CA1264454A - Method and apparatus for biological treatment of waste gases - Google Patents

Abstract

A B S T R A C T
Method and apparatus for removal of malodorous and/or toxic components from wast gases by biological treatment of the same. Such removal is accomplished by passing the gas stream through a humidifying temperature conditioning section and then through a filter bed. The filter bed consists of a mixture of an organic biological active material, like compost, an inert fraction and additives for neutralization and organic buffening.
Also disclosed is the inoculation of the filter bed with specific micro-organisms adapted to remove constituents of the gas stream which are difficultly biodegradable, e.g.
methylenechloride.

Description

~2~
., , Method and apparatus for biological treatment o~ waste gases.

The lnvention relates to a me~hod of bio-logically filtering gaces, in particular waste gases, by passing them through a layer of carrier material whlch has been provided with appropriate micro -organisms, and to an apparatus for carrying out the me~hod.
For recent years biological ~iltrat~on $s increasingly used for cleaning gas streams origi-nating from various industrial activities like production processes, waste water transport and waste water treatment in order to reduce nuisance, trouble or damage to ~he envi-ronment caused by malodorant and/or poisonous components.
In view of the material often used for bioLogical filtration, one also speaks of compost 11tration. In biological filters other materials like peat, wood chips or fit-branches are used as the so called carrier materials.
In the biological method the removal of har~ful components from the gas takes place by activity of micro-organisms, malnly bacteria and fungi, which are present on or in the carrier material. The Garrier materials for that purpose~ are disposed in a container. The assembly of container, carrier materials and micro-organisms usually is called a biological fil~er. The gas to be treated is introduced into the container by a fan, the gas passing through the carrier material from abo~e to helow or vice versa. The components to be~removed from the gas are adsor-bed at the carrier material and converted by the micro-; organisms present on or in;the~-carrier materlal into sub- -~; stances whish are not harmful to ~he environment.~
A method and apparatus of the kind mentio-ned in the preamble are disclosed in the Dutch patent appll-cation 81.04g87 and in the German patent applicatlons Z.445.315, 2.558.256 and ~.605.606.
The Dutch patent applicatlon Bl.04987 -~ discloses a biological filter for cleaning gases, wherein ;~ 35 rP1~tlvely large inactive particles of the ~tartlng com-post are added to the active compo~t with ~he primary ob~ect of reducing the pressure drop across ~he ~lter ~n~ hance ,~

the energy consumption.
According to the Weetgerman patent application 2.445.315 the activity of the filter is in-creased hy mixing the carrier material with bentonlte earth~ Thereby hea~y metals which adversely affect the biological activity of micro-organismC; arechemicallybound to bentonlte so as to maintain the required biological activity.
It is noted, that the carrier material in biological filtexs usually is oomposed of particles of such size that besides a reasonable adsorbing surface also an acceptable flow resistance is suaranteed. A too small adsorbing surface leads to a too large and uneconomical volume of the filter, whereas a too large filter resistance is accompanied by a too large energy consumption in passing the ga5 stream through the filter.
Furthermore lt is notad that for opti-mal operation of the biological filter a water content of 40-60% in respect of the weigh~ of the carrier material is necessary. In the known methods and apparatuses humidity control, if any, is realized by spraying water across the upper surface of the filter~
It has now appeared ~hat by ~praying water onto or into the carrier material it is not or not sufficiently possible torealize an adequate humidity control.
25~ In particular the degree of humidity of ~he most active portion of the filter,~i.e. the lower side', when the flow throuyh the filter is~from below to above, cannot suffi ciently be ccntrolled by the known method. The known method, e.g. by sprayin~ water externally or internally, is only ~capable of restorin~ to the desired water content of the ` carrier ma~terial, whlc~Ycalamities have dried too muchO
At too low~a mo~sture cvntent of the carrier material the biolo~ical activity will ~trongly dlminish or even totally disappear.
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Furthermore, shrlnk cracks develop in the carrier material in consequence of drying up resultlng in short circultlng the gas flow ln the bed so ~hat a considerable portion of the waste gas 1~ withdrawn ~rom the contemplated biologlcal treatment ShrlnX cralcks can be avolded to a large extent by the choice of a ~ui~able .

