A biofilter is a biologically acting fixed bed reactor for the purification of air or water. It is not a filter in the actual sense, since the main purpose is the separation of gaseous or dissolved substances and not of solid particles. In contrast to the biodiesel bed reactor on the one hand, where a so-called biological lawn forms on internals in the reactor which is continuously rinsed, and the bioscrubber on the other hand, where the microorganisms are predominantly suspended in a washing liquid, the microorganisms in the biofilter are fixed on a matrix which partly provides the nutrient supply.
The idea of purifying exhaust air by biological means existed as early as the 1920s, with technical application taking place in the 1960s at the latest. Over the years, biofilters have been optimized for a variety of applications.
How it works
Biofiltration is a relatively simple and cost-effective method of cleaning exhaust air containing odours and VOCs. Microorganisms break down pollutants and odorous substances into harmless products such as carbon dioxide and water. Biofilters are mainly used for exhaust air purification. However, biofilters are also used for special tasks in water and wastewater treatment.
The following information refers to exhaust air purification. The processes for the biofiltering 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 by microbial degradation. The degradation of the substances contained in the air is carried out by various organisms, such as bacteria and fungi. In simple terms, the microorganisms convert the pollutants into carbon dioxide and water with the help of oxygen, i.e. it is a substance-destroying process.
This reaction can only take place if the pollutants are transferred from the gaseous phase to the aqueous phase, as the water forms the habitat of the microorganisms. Therefore, the transfer of the pollutants into the aqueous phase is an important factor in all biological processes. The microorganisms which are best able to adapt to the prevailing conditions and the food supply, i.e. the pollutants to be cleaned, prevail. This is always a mixture of different heterotrophic species, which use the pollutants in the air as a source of carbon and energy.
Types and mode of operation
Biofilters are operated in many different designs, such as panel filters, container filters, stack filters, honeycomb filters and tower filters. In all designs, the exhaust air flows through a filter material bed. In some cases, an air washer is arranged upstream of the biofilter, in which the air is brought to a saturation content of almost 100 % relative humidity. This is to prevent the material from drying out. In addition, the removal of particles from the exhaust air may also take place in the air scrubber as required. The water vapour-saturated and dust-free raw gas is then fed to the biofilter, which contains the filter material. This is always kept moist by additional irrigation. The microorganisms are immobilized on the filter material. When the air flows through the filter layer, the exhaust air substances sorb on the surface of the material and are thus available to the microorganisms for decomposition. In order to ensure high microbial activity in the filter, optimum conditions for the microorganisms must be maintained with regard to pH value, moisture of the filter material, temperature and nutrient supply. Practice shows that the mixed microbial populations developing in biofilters are very robust if certain parameters are maintained.
A wide range of requirements are placed on the filter material. It should have a large specific surface area and thus a large growth surface for the microorganisms, be able to store moisture well, cause a low pressure loss when flowing through, have a certain buffer capacity against pH-value fluctuations, allow a uniform flow through the filter bed and have a low rotting rate. In addition, the microorganisms should be supplied with inorganic nutrients and trace elements. The following substances, also as mixtures, are suitable as filter material:
- Compost from bark or garbage
- Heather, brushwood or coconut fibres
- Peat products
- Paper granules
In addition to loosening, inert aggregates such as expanded clay, polystyrene or foam are added to these materials.
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 waterlogged in places, thereby enabling a uniform flow through the filter bed. This can be achieved mainly by encapsulating the biofilters. Disadvantages are often the large space requirement of these systems, the cost-intensive fan energy for pressure increase and the permanent watering. Compared to other processes, such as ionisation with ionisation tubes, the constant biological cleaning process is often advantageous due toCO2 savings and numerous economic aspects, such as medium purchase costs, long filter service life and medium operating costs.
Process engineering basics
The process engineering basis is essentially a biochemical oxidation and thus degradation and conversion of the substances by bacteria, fungi and yeasts into harmless and odourless substances. The prerequisites are that the harmful substances are water-soluble, biodegradable and non-toxic to the microorganisms. The microorganisms, which settle on suitable filter material, utilise the VOC substances contained in the exhaust air for their metabolic processes. The decomposition takes place under aerobic conditions and sufficient oxygen must be present. The latter is usually given by the oxygen contained in the exhaust air. Exhaust air saturated with water vapour must be used, as the filter material must be moist.
Areas of application for biofilters for exhaust air purification
Biofilters are mainly used for exhaust air purification in downstream plants:
- Wastewater treatment (both municipal and industrial)
- Waste recycling, composting plants, MBAs
- Surface coatings with solvents (metals, wood and plastics)
- Food processing, mushroom growing, smokehouses
- Oil mills and maltings
- Agricultural facilities
- Biogas plants
- (passive) landfill degassing
- Rendering plants
- Feed production plants
- Exhaust air of the sludge drying plants
- Odorous production exhaust air
Biofilter for odour elimination
The main field of application of biofilter plants traditionally lies in the purification of odour-contaminated exhaust air. The microbial degradation of odorous and harmful substances toCO2 and H2O or odourless compounds takes place at ambient temperature, so that no additional energy and additives have to be used. Consequently, the operating costs are very low with this process. In many areas, biofiltration is state of the art. In Germany and Europe, thousands of biofilter systems are used to reduce odours from a wide variety of emission sources. Odour problems frequently occur in the vicinity of sewage treatment plants, landfill sites, foundries, breweries, the food industry, rendering plants, waste processing plants, agricultural operations and slaughterhouses. Biofiltration is the most cost effective and reliable method of odour removal – achieving efficiencies of up to 99%.
