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BioSal Anlagenbau GmbH
Gewerbegebiet: An den Angerwiesen 6
D-04651 Bad Lausick
Germany
Tel:  ++49 (0) 34345-25151
Fax:  ++49 (0) 34345-25153
e-mail: info@biosal.eu.

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information about biofilter plants

No Chance, not only for odours

Combined Biofilter-systems for Industrial Exhaust Air Treatment

Dieter Sattler, Jörg Sattler, Dr Ralf Schneider

For cleaning exhaust air polluted with volatile and/or odouous components (e.g. VOC, solvents, H2S, Ammonia) several methods have been developed, which could establish with different intensity on the market. In the following a brief overview about the possibilities will be given, after that details on the method of biofiltration will be shown.

In Table 1 different methods are shown as an example while no plants for smoke gas cleaning were named (Desulphurization, Denitrification). The most decisive factor for choosing one of the named methods is besides the achieved efficiency also the investment and operational costs.

While the oxidative methods of exhaust air cleaning use air oxygen to convert the pollutants (mostly organics) and the odour components to ecologically harmless and non-odourous end products. Thermal methods such as catalytic or thermal re-combustion or thermal/regenerative re-combustion respectively use oxygen to convert the organic pollutants and odour components completely to

  • carbon dioxide,
  • water,
  • NOx (if the substances comprise nitrogen),
  • SOx (if the substances comprise sulphur),
  • Halogenized Hydrocarbons (if the substances comprise halogens).

They need more or less high input of primary energy by initial incineration or support heating, or other heating of the exhaust air stream respectively. This normally can be seen in the operational costs. Furthermore mostly high concentrations of pollutants are necessary for the system to work efficiently by using the energy content of the pollutants. Otherwise an unnecessarily big amount of carrier gas (air) must be heated up with high costs.

Investment costs for thermal and catalytical oxidative processes are comparably high, caused among other things by high-grade, thermally and chemically resistant reactor materials, expensive safety features and increasing prices for catalysts. Besides that, especially concerning the investment costs for these methods, a smoke gas cleaning (NOx, SOx, HX) stage, and, if using the catalytic re-combustion, a gas scrubber stage (HX-sorption) is needed.

Method

Principle

catalytic re-combustion

oxidation

thermal re-combustion

oxidation

thermal regenerative re-combustion

oxidation

microbial oxidation in a biofilter

biooxidation

Absorption in a bioscrubber with adjacent microbial oxidation

separation, followed by biooxidation

UV-induced oxidation

oxidation

gas scrubbing with or without regenerative washing media

accumulation / separation

Adsorption on porous adsorbants, e.g. activated carbon, adsorbing resins, molecular screens or similar

accumulation / separation

direct and indirect condensation processes

accumulation / separation

Table 1: Chosen methods for eliminating volatile and/or odourous components from exhaust air streams.

 

Oxidation with UV-radiator

In recent years oxidative methods by means of UV-radiation sources are offered more frequently. Using Hg-middle pressure radiators (UV-A, UV-B, and parts of UV-C) the air pollutants and odour components are to be oxidized by creating short-term animated oxygen molecules. Besides that the pollutants themselves absorb the light quantums in a certain amount what makes them more reactively animated short-chain radicals.

The method of the UV-oxydizing is more suitable in the gas phase with low gas velocities in the reactor and thereby a longer stay in the reactor. It is also necessary to provide a very dust-free and dry air for a good transmission of the UV-rays and thereby to achieve a high efficiency of the method. Furthermore the operation of the Hg-middle pressure radiators and the created amounts of ozone through that can cause a secondary pollution when the ozone does not react correctly with the original pollutants.

The life cycle of the middle-pressure radiators used for the UV-ray creation is highly limited to only 1000 to 1500 operating hours. Exchanging the radiators that often would cause very high operational costs if used as a continuous processing. The specific power consumption of a plant for UV-oxydizing is not far from a plant with catalytic re-combustion.

Gas scrubbing, Adsorption, Condensation

Using the methods gas scrubbing, adsorption or condensation shown in table 1, the pollutants and odour components are being accumulated and then separated from the gas stream. Then the separate disposal of the eliminated substances and regenerating or exchanging the used adsorbing materials, washing media or filter materials must be considered in the operational costs. Thereby high costs can occur. These methods, however, are useful if valuable substances can be won from the process (e.g. adsorption or condensation: solvent regeneration from exhaust air with high solvent concentrations).