~ ~ 6 carrier material.
A too high moisture content in the car-rier mar~rial results in the de~elopment of anaerobic zones.
Such zones have an increased flow re~istance precluding a homogeneous flow of gas, which results in a considerable decrease of the average residence time ofthe gas in the filter bed.
Furt~.ermore the anaerobic zones pro-duce volatile metabolic products leaving the bed with the waste gas. Since theproduced metabolic products have malodo-rant properties, they contribute so the undesired malodorant nuisance.
It is noted that the micro~organisms responsible for decomposing the mainly organic components which are present in the gas to be treated, require various inorganic nutrients for their metabolism. As basic material various kinds of compost axe satisfactory for this purpose.
These materials, however, have the common disadvantage of being strongly subject to aging. Some kindsofcompost have a tendency of lumping accompanied by a strong decrease of the specific surface area ~surface area per unit of volume) aging phenomena of the carrier material can be caused by local loss of moisture by a too low relative h~midity of the air and~or the occurrence of t`amperature gradients in the carrier material.
The occurrence of temperature gradients in the carrier material is inherent to the microbiological I
activity.
This activity may be locally different.
Where this activity is high, the temperature will be ~ slightly higher than elsewhere as a result of the realised ;; energy of ~xidation. ~ ~ ;
a result of these temperature gra-dients will develop alao gradients in the maximum water vapour pressure. This lmplies that biologically acti-ve zones have a tend~ncy of dry:ing up whlle l~ss active zones take up thls excess water by condensatlan.
As a result active zones will expe-rience a loss of activity and wet, mostly lnactive zones will take up still more water.

~L26~45~
Aging phenomena in the carrier material mostly manifest themselYes by the deYelopment of shrink cracks in the carrler material in which drying occurs, and by the deve-lopment of wet, of~en anaerobic zones. Such aging phenomena 5 are mostly irreve.rsIble, which means, that a shrink crack once developed will not disappear automatically.
It has been known that aging phenomena of this kind can be prevented by continuously moving the carrier material throughout the bed and mixing it up outside of the bed lO as disclosed by the German patent application 2.445.315.
Another problem presented by the known methods and apparatuses is that in the microbiological conversion of certain organic components in the waste gases acid components, : a.o. acetates, are produced as intermediate products. Such l5 acid compo~ents contribute to acidiflcation of the carrier material and as a result the microbiologtcal activity will drastically decrease or even totally dlsappearO
Furthermore the decomposition of other organic compoments by acid-intolerant organisms can be considerably 20 hampered. This, for example, is the case with the decomposi-tion of toluene.
In the use of biological filters for cleaning waste gas strongly varying loadings often occur due to dis continuous processes.
~he active micro-organism~ in the filter con-sequently are subject to loadings fluctuating in time. In such cases the concentrations of polluting components in the effluent gas may strongly vary unless one proceeds to the use of a strongly oversi~ed filterbed.
~ It can be generally stated that these fluc-tuations may adver:sely affect maintaining an optimal mlcro-biological aotivity. In discontinuous gas discharges no loading at all takes place at the filter during certain periods of the day. Though the nIc~organ1sms may well survive 35 during perIods of shut-down, also of lon~er duration,for example, weeks, it still means, that the decomposlng capa-city of the mlcro-organi~m~ is not fully exploited.
The lnvention now has the object of providing a method and apparatus for blologically flltering gase~
wherein the above mentioned disadvantages are removed ln an ~.264at~