Biofilter for solvent elimination
With the redefinition and tightening of the emission limits of the 31st Federal Immission Control Ordinance, biofilter systems are increasingly being used in the purification of solvent-contaminated exhaust air from a wide range of industries, such as furniture painting, surface coating, the chemical industry, electroplating, plastics processing, painting of car parts, etc.
Most solvents provide a well-suited carbon source for the microorganisms, which they can oxidize and convert into non-harmful compounds for their energy production. This process does not require any supporting fuels such as oil or gas. The advantages of biofiltration compared to thermal processes are thus theCO2-neutralmode of operation and the low operating costs.
From a hydrocarbon concentration of more than 1,500 mg/m3 , biological processes for solvent elimination are only suitable to a limited extent – in this case, other methods of exhaust air purification should be used, such as regenerative post-combustion.
Biofilter for manholes
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 the escape of strong odours from the sewer system, especially at the outlet of pressure pipes. It contains an integrated dirt trap and a filter filling (mixture of peat and fillers for microorganisms). The accumulating surface water is collected via a collection funnel under the sludge trap and discharged via a downpipe. A rubber seal prevents the escape of unpurified air. Modern filters no longer obstruct the passage of air. Modern paper based filters are maintenance free and will operate for 5 to 6 years without constant monitoring and maintenance. The efficiency is 99%.
Biofilter for landfill gas treatment
If the methane content in the landfill gas decreases and the volume flow rate decreases accordingly, autothermal recovery or treatment of the gas is usually 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, through which gas flows only due to 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, especially ammonium, can have an inhibitory effect.
Biofilter for the reduction of bioaerosols
In agricultural livestock production, biofilters are mainly used for odor removal from the exhaust air of pig barns. However, biofilter use can also minimize bioaerosols. The secondary emissions of microorganisms escaping from the filter are significantly lower compared to the separated microorganisms.
General conditions of biological exhaust air purification
The most important requirements for the functioning of biofilters are:
- Maintaining favourable 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 to avoid unfiltered passage through the material in cracks and crevices.
- Substances in the exhaust air that are to be cleaned must be water-soluble.
- Exhaust air substances to be cleaned must be biodegradable.
- Supplemental nutrients in the filter material.
Wastewater treatment in biofilters
In most cases, wastewater can be purified biologically, but it should be noted that the water quality depends on the ingredients. The water is purified biologically by adsorption, by natural precipitation reactions, by plants and microorganisms and by bacteria. This process is used in the constructed wetlands to purify unpurified water and thus significantly improve water quality. In the technical wastewater treatment plants, the wastewater is purified in a biological manner, 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. Due to the degradation processes, a slow mineralisation of the filter material takes place and the available nutrients are consumed. Before replacement, it should be tested whether regeneration of the filter material is still possible (biological activity, water absorption/holding capacity, …).
There are mechanical and biochemical methods for regeneration. Often mechanical methods such as loosening of clumped material, separation of the fines (with a sieve drum), addition of fresh material are sufficient. For biochemical regeneration, if necessary, nutrients and additives can also be added, for example to adjust the pH value (with lime or sulphur, preferably where the raw gas first enters). These additions should reach the filter material surface.
Replacement 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 desiccation, mineralisation or over-wetting of sub-areas can no longer be remedied, or
- the effectiveness of the biofilter decreases despite care and maintenance.
Recommendations for disposal should be included in the product description at the time of delivery. The Closed Substance Cycle Waste Management Act (Kreislaufwirtschaftsgesetz) and its subsequent regulations such as the Biowaste Ordinance (Bioabfallverordnung) and the Fertiliser Ordinance (Düngemittelverordnung) must be observed.
- Klaus Fischer: Fremdstoffabbau in der Luft. In: Johannes C. G. Ottow, Werner Bidlingmaier (Eds.): Umweltbiotechnologie. Gustav Fischer Verlag, Stuttgart/ Jena/ Lübeck/ Ulm 1997, ISBN 3-437-25230-5, pp. 317-349.
- Hermann Bubinger, Hans-Gerd Schwinning; Fundamentals and Application Examples of Biofilter Technology; In: WLB Wasser, Luft und Boden, 5, 1992, p. 66
- Hermann Bubinger, Hans-Gerd Schwinning; Fundamentals and application examples of biofilter technology; In: WLB Wasser, Luft und Boden, 5, 1992, pp. 66-70
- [http*//www.bionik.at/anwendungsgebiete.php applications-for-biofilters]; retrieved 25 December 2010
- VDI 3477:2014-05 (draft) Biological exhaust gas cleaning – Biofilters. Berlin: Beuth Verlag, pp. 27-30.
- Uwe Walter: Biofilter systems for the degradation of methane-containing gases. Hazardous Substances – Clean Air 73 (2013) No. 5, pp. 183-186.
- Jochen Hahne: Biofilters in animal husbandry. Gefahrstoffe – Reinhaltung der Luft, Vol. 73 (2013) 5, pp. 187-191.
- VDI 4255 Blatt 2:2009-12 Bioaerosols and biological agents; Emission sources and control measures in livestock operations; Overview. Beuth Verlag, Berlin. S. 21.
- Torsten Herold, Mirko Schlegelmilch, Bernd Dammann, Jan Streese, Rainer Stegmann, Andreas Hensel: Germ retention during the treatment of intensive rotting exhaust air in a bioscrubber/biofilter system. Gefahrstoffe – Reinhaltung der Luft, Vol. 61 (2001) 6, pp. 255-260.
- Andreas Oberhammer; Method for the uniform moistening of a filter mass; Patent specification; 13 January 1997; Retrieved 25 December 2010
- Andreas Oberhammer; Filter mass for a biogas filter; German Patent Office: 11 March 1993; Retrieved 25 December 2010
- VDI 3477:2014-05 (draft) Biological exhaust gas cleaning – Biofilters. Berlin: Beuth Verlag, pp. 16-17.