation: Le Hoang Viet