Biofilter

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Exhaust pipe of the biofilter of the Oberostendorf household waste dump in Ostallgäu

A biofilter is a biologically active fixed bed reactor for purifying air or water. This is not a filter in the true sense of the word, as the main purpose is to separate gaseous or dissolved substances and not solid particles. In contrast to Biorieselbettreaktor one hand, in which is formed on internals in the reactor a so-called biological grass, which is continuously purged, and the bioscrubber the other hand, in which the microorganisms are mainly suspended in a washing liquid, the microorganisms in the biofilters on a matrix partially ensures the supply of nutrients, fixed.

The idea of ​​biologically cleaning exhaust air already existed in the 1920s and technical use took place in the 1960s at the latest. Over the years, biofilters have been optimized for a wide variety of applications.

functionality

Biofiltration is a relatively simple and inexpensive method of cleaning odorous and VOC- containing exhaust air. Microorganisms break down pollutants and odorous substances into harmless products such as carbon dioxide and water. Biofilters are mainly used for exhaust air purification. Biofilters are also used for special tasks in water and wastewater treatment. The following information relates to exhaust air cleaning. The processes for the biofiltration of water are based on the same principles and are adapted to the liquid medium.

Biological exhaust air purification uses microorganisms to remove pollutants from the air through microbial degradation. The substances contained in the air are broken down by various organisms such as bacteria and fungi. Put simply, the microorganisms convert the pollutants into carbon dioxide and water with the help of oxygen, which means that it is a substance-destroying process. This reaction can only take place if the pollutants are transferred from the gas to the water phase, since the water forms the habitat of the microorganisms. This is why the transfer of the pollutants into the aqueous phase is an important factor in all biological processes. Those microorganisms prevail which can best adapt to the prevailing conditions and the food supply, i.e. the pollutants to be cleaned. It is always a mixture of different heterotrophic species that use the pollutants in the air as a source of carbon and energy.

Design and mode of operation

Biofilters are operated in many different designs, such as surface filters, container filters, floor filters, honeycomb filters and tower filters. With all designs, the exhaust air flows through a filter material bed. In some cases an air washer is arranged in front of the biofilter, in which the air is brought to a saturation level of almost 100% relative humidity. This is to prevent the material from drying out. In addition, the removal of particles from the exhaust air can take place in the air washer, if required. The water vapor-saturated and dust-free raw gas is then fed to the biofilter in which the filter material is located. This is always kept moist by additional irrigation. The microorganisms are immobilized on the filter material. When flowing through the filter layer, the substances contained in the exhaust air sorb on the surface of the material and are thus available for the microorganisms to break down. In order to ensure high microbial activity in the filter, optimal conditions for the microorganisms in terms of pH value, humidity of the filter material, temperature and nutrient supply must be maintained. Practice shows that the mixed microbial populations that develop in biofilters are very robust if certain parameters are observed.

Filter material

Various requirements are placed on the filter material. It should have a large specific surface and thus a large growth area for the microorganisms, which can store moisture well, cause a low pressure loss when flowing through, have a certain buffer capacity against fluctuations in the pH value, enable a uniform flow through the filter bed and have a low rate of rotting . In addition, the microorganisms should be supplied with inorganic nutrients and trace elements. The following substances, also as mixtures, are suitable as filter material:

In addition to loosening these materials, inert additives such as expanded clay , styrofoam or foam are added. The filter material is not only a carrier for the microorganisms, but also a nutrient supplier.

Advantages and disadvantages of biofiltration

When operating a biofilter, one of the main problems is to prevent the filter material from drying out or becoming wet in places, thereby enabling a uniform flow through the filter bed. This can be achieved above all by encapsulating the biofilter. The large space requirements of these systems, the cost-intensive fan energy for increasing the pressure and the permanent irrigation are often disadvantageous. Compared to other processes, such as ionization with ionization tubes, the constant biological cleaning process is often advantageous due to CO 2 savings and numerous economic aspects such as average acquisition costs, long filter service life and average operating costs.

Process engineering basics

The procedural basis is essentially a biochemical oxidation and thus degradation and conversion of the substances by bacteria , fungi and yeast into harmless and odorless substances. The prerequisites are that the harmful substances are water-soluble, biodegradable and non-toxic for the microorganisms. The microorganisms sessile on suitable filter material utilize the VOC substances contained in the exhaust air for their metabolic processes. The degradation takes place under aerobic conditions and sufficient oxygen must be available. The latter is usually given by the oxygen contained in the exhaust air. Exhaust air saturated with water vapor should be used, as the filter material must be moist.