efficient ~anner.
For this purpose the method of the lnven~ion i5 characteriz~d ln that the gases prior to being passed through the carrie~ material are brought into intimate contact with water in such amannerthat the gases contain the quantity of watPr required for the optimum activity of ~he micro-organisms , the gases are brou~ht to thQ temperature required for that purpos~, and any water soluble substances present in the gases dissolve at }east partially.
The gases to be treated are brought into intimate contact wi~h water by spraying.
This brings the gas streams to the right humidity and to the required temperature as is necessary for the micro-organisms o~ the carrier material in the filter to function optimally.
Furthermore as a result of spraying the gases to be treated in a socall~d pretreatment chamber the water soluble components of the gas stream will at least partially dissolve in the water so as to be removed from the gas stream. Thereby the components which are toxic for the micro-organisms can be removed from the gas stream during this pretreatment so as not to affect the activ~ty of the micro-organisms.
In case of non-toxic but water soluble componen~s decomposable by micro-organisms a considerable portion thereof can be removed from the gas stream by dissolving~ For decomposiIIg the so dissolved components ln accordance with the lnvention now a specific biological population, for example, ac~iYesludge, canadvantageously be introduced into the spray water.
In this manner a lager quantityocomponents can be removed per unit of time from the gases to be treated so as to increase the efficiency of the b$ologlcal 3~ filter which subsequently is contacted by the gas.
Research at the Eindhoven University of Technology on which the invention 1~ based, has shown that adding certaln additional materials to ~he carrler materlal yields very favourable results.

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The additional materials of the i~Yention can be sub-di~ided into materials pre~enting the development of shri~k cracks in the carrier material and reducing the fl~w resistance of the biological filter on the one hand, and additional materials counteractiny -aeidificatlon of the filtermaterial on the other hand.
In particular it has been shown in accor-dance with the inventlon that modifying the structure of the carrier material by adding additional material of the first type may prevent the very disadvantageous aging phenomena.
As a result of adding the additional material, which current-ly is inert material to the carrier material, ~or example, compost, and by mixing it therewith the development of shrink cracks can be counteracted.
As additional material of the first-mentio-ned type, which mainly is inert, and has a diametrical par-ticle size of 3-10 mm, the following substances having a more or less rigid structure can successfully used in accordance with the invention: organic materials, like polyethylene, ~ 2Q: polystyrene, particles of ground automobile tyres, as well as ,~f' inorganic materials like fired clay particIes, ground lava bits, ground coalcin~er particles and pelletized flue gas particIes, perlite and active coal.
The proportion of mixing the additional material with the carrier material is be~ween 30-70% and 70-30% on the basis of volume.
The additional material should pxeviously be thoroughly mixed with the carrier material, for ex~mple compost, so as to obt in a loose structure having a large specific surface.
~ This mixing can take place simultaneously ; with other additions if necessary or preferable. In this respect, for example, mlcro-organisms which by nature are not-present in or on the carrier materlal can be applied to the carrier material by inoculatlon~
~;~ Furthermore, the research at the Elndhoven UniversityofTechnology has ~hown that by thelchoice of 2 sultable additional material also the flow resl~tance can be consider~bly decreased. So, fox example, the 1OW resi~tance at a surface loading of 200 m3/m2/hour is about 8 mm head : ,~

5~
.

of water per meter of bed height when using a mixture of 50%-60%
polyethylene particl~s (low density nr. 1500, particle diameter about 4 mm) and 50%-40% compost.

In contrast therewith there is at a same value of sur-face loading a pressure drop of about 120 ~n head of water per meter of bed height when no polyethylene particles are added.

Porous additional matsrials having a high internal porosity and hydrophilic properties are advantageous, since such materials may function as a buffer for excess moisture in the carrier material, which excess moisture in case of excessive loss of molsture can be yielded by such materials. This effects a certain degree of humidity control in t.he carrler material.
The disadvantage of acidification of the carrier ma~e-rial can be advantageously removed by the use of additional mate-rials of the second typa counteracting the acidification of the filter material. For this purpose a pH-reducing alkaline sub-stance is added to the carrier material and intimately mixedtherewith. ~dvantageously~ for example, marl, limestone and cal-cium ca~bo~ate in a weight proportion of 2-40% in respect of the carrier material can be considered for preventing acidification of thé carrier material. It has appeared that ln particular cases (for example, th~ micro-biologlcal decomposition of methy-lene chloride) considerably higher weight proportions should b~
applied, which may range up to a value of 40~ ln respect of the total filling quantity.