Microbiological Oxidation of Organic Exhaust Air Contents with Biofilters

Although the word "biofilter" has got a double meaning and has become a common word in everydays language it is necessary to say that this exhaust air cleaning method does not work like a filtration, meaning the separation of particles from a gas phase. During the conversion of air-carried pollutants inside the biofiltration media gaseous organic substances are broken down in numerous single enzymatic reactions with complex kinetics without adding extra energy. The process is aerobic and biological. The biological breakdown in the biofilter is based on the activity of microorgansims (destruents) which are able to biochemically oxidize organic and also some inorganic components to harmless or, if needed, non-odourous substances. Most transport and reaction models are based on the idea that the microbes are to a high degree immobilized in a biofilm on the porous surface of solid materials (biofilter media) (picture 1).

Pic. 1: Simplified representation of the substance transport and the microbiological conversion in the biofilter

The whole process with several stages can be (simplified) devided into:

  • mostly convective transport of the molecules from the gas phase to the phase border gas phase/water phase
  • Diffusion of the molecules into the water phase and absorption
  • further transport of the molecules into the biofilm
  • microbiological conversion of the pollutants
  • release of reaction or metabolism end products into the water phase/gas Phase (revers 3. to 1.)

Involved in the microbiological breakdown in the biofilter are mostly heterogenous populations consisting of bacteria, actinomycetes and funghi. Their composition is mostly affected by the substances in the exhaust air stream, which are nutrient and energy source for these microbes. In most biofilter materials it can be monitored that the microorganisms (MO) settled on the materials are to a high degree funghi and actinomycetes. The natural potential of different microorganisms can with time be adapted to substances that are normally hard to decompose.

The following factors do, besides the type of the air-carried pollutants, influence the composition and the activity of the microflora and thereby the allover efficiency of the biofilter itself:

  • Sufficient availability of oxygen: Oxygen is available in water, carbon dioxide and numerous organic compounds. But most of the microbes can only use molecular or solved oxygen, which has to be provided continuously.
  • Nutrient availability: Optimum C:N:P-ratio, complementary sulphur and trace elements.
  • Water availability: Water is a basic requirement for every biochemical reaction taking place in the cells. In the biofilter it is not the total water content which is decisive, but its availability for the microbes. Is there no water available, the active and passive nutrient transport into the cell as well as the transport of metabolism end products from the cell is being stopped. In the end the metabolism or the microbial activity respectively is stopped completely.
  • Optimum temperature range: Regarding their optimum growth rates the microorganisms can simplified be divided into three groups:

Psychrophilic MO 15 35C

Mesophilic MO 20 50C

Thermophilic MO 40 70C.

For the exhaust air cleaning in biofilters predominantly mesophilic and thermophilic microorganisms are meaningful. Which of the named types of microorganisms settle on the media while cleaning a specific air flow depends mainly on the temperature of the raw gas, in smaller filters also on the ambient air temperature.

  • pH-value: metabolism, growth and breeding of the MO depend on the pH-value. In the neutral pH-range the widest spectrum of species is representative, because most MO do find the best conditions here. Mould funghi and yeasts (acidophilic) can be found in a more acidic range.
  • Biofilter material: The carrier material in the biofilter for the settling of the microbes plays a decisive role for the necessary efficiency and the performance of the filter. Biofilters normally are equipped with biologically active organic material such as root wood, bark mulch, humus, quality compost or peat moss products. Additionally also bigger amounts of highly-resistant support material can be brought in, e.g. filled out clay, polystyrol balls, porous glass or ceramics. When this highly resistant material is added to the filter material in bigger amount it may happen that adding nutrients for the microbes is essential.

According to the german norm VDI 3477 high-quality filter material for biofilters should have the characteristics shown in table 2.

 

Filter material characteristics for Biofilters

Homogenous structure (even flow distribution, low pressure loss)

Sufficient porosity and void volume (low pressure loss, low filter resistance, good drainage and oxygen supply)

High share of organic substances (big additional nutrient supply, long filter life periods)

Good nutrient availability (ensuring the demand for C, N, P, S and trace elements)

Big surface (optimum density of settling, high ability for biosorption)

Good water balance ability (constant and even moisture)

Low rotting velocity (long stand structure, structure stability, long maintenance intervals)

Low self-smell

Low price

Table 2: Characteristics of high-quality filter materials for

biofilters according to the german norm VDI 3477

Design of Biofilters

Besides the composition of the raw gas stream, the following parameters are important for the design of a biofilter:

Gas throughput [m/h]

Gas area load [m*m-2*h-1]

Gas volume load [m*m-3*h-1]

specific resistance [Pa/m]

Relative moisture of the filter material [%]

Optimum filter working temperature [C].

For exhaust air cleaning with biofilters a comparably big filter volume is necessary to provide sufficient staying times for the gas in order to be processed by the microbiological breakdown.