Areas of application for biofilters in exhaust air purification

Biofilters are mainly used for exhaust air purification in the following systems:

  • Wastewater treatment (both municipal and industrial)
  • Waste recovery, composting plants, MBAs
  • Surface coatings with solvents (metals, wood and plastics)
  • Food processing, mushroom cultivation, smokehouse
  • Oil mills and malt houses
  • Agricultural equipment
  • Biogas plants
  • (Passive) landfill degassing
  • Rendering plants
  • Feed production plants
  • Slaughterhouses
  • Exhaust air from the sludge drying systems
  • Odor-polluted production air

Biofilter to remove odors

The main area of ​​application of biofilter systems is traditionally in the cleaning of odorous exhaust air. The microbial degradation of the odorous and pollutants to CO 2 and H 2 O or compounds that can no longer be perceived by smell takes place at ambient temperature, so that no additional energy or additives have to be used. The operating costs are consequently very low with this method. Biofiltration is state-of-the-art in many areas. In Germany and Europe, thousands of biofilter systems are used to reduce odors from a wide variety of emission sources. Odor problems are common in the vicinity of sewage treatment plants, landfills, foundries, breweries, the food processing industry, rendering facilities, garbage processing plants, farms and slaughterhouses. Biofiltration is the most cost-effective and most reliable method for eliminating odorous substances - efficiencies of up to 99% are achieved.

Biofilter for solvent elimination

With the redefinition and tightening of the emission limit values ​​of the 31st Federal Immission Control Ordinance, the cleaning of solvent-contaminated exhaust air from various branches of industry such as B. furniture painting, surface coating, chemical industry, electroplating, plastics processing, painting of car parts, etc. also increasing use of biofilter systems.

Most solvents represent a well-suited carbon source for the microorganisms, which they can oxidize for their energy production and convert into non-harmful compounds. No auxiliary fuels such as oil or gas are required in this process. Advantages of biofiltration compared to thermal processes are therefore the CO 2 -neutral functionality and the low operating costs.

From a hydrocarbon concentration of more than 1,500 mg / m 3 , biological methods for solvent elimination are only suitable to a limited extent - here, other methods of exhaust air purification such as regenerative post-combustion should be used.

Biofilter for sewer shafts

One area of ​​application is, for example, the biofilter for sewer manholes , which is hung in a sewer manhole under the dirt trap and is intended to prevent strong odors from escaping from the sewer system , especially at the outlet of pressure pipelines . It contains an integrated dirt trap and a filter filling (mixture of peat and packing for microorganisms). The surface water that arises is collected in a collecting funnel under the sludge trap and drained off via a downpipe. A rubber seal prevents uncleaned air from escaping. Modern filters no longer hinder the passage of air. Modern paper-based filters are maintenance-free and work for 5 to 6 years without constant monitoring and care. The efficiency is 99%.

Biofilter for landfill gas treatment

If the methane content in the landfill gas falls and the volume flow decreases accordingly, autothermal recycling or treatment of the gas is generally no longer possible. In such cases, microbial methane oxidation in the biofilter is an option. Biofilters for methane degradation are operated both as passive filter systems, which are only flowed through due to the pressure differences between the landfill body and the atmosphere, and as active biofilter systems. It should be noted that due to the low water solubility of methane combined with correspondingly low reaction rates, the residence times must be selected to be correspondingly long. The formation of extracellular polymeric substances by the ubiquitous methanotrophic bacteria can lead to gelatinization of the biofilter. In addition, certain substances present in the landfill gas or in the biofilter, in particular ammonium , can have an inhibiting effect.

Biofilter to reduce bioaerosols

In agricultural livestock farming, biofilters are mainly used to separate odors from the exhaust air from pig stalls. However, bioaerosols can also be minimized through the use of biofilters . The secondary emissions of microorganisms emerging from the filter are significantly lower compared to the separated microorganisms.