The research at the Eindhoven Un1versity of T GhnOlOgy besides the previously mentioned two types of additional material has shown active carkon to be very suitable as additionaI mate-rial.

Active carhon added to the carrier material in a quan-~ity of 1-50 kg/m3 appears to be very favourable in case the pre-.
...

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sent biological filters of the invention are sub~ected to various loads as in various and/or discontinuous processes. The active carbon in such case is or is not mixed with the carrier material.
These carbon particles have adsorbing properties for organic com-5 ponents which are present in the waste gas to be treated.

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- 7a -~.1 .,., .. ,,.,. ,; :~ ;~
' ` ~ ' '', ~ ' -8- ~26~4 Besides acti~e carbon other substances having adsorbing properties may be used, for ex~nple polyethylene glycol.
Besides their good adsorbing properties such substances should also have good desorbing propertie~.
Thereby it is possible that during a per1od of incr~ased supply the adsorbens temporarily adsorbs the concentration excess anddesorbsit at lower loadson b,ehalfofthe micro-biological decomposition. This permits in principle a considerable reduction in the filter volume requirled at discontinuously discharging plants.
When Yery concentrated waste gas streams are discharged during a short period, in order to reduce the volume of the filter a quantity of adsorben~ (active carbon) is required which will be in excess o 50 kg/m3 and will be at maximum 250 kg/m3.
At such high concentrations of carbon these particles can also take overthe function of inertcarrier material.
It i5 noted that in accordance with the present method the temperature of the spray water is chosen so that the temperature of the gases to be treated is between lO and 40C.
In accordance with the invention ~he relative humidity of the gases after pretreatment with water ~s usually 95-100%.
The invention also relates to an apparatus for carrying out the method of the invention for biologically flltering gases, in particular waste gases~`
Such apparatus c~mprlses a container lncluding a biological filter unit, a supply conduit for the gases to be treated at the lowex side and a discharge conduit for the gases to be treated at the uppPr side.
The apparatus of the invention is characterized ~n that at least one f~lter unit ls arranged in the contai-ner and a pretreatment chamber for the gases to be treatedi9 di~posed underor at the lower sideo~the lowermost filterunit.
Usually the contain2r ~s cylindrlcal, though it may be rectangular or be differently shaped.
In the apparatus of the lnvention the pretreat-ment chamber and the lowermost filter unit are separated -9- ~26~45~