In recent years a variety of different biofilter designs was developed, which can be divided by their construction into:

  • Area/windrow biofilters
  • Storey biofilters
  • In-vessel biofilters
  • Tower biofilters with filter layers up to 6m
  • Rotating biofilters
  • Dynamic biofilters

In-vessel Biofilters in Modular Design

Self-contained biofilters have got the great advantage compared with open systems that more control of the moisture balance for the filter material is possible. Moreover the filter bed is reliably weather-protected. BioSal-biofilters (BioSal®-FIL system) are manufactured as containers with one or more layer walls (optional hook lift design) with or without integrated functional unit (control panel and ventilation etc.). Picture 2 shows an overview sketch of such a biofilter plant. The air flow in the biofilters can optional be lead vertically from the upper side to the bottom or opposite. A flow from the top to the bottom has got the advantage that in general the moisturizing water can be distributed better in the filter.

Picture 2: Biofiltration plant for use in a WWTP: External functional unit for ventilation and air conditioning.

The container biofilter plants are designed modular and can also be added supplementary according to the exhaust air amounts. This makes a step by step investment possible.

To support the filter material corrosion resistant rests (grids) are brought in to provide a good air distribution in the filter. A drainage system is integrated in the rest to drain off excess or condensed water. By means of special constructions on the filter side a short-cut flow of the exhaust air is avoided. That prevents a so-called "breakthrough" of the raw gas through the filter. The filter walls are made of stainless steel, plastics, or steel with several corrosion resistant layers. The outer support construction of the biofilter containers normally consists of colour-coated or hot-dip galvanized steel.

Biofilters can be equipped with different filter materials. The standard biofilter material is a well-tried combination of shredded root wood, bark mulch, quality compost, and dynamically produced substrate. These components are brought into the container in layers with different thickness. The filter material is built in that way, that the lowest achievable pressure loss and the best air flow can be provided. This long-term experienced substrate combination according to the german norm VDI 3477 makes it possible for the microbes to find an optimum settling surface, grow to a high population density as well as providing a good water balance and nutrient ability. Besides the standard mixture other materials can be added, that can be other organic substrate combinations or biofilter materials with inorganic supporting substrates.

The service ability life of the standard filter material amounts at least two years. The natural ageing of the material is caused by sintering and microbiological degradation. The used material, which is similar to compost, is normally free of poisonous substances or other pollutants and can be disposed without problems.

Usually the microbes on the filter material surface adapt to the specific emission spectrum as well as to the temperature range of the exhaust air stream after an adaption time of about two or three weeks. Moreover it is possible to initially inoculate the biofilter material with a biologically active nutrient solution.

The pictures 3 and 4 show further application for these plants.

Picture 3: Biofilter plant for use in a paint shop: Integrated functional unit for ventilation and air conditioning

Picture 4: Biofilter plant for use in a plastics factory

Combinations of Biofilter plants with upstream or downstream process stages

The cooperating companies of the BioSal Group BioSal Anlagenbau GmbH and BEV Sattler Umweltanlagenbau e.K. develop and manufacture combined exhaust air treatment plants for throughputs of 100 m/h up to >50,000 m/h. Chosing the specific plant combinations is done while considering the legal and official regulations, as well as local and economical conditions. This depends mainly on the raw gas parameters, such as input temperature, input air humidity, dust content, concentration of gaseous pollutants, and the aimed parameters or the plant efficiency respectively. The combined exhaust air treatment plants with the basic component biofilter usually consist of several modular connected single components adjusted to the loads in the raw gas (picture 5):