Framework conditions for biological exhaust air purification

The most important prerequisites for the functioning of biofilters are:

  • Maintain favorable temperatures (5 to 40 ° C, in exceptional cases up to 55 ° C).
  • Moistening of the filter material to prevent the filter material from drying out.
  • Homogeneous filter material in order not to allow unfiltered passage through the material in cracks and crevices.
  • Ingredients to be cleaned in the exhaust air must be soluble in water.
  • Ingredients to be cleaned in the exhaust air must be biodegradable.
  • Supplementary nutrients in the filter material.

Wastewater treatment in biofilters

In most cases, wastewater can be treated biologically, but it should be noted that the water quality depends on the ingredients. The water is cleaned biologically by adsorption , natural precipitation reactions , plants and microorganisms as well as bacteria . This process is used in the plant-based sewage treatment plants to purify uncleaned water and thus significantly improve the water quality. In the technical wastewater treatment plants , the wastewater is treated biologically, among other things.

Regeneration, replacement, disposal

Depending on the material, the service life is two to five years, longer with higher inert contents in the filter material. The degradation processes slowly mineralize the filter material and the existing nutrients are used up. Before replacing it, it should be tested whether regeneration of the filter material is still possible (biological activity, water absorption / retention capacity, ...).

There are mechanical and biochemical methods of regeneration. Often, mechanical elements such as loosening up clumped material, separating the fine grain fraction (with a sieve drum), adding fresh material are sufficient. For biochemical regeneration, nutrients and additives can, if necessary, be added to adjust the pH value, for example (with lime or sulfur, ideally where the raw gas first enters). These additions should reach the surface of the filter material.

An exchange of filter material is necessary if

  • the volume flow is no longer achieved due to the increase in differential pressure ,
  • an uneven flow due to drying out, mineralization or over-humidification of partial areas can no longer be remedied or
  • the effectiveness of the biofilter decreases despite care and maintenance.

On delivery, the product description should contain recommendations for disposal. The circular economy law and its post-statutory regulations such as B. Biowaste Ordinance , Fertilizer Ordinance .

Individual evidence

  1. ↑ Klaus Fischer: Degradation of foreign matter in the air . In: Johannes CG Ottow, Werner Bidlingmaier (Ed.): Environmental biotechnology . Gustav Fischer Verlag, Stuttgart / Jena / Lübeck / Ulm 1997, ISBN 3-437-25230-5 , pp. 317-349.
  2. ↑ Hermann Bubinger, Hans-Gerd Schwinning; Basics and application examples of biofilter technology ; In: WLB Wasser, Luft und Boden , 5, 1992, p. 66
  3. ↑ Hermann Bubinger, Hans-Gerd Schwinning; Basics and application examples of biofilter technology ; In: WLB Wasser, Luft und Boden , 5, 1992, pp. 66-70
  4. ↑ [http * // www.bionik.at/anwendungsgebiete.php application areas for biofilters]; Retrieved December 25, 2010
  5. ↑ a b c d e VDI 3477: 2014-05 (draft) Biological exhaust gas cleaning - biofilter . Berlin: Beuth Verlag, pp. 27-30.
  6. ↑ a b c Uwe Walter: Biofilter systems for breaking down methane-containing gases . Hazardous substances - keeping the air clean 73 (2013) No. 5, pp. 183–186.
  7. ↑ Jochen Hahne: Biofilters in animal husbandry . Hazardous substances - keeping the air clean, Volume 73 (2013) 5, pp. 187–191.
  8. ↑ VDI 4255 sheet 2: 2009-12 Bioaerosols and biological agents; Sources of emissions and mitigation measures in farm animal husbandry; Overview (Bioaerosols and biological agents; Emission sources and control measures in livestock operations; Overview). Beuth Verlag, Berlin. P. 21.
  9. ↑ Torsten Herold, Mirko Schlegelmilch, Bernd Dammann, Jan Streese, Rainer Stegmann, Andreas Hensel: Germ retention in the treatment of intensive rotting air in a bio-washer / bio-filter system . Hazardous substances - keeping the air clean, Volume 61 (2001) 6, pp. 255–260.
  10. ^ Andreas Oberhammer; Method for uniformly moistening a filter mass ; Patent specification; January 13, 1997; Retrieved December 25, 2010
  11. ^ Andreas Oberhammer; Filter mass for a biogas filter ; German Patent Office: March 11, 1993; Retrieved December 25, 2010
  12. ↑ a b c d VDI 3477: 2014-05 (draft) Biological exhaust gas cleaning - biofilter . Berlin: Beuth Verlag, pp. 16-17.