from each other by a support layer permeable for the gas to be treated. The support layer us~ally is a perfcrated metal or plastics plate~ The pretreatment chamber is proYi-ded withaspraying water facility for intimately contactinS
5 the waste gases with water.
In a favourable emhodiment the pretreatment chamber is provided with a contact bed arranged above the supply conduit for the gases to be treated and resting on the gas-permeable support layer, a discharge conduit for 10 spraywater under the support layer and the spraylng Eacility for the water above the contact bed. The contact bed enhances the intimate contact between the yases to be treated and the spray w~ter. The contact bed advantageously can be a gravel bed, ~hough other suitable materials also can be used.
To reduce the quantity of water used, it is advantageous, when the spraying facilitiesareconnected to a dlscharge conduit for spraywater via a cixculation conduit including a circulation pump for circulating the spraywater.
For adjusting the temperature of the spray-20 water it is favourable when the circulation conduit includesa heat exchanger. Adjacent to the pretreatment chamber is a filter unit, which, in a simple embodiment, consists of a gas-permeable support plate and a biologically active zone.
Currently, the gas-permeable plate ls a perforated metal or 25 plastics plate.
In order to obtain a more homogeneous dis tribution of the gases to be treated across the biologically active zone use can be made of a contact material on which the biologically active zone rests. As a contact material 30 advantageously lava bits or graveI are used, though other ~uitable materials al o can be considered for this purpose.
According to the invention the biologically a~tive zone can be composed of a suitabl~ carrier material, for example compost, peat, wood chips, etc. Preferably, the 35 bIological active zone is composed of such carrier material and an appropriate additlonal material as di cussed pr~-viou~ly.
Advantageously, spraying acilities~re dispa~-sed in the upper portion of the filker unit ln order to 40 prevent ~he biologlcally active zone from clrying up ~n case 1 264~5~
of calamities.
There can be circumstances requirin~ that the apparatus has a plurality of filter units disposed above each othPr and separated from each other by a gas-5 permeable suppoxt plate being a perforated metal or plasticsplate as mentioned previously.
A favourable embodiment of the apparatus is illustrated in the single drawing.
The container 1 which in thi5 embodiment is 10 cylindrical is composed of a pretreatment chamber 2 for the waste gases and a filter unit 3. In the pretreatment chamber is provided the supply conduit for the gases to be treated 4 and the support layer 9 which in this embodiment is a perforated metal plate, The pretreatment chamber may be 15 arranged separately from the container.
On the support layer 9 a gravel bed is pro-vided for enhancing the contact between the upward gas-stream and the downwardly sprayed liquid.
Up in the pretreatmbnt chamber 2 spraying 20 means 5 are provided. Down in the pretreatment chamber 2 the spray water may be tapped through the discharge conduit 20 and the valve 21 or it may be circulated through the circulation conduit 19 including the circulation pump 6 and the heat ~xchanger 7. In the heat exchanger 7 the spray 25 water can be brought to the desired temperature, i.e~ when a higher temperature is desired, the spray water can be heated in the heat exchanger or when a lower temperature is desired, the spray water can be cooled.
Above the pr~trea~lent chamber 2 is prov~ded 30 the filter unit 3 separated by the gas permeable support :: plate 10 on which rests the biologically actlve zone 11.
The biologically active ~zone 11 can be composéd of~the car-rier material alone but~preferahly of a mixture of carrier mat~riaL and~the previously dlscussed additional materials.
Above the blologically active zone 11 are provided sprayers 12 which in case of emergency, for example, when the spraying means 5 in the pretreatment chamber ~
should fall, can ecure the moistening o~ the biolo~lcally active zone.
On the filter unlt 3 is fastened the top section 14 through the fastenin~ means 13, for example a screw means.
Th~ biologically filtered gas can now be dis-charged to the atmosphere through the discharge 15 and the valve 16 or it can be conducted through the conduit 17 and the valve 18 to a measuring and sampling apparatus.
On the filter unit 3, if desired, a plurality of filter units can be arranged which then can be fastened ~ogether by a similar fastening means as 13, for example.
For convenience, such fas~ening means can be a screw mean~.
The use of more than one filter unit can be ne-cessary in cases wherein the waste gases to be treated con-tain components requiring for their decomposition different lS conditions possibly including dlfferent miorQ-organisms or when the waste gases to be treated include one certain com-ponent in such a high concentration that the capacity of one filter unit is inadequate for sufficiently decomposing it.
By a uniform construction of the filter units the apparatus of the invention, if desired, can be adapted to biologically cleaning various waste gases of differant compositions by simple fastening the said filter units on each other.
~; 25 The principle of ~ultiplé filter units can also ; be applied by dividing a gas stream to be treated into two of more equal streams and by conducting the separate streams to separate filter units~isposed above each other in one column.
: 3a ~hen the method is effected in such an appara-U5, a considerable further reduction of the pressure drop of ~he gas stream through the fiLter can be reached. This decrease of the pressure drop for two units arranged inpa-rallel theore~ically amounts to a fa~tor 4 in comparison with the case of two units ln series: namely a ~ac~or 2 ~or ~hs reduction of the gas loadlng per m~ o~ filterarea traversed and a ~ac~or 2 for the reduction of the filter helght.
The method of the lnvention will now be ex-plalned by way o~ the followlng examples.

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Exam~le I.
An apparatus accordiny to the in~ention and ln-cluding a pretreatment chamber and five filter units arran-ged ln series was continuously fed with a synthetic wast~
gas from a paint spraying shop. The diameter of a filter unit was 15 cm, the filling height was 60 cm, the ~olume o carrier material thus being about 11 1 per filter unit.
The composition of the filter material was 29% by weight of peat compost,59%by weight ofpol~ethylelle particles tlowdensitY
nr.1500,average particle diameter about4mm),2~by weight ofmarl.
The filter material was inoculated with an active sludy~
suspension and a toluene decomposing organism which was iso-lated from a ground sample of a petrol filling station.
The synthetic waste gas contained a mixture of the components aethyl-acetate, butyl-acetate, butanol and toluene.
The gas output practised was 67 l/min corresponding to a gas load of 220 m3/m2/hour, the temperature was about 20C.
The measured total pressure drop across the filter units at the said gas output was 26 mm head of water.
Under stationary conditions the following con-centrations were measured: 3 Gasconcentration in mg/m Component toluene butylace- aethylace- butanol tate tate supply pretreatment 958 218 205 127 25 chamber discharge pretreat- 941 214 200 0 ment chamber discharge firstfilter 936 89 95 0 unit d'schar~e secondfilte~ 928 17 38 0 discharge thirdfilter 908 0 0 0 unlt ;
dlscharge fourth filter 872 0 0 0 unit discharge fif h filter 810 0 0 0 ., . ~ _ ~ _ ,, . ~ ~ , ,. , ~ , ~6~5~
Example II.
A heaYily loaded apparatus includlng a pre~
treatment chamber and a one-stage filter unit was fed wlth a synthetic waste gas which containedaethyl-acetate as a polluting cQmponent. The filter unit had a diameter of 15 cm and a filling height of 100 cm, thus a filling volume of about 18 1. The filling material of the filter unit consis-ted of 7920 g of peat compost' 108 g of polystyrene parti-cles ~diameter about 3 mm) and 216 g of marl.
The said filling material was inoculated with micro-organisms from an active sludg~ suspension taken from a waste water treatment plant.
The filter unit was daily intermittently loaded: during 8 hours per 24 hrs a waste gas at a volume output of 100 l/min and therefore a gas load of 340 m3/m2j hour, and having an aethyl-acetate inle~ concentratlon of 825 mg/m3 was fed. During the remaining 16 hours aeratlon of the filter with clean air (air output 10 l/min) took place.
Eight days after start-up of the filter the dynamic behaviour was studied. For that purpose the concen-tration of aethy~acetate in the waste gas from the filter unit was measured after application of the load.
This concentration appeared to increase sub-stantially immediately from zero value until after about 30 min a stationary waste gas concentration of about 320 mg/
m3 was reached. Thls concentration was maintained during tha remaining period of 7~ hours for which the filter was loaded.
:
To the fi}ling material in ~he apparatus des-cribe~ ln Example lI 549 of active carbon was ~dded to be mlxed therewith.
Also in this case the filter was dally inter~
mittently loaded in accordance with ~he data in Example II, lt being under~tood that at the s~me tlme the lnle~ concen-tration ofaethyl-ac~tate ln the ~ynthetic waste ga~ to be cleaned had been further lncreased to about 1610 mg/m3.

A~ter the filter in this way had been loaded for about 14 days, the dynamic behaYiour wasagain studied.
After application of the load the concentration of aethyl-acetate in the waste gas during the first 30 mln appeared to ~e substantiallyzero to slowly increase subsequently.
This increas~ continued during the remaining time of the loaded period. At the end of this periodS after 8 hours, the ethylene-acetate concentration in the waste gas was about 700 mg/m3.

