Biofilter

Biofilters are bioaffecting fixed bed reactors for purification of air or water. This is not to filter in the proper sense, since the main purpose is the separation of gaseous or dissolved substances, and not of solid particles.

The idea of ​​clean air in a biological way, was already known in the 1970s, but was only converted in the 1980s thanks to intensive research into practice. Based on a microbiological method arose from it over the years, a modular system concept with almost universal application.

Operation

Biofiltration is a relatively simple and inexpensive method for the purification of geruchstoffhaltiger and VOC -containing exhaust dar. microorganisms dismantled for pollutants and odors into harmless products such as carbon dioxide and water. Biofilters are used mainly for gas purification. But also biofilter in water and wastewater treatment are used for special tasks. The following information relates to the gas purification. The procedures for biological filtration of water samples based on the same principles and are adapted to the liquid medium.

Biological waste air cleaning 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 simplified terms, the microorganisms convert the pollutants with the help of oxygen into carbon dioxide and water vapor, that is, it is a substance- destructive method. This reaction can only take place if the pollutants are transferred from the gas to the water phase, as the water is the habitat of the microorganisms. Therefore, the transition of contaminants in the aqueous phase is an important factor in all biological processes. There sit those microorganisms that are best adapted to the prevailing conditions and to the food supply, so that can adjust to clean pollutants. It always is a mixture of different heterotrophic species which utilize the pollutants in the air as a carbon and energy source.

Designs and operation

Biofilters are operated in many different designs, such as surface filter, container filter stage filter, the honeycomb filter, and the filter tower. In all designs, the exhaust air flows through a filter bed of material. Before the biofilter, an air washer is disposed in some cases in which the air is brought to a saturation level of nearly 100% relative humidity. This is to prevent the drying of the material. In addition, the removal of particles takes place from the exhaust air in the air washer and may vary as necessary. The saturated water vapor and dust- free crude gas is then fed to the bio-filter in which the filter material is located. This is always kept moist by an additional irrigation. The microorganisms are immobilized on the filter material. As it flows through the filter layer, the exhaust constituents sorbed on the surface of the material and are thus the microorganisms to degrade available. To ensure a high microbial activity in the filter, the optimum conditions for the microorganisms with regard to pH, moisture of the filter material, temperature and nutrients must be complied with. The practice shows that the evolving mixed microbial populations in biofilters are very robust when certain parameters are met.

Filter material

At the filter material varied demands. It should have a large specific surface area and hence a large growth surface for the microorganisms having the moisture can store well, causing a low pressure loss as it flows through, comprising a certain buffering capacity to pH fluctuations, which allow an even flow through the filter bed and have a low rotting. In addition, the microorganisms are to be supplied with inorganic nutrients and trace elements. The following materials, as well as mixtures are suitable as filter material:

• Composted bark, or waste
• Heather, twigs or coconut fiber
• Peat
• Paper granules

In addition to loosening these materials are added inert aggregates, such as expanded clay, polystyrene or foam. The filter material is not only a support for the microorganisms, but also a source of nutrients.

Advantages and disadvantages of biofiltration

In the operation of a biofilter, a major problem is to avoid places, dehydration or water logging of the filter material, thereby enabling an even flow through the filter bed. This can be achieved mainly through the encapsulation of the biofilter. The disadvantage is often the large space requirement of these systems, the cost-intensive fan energy to increase the pressure and the permanent irrigation. Compared to other methods, such as the ionization ionization tubes, the constant biological cleaning process is often caused by CO2 savings and numerous economic aspects, such as average acquisition cost, long filter life and average operating cost advantage.

Procedural Basics

The procedural basis is essentially a biochemical oxidation and, therefore, degradation and transformation of substances by bacteria, fungi and yeasts into harmless odor and non-interfering substances. Requirements are that the harmful substances are not water soluble, biodegradable and toxic to the microorganisms. The sessile on a suitable filter material microorganisms utilize the information contained in the exhaust VOC substances for their metabolic processes. The degradation takes place under aerobic conditions and sufficient oxygen must be present. The latter is given by the oxygen contained in the exhaust air generally. Saturated steam exhaust is to be used as the filter material must be moist.

Applications of biofilter exhaust air purification

Biofilters are mainly used for air purification in subsequent installations:

Biofilter for odor removal

The main application of biofilter is traditionally in the purification of waste air containing odorous substance. Microbial degradation of the odor and pollutants to CO2 and H2O and the odor is no longer perceptible compounds is carried out at ambient temperature, so that no additional energy and additives must be used. Operating costs are therefore very low in this method. In many areas, biofiltration is known in the art. In Germany and Europe, thousands of biofilter systems are used to reduce odors from a variety of emission sources. Odor problems often occur in the vicinity of sewage treatment plants, landfills, foundries, breweries, food processing, animal waste processing sites, waste treatment plants, agricultural farms and slaughterhouses. Biofiltration represents the most cost-effective and reliable method for odor elimination - there are efficiencies achieved up to 99 %.

Biofilter for solvent elimination

With the redefinition and tightening emission limits of 31 federal immission control regulation come from the cleaning of solvent- contaminated extract air from a variety of industries such as Furniture coating, surface coating, chemical industry, electroplating, plastics, paint auto parts, etc. also increasingly biofilter used.

Most solvents provide for the microorganisms is a well suitable carbon source they oxidize for their energy and can not convert harmful compounds. In this process, no supporting fuel such as oil or gas, are needed. Advantages of biofiltration compared to thermal methods are thus the CO2-neutral operation, and low operating costs.

From a hydrocarbon concentration of more than 1.500 mg / m³ biological processes to solvent elimination are only conditionally suitable - this should be resorted to other methods of air purification such as the regenerative afterburning.

Biofilter for manholes

One application, for example, the biofilter for manholes, which is hung below the strainer into a manhole and to prevent the escape of strong odors from the sewer, especially at the outlet of pressure pipelines. It features an integrated strainer and a filter filling ( mixture of peat and packing for microorganisms). The resulting surface water is collected on a collecting funnel under the sludge trap and discharged via a downpipe. A rubber seal prevents the escape of untreated air. Modern filter no longer obstruct the air passage. Modern filter paper-based are maintenance free and work 5 to 6 years without constant monitoring and care. The efficiency is 99%.

Demands on the air for biological waste gas purification

The most important prerequisites for the functioning of biofilters are:

• Compliance favorable temperatures ( 5 to 40 ° C in 55 ° C).
• Wetting the filter material in order to prevent drying out of the filter material.
• Homogeneous filtration material to allow no unfiltered passage through the material in cracks and crevices.
• For cleansing ingredients of the exhaust air must be water soluble.
• To clean the exhaust air constituents to be biodegradable.
• Additional nutrients in the filter material.

Wastewater treatment in biofilters

Waste water can be purified in most cases by biological means, It should be noted that the water quality of the ingredients is dependent. The water is purified by adsorption by natural precipitation reactions by plants and microorganisms and also by bacteria in a biological way. This method is applied to the constructed wetlands to purify untreated water and thus improve water quality significantly. The technical treatment plants the waste water is cleaned, among other biological means.