  • Aerosols and dust separation
  • Condensate separation and air cooling: In almost every warm gas stream with high input air humidity it is recommended to use an upstream combination of air cooling and condensate separation. That prevents water, carried in the saturated moist raw gas stream, from condensating massively on colder, following plant components to cause problems for an unproblematic processing.
  • Exhaust air scrubber: Single or multi-stage vertical counterstream scrubbers with or without fillings/packages for absorbing or preliminary separating of pollutants from the raw gas are possible. If required the parameters of the washing solution can be registered online for aim-directed dosage of chemicals, pH-value control or separation of products / saturated washing solution. The most common applications are up to now the acidic scrubbing of NH3 as well as the scrubbing of H2S (basic chemicals) out of raw gas streams. Besides that also counterstream scrubbers can be operated with specific absorber fluids, e.g. washing solution based on Polyethyleneglycoldibutylether for the air scrubbing of hydrocarbon polluted exhaust air.
  • Air conditioning upstream, prior to the input to the biofilter: In most BioSal-FIL biofiltration plants the input air is humidified up to 95% by an upstream air humidifier including a rectifier and drop separator. The horizontally placed air humidifiers have got a circulating water line with spray nozzles as well as an automatic supplementary water supply to equal the the water carried away with the air stream. The high-performance spray nozzles saturate the air with water. The thermostate controlled heating circuit acts as a frost protection in the water storage.
  • Modular biofilters: Optional in hook lift design for easy lorry transportation.
  • Adsorber stage: In particular applications short-term peaks of pollutant emissions can occur (concentration peaks), which - among other things can be able to break through the biofilter bed, which is designed for normal concentrations, in reduced amounts. If required a downstream adsorber stage with steam resistant adsorbents can be integrated in the plant solution for a reliable catching of the concentration peaks. Adsorbents can be activated carbons or cokes, adjusted to the emission spectrum and the gas humidity. Mostly containers or cylindrical adsorber stages made of steel, stainless steel or plastics are used.
  • Regenerative thermal or catalytic oxidative downstream combustion: This system can be integrated into selectively registrated air streams with high share of volatile halogen hydrocarbons (e.g. methane), when the biofiltration stage cannot degrade them or even an adsorption stage cannot catch these compounds. The basic conditions for an economically sound operation of such combustion plants are determined by the total volume stream and the input concentration. Ideal would be an autothermal operation, that means where the energy is set free during the combustion of the pollutants and at the same time is enough to keep the combustion temperature level. The energy demand depends on the heat regeneration efficiency, which at an efficiency of 98% needs a total carbon concentration of 1,000mg/m. The necessary heating energy in this case can be taken totally from burning the hydrocarbons. If the thermal plant is only used for a temporary catching of pollutants, which are usually not in the gas stream, it can be on demand switched off or by-passed by means of an online methane probe or an online FID.
  • Exhaust air chimney: The in-vessel construction of the BioSal biofilter containers makes it possible to register the cleaned gas without problems and free of any losses. This way the cleaned gas can be let to an exhaust air chimney.
  • Functional unit: This is either an external or integrated unit, which is frost and sonic protected, containing the electrical plant technology, ventilator(s), control and switching board including SPS process control. Important plant parts such as pumps or ventilating fans can be carried out double for process safety reasons.

Picture 5: Example for configuration of a modular exhaust air treatment plant, here used in an exhaust air treatment for composting municipal solid waste (german: MBA).

Picture 6: Functional scheme of a modular BioSal-AMMON counterstream scrubber.

Counterstream scrubber for ammonia removal from exhaust air

When exhaust air streams carry higher concentrations of ammonia (NH3), e.g. in exhaust air streams from the mechanica biological waste treatment (MBA), it is recommended to add a counterstream scrubber to the exhaust air treatment prior to the biofilter, in order to avoid a negative effect on the microbiological degradation process (picture 6). The objective of the scrubber process is to convert the ammonia by means of sulphuric acid to the useable product ammonia sulfate. For efficiency reason the scrubbers are mostly designed in two stages.

The reaction in the scrubber is:

2 NH3 + H2O4 à (NH4)2SO4.

After the scrubber pre-cleaning the exhaust air is, for example, led to the biofilter to remove the other pollutants.

During the conversion of the ammonia the ammonia sulfate in the scrubber solution concetrates more and more until the scrubber solution is saturated and is then exchanged by new water. The product a saturated ammonia sulfate solution can be used as a liquid fertilizer in agriculture or as an amendment for glueing industrial chipboards for example. It can also be used in producing fire extinguishers.

By means of an integrated online pH-value electrode configuration in a flow cell a delay-free determination of the pH-value and thereby an optimum control of the dosage of the ammonia equivalent acid concentration takes place. That avoids a too high adding of acid to the scrubber solution, because only the needed amount of acid is added (according to the ammonia input concentration). Thereby ammonia concentration peaks can be caught from the air stream without problems, while during lower gas loads there is almost no consumption of acid.

The compact counterstream scrubbers can also be easily integrated into already existing exhaust air treatment plants because of their modular design. For the installation only a fresh water and electrical power supply, and a foundation if necessary, are needed.

Preliminary test runs possible

A combined biological chemical exhaust air treatment plant consisting of an exhaust air condensator, counterstream scrubber, biofilter (static or dynamic operation), and adsorber is available for interested users for throughputs of up to 3000 m/h.

For further information you can use the following adress. Enterprises, offering cleaning devices or plants for exhaust air are named under the product group 3.11.1.

_______________________________________

BioSal Anlagenbau GmbH

An den Angerwiesen 6

04651 Bad Lausick

Germany

Tel.: +49 (0) 34345 25151

Fax: +49 (0) 34345 25153

www.biosal.de

info@biosal.de

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