Example_IV, An apparatus according to the invention incl1~-ding a pretreatment chamber and three filter units arranged in series was discontinuously ~ed with a waste gas from a pharmaceutical plant. The diame~er of a filter unit was 150 cm, the filling height was 100 cm, the volume of carrier material thus being 1,77 m3 per filter unit. The composition o~ the filling material in the filter unit was 21~ by weight of peat compost, 39~ by weight of marl and 40~ by weight of active carbon.
Once a week a waste gas was treated by the fil-ter, said gas a.o. containing a methylene chloride concen-tration of ~00 mg/m3 after the pretreatment chamber. The gas output practi ed was 350 m3/hour corresponding to a gas load o~ 200 m3/m2/hour. The discharge time of the waste gas was S~hours. In these S hours thè methylene chloride was quan-titavely captured in the upper filter unit by adsorption at the active carbon present in the filling material.
~After the dlscharge of the waste gas the methylene chloride was desorbed~from the active carbon by passing an output o~
50 m3/hour of clean ~oom air through the filter. The desor~ed ; methylenechloridewas adsorked by the biologically active frac-.
tion of the filling material and subsequently biologically decompo~ed.
The biological decomposing capacity of the fil-llng ma~erial for^recalcltrant component methylene chlorlde was obtained in that be~ore the start-up a suspension of methylene chloride decompQslng micro-organisms, which had been especially prepared for this purpose, was ~ixed through the fillina material of the upper filter section.

15 ~2~4~

The biological decQmposin~ capacity of methylene chloride at 20C was 30 g/~3/hour.
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Claims (36)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of biologically filtering gases containing pollutants by a bed type filter material containing a carrier material which has been provided with a biologically active zone of micro-organisms which are stationary on the surface of the carrier material supported on a gas permeable support layer of perforated metal or plastics plate, in which the gases are initially water saturated prior to their entrance into the filter material by bringing the gases into intimate contact wlth water in such man-ner that the gases contain the quantity of water required for the micro-organisms, to optimally function, the water saturated gases are then directed into the filter material and passed through it, whereby the pollutants in the water saturated gas come in direct contact with the micro-organism on the surface of the carrier material.

2. The method of claim 1, in which materials are added to the carrier material which counteract the development of shrink cracks in the carrier material and which reduce the flow resistance of the blologlcal fllter, the material being selected from polyethylene particles, polystyrene particles, particles of ground automobile tire fixed clay particles, ground lava bits, coal cinder particles, pelletized flue dust, perlite pellets, and active carbon.

3. The method of claim 2, in which the particles of the material have a diameter of 3 to 10 mm.

4. The method of claim 2, in which the material is intimately mixed with the carrier material in a proportion lying between 30-70% and 70-30% on the basis of volume.

5. The method of claim 2, in which microorganisms are applied to the carrier material which microorganisms by nature are not present in and/or on the carrier material.

6. The method of claim 1, in which materials counter-acting acidification are added to the carrier material.

7. The method of claim 6, in which said material coun-teracting acidification is selected from limestone and calcium carbonate.

8. The method of claim 6 or 7, in which the material is added in a weight proportion of 2-40% relative to the carrier material.

9. The method of claim 1, in which a strongly adsorb-ing material for the substances to be removed is added.

10. The method of claim 9, in which said material is active carbon, and is added in a quantity of 1-250 kg/m3.

11. The method of claim 1, 2 or 3, in which the water is sprayed for the gases being brought into intimate contact with the water.

12. The method of claim 1, 2 or 3, in which the weight ratio of spraywater to gas is from 1:10 to 10:1.

13. The method of claim 1, 2 or 3, in which the gases are brought to the temperature required for the micro-organisms to optimally function, the temperature of the water is selected so that the temperature of the gases to be treated is between 10 and 40°C.

14. The method of claim 1, 2 or 3, in which the rela-tive humidity of the gases after pretreatment with water is 95-100%.

15. The method of claim 1, 2 or 3, in which in the water a specific microbiological population is introduced for decomposing part of the substances dissolved in the water.

16. The method of claim 1, 2 or 3, in which the water soluble substances present in the gases after dissolving in the water are removed from the gases to be filtered.

17. The method of claim 1, 2 or 3, in which the filter material includes a gas permeable support layer of perforated metal or plastics-plate and a biologically active zone of micro-organisms supported on a gravel bed carrier material.

18. Apparatus for biologically filtering gases con-taining pollutants, by a fixed bed type filter material a carrier material which has been provided with a biologically active zone of microorganisms which are stationary within the fixed bed on the surface of the carrier material, wherein the gases are ini-tially water saturated prior to their entrance into the filter material by bringing the gases into intimate contact with water in such a manner that the gases contain the quantity of water required for the microorganisms to optimally function the water saturated gases are then directed into the filter material and passed through it, whereby the pollutants in the water saturated gas come in direct contact with the microorganisms on the surface of the carrier material, the apparatus comprising a container including at least a biological fixed bed type filter unit including the fixed bed having a gas permeable support layer of perforated metal or plastics plate and the carrier material, a supply conduit for the gases to be treated at the lower side and a discharge for the treated gases at the upper side, in which at least one filter unit is arranged in the container, means for moisturizing the gases prior to entry of the gases into the fixed bed, and the fixed bed located downstream of the moisturizing means.

19. Apparatus of claim 18, in which the filter unit comprises a gas permeable support layer of perforated metal or plastics-plate and a biologically active zone of microorganisms supported on a gravel bed carrier material.

20. Apparatus of claim 18, in which the moisturizing means includes a pretreatment chamber arranged separately from the container.

21. Apparatus of claim 18, in which the container is cylindrical.

22. Apparatus of claim 18, in which the moisturizing means includes a pretreatment chamber and the lowermost filter unit separated from each other by a support layer permeable for the gas to be treated.

23. Apparatus of claim 22, in which the gas-permeable support layer is a perforated metal or plastics-plate.

24. Apparatus of claim 18, in which the moisturizing means includes a pretreatment chamber provided with spraying facilities for the water.

25. Apparatus of claim 18, in which moisturizing means includes a pretreatment chamber provided with a contact bed arranged above the supply conduit for the gases to be treated and resting on the gas permeable support layer, a discharge conduit for spraywater under the support layer and the spraying facili-ties for the water above the contact bed.

26. Apparatus of claim 25, in which the contact bed is a gravel bed.

27. Apparatus of claim 25, in which the spraying means are connected to a discharge conduit for spraywater via a circu-lation conduit including a circulation pump for circulating the spraywater.

28. Apparatus of claim 27, in which the circulation conduit includes a heat exchanger.

29. Apparatus of claim 18, in which the filter unit comprises a gas-permeable support plate and a biologically active zone.

30. Apparatus of claim 29, in which the gas-permeable support plate is a perforated metal or plastics-plate.

31. Apparatus of claim 29, in which the biologically active zone rests on a contact material.

32. Apparatus of claim 31, in which said contact mate-rial is selected from lava bits, and gravel.

33. Apparatus of claim 29, in which the biologically active zone is composed of a carrier material and materials which counteract the development of shrink cracks in the carrier mate-rial and which reduce the flow resistance of the biological fil-ter, material being selected from polyethylene particles, polystyrene particles, particles of ground automobile tires, fixed clay particles, ground lava bits, coal cinder particles, pelletized flue dust, perlite pellets, and active carbon, materi-als counter acting acidification or strongly absorbing material for the substance to be removed.

34. Apparatus of claim 18, in which spraying facili-ties are disposed in the upper portion of the filter unit.

35. Apparatus of claim 18, in which more than one fil-ter units present which units are separated by a gas-permeable support plate.

36. Apparatus of claim 35, in which the support plate is a perforated metal or plastics-